Fibroblast Growth Factor 23 (FGF-23) Concentrations in Cats with Early Nonazotemic Chronic Kidney Disease (CKD) and in Healthy Geriatric Cats

Establishing a Reference Interval for Fibroblast Growth Factor (FGF)-23 in Cats

Fibroblast growth factor (FGF)-23 is a phosphaturic hormone. An association between increasing FGF-23 levels and progression of chronic kidney disease (CKD) was documented in cats, dogs, and humans. The information regarding reference intervals (RIs) of FGF-23 in cats is limited. We aimed to establish RIs in a large cohort of clinically healthy cats and to investigate correlations with sex and age. A total of 118 cats with unremarkable complete blood count and serum chemistry profile were included. Clinically sick cats, cats with concurrent diseases, suspicion of CKD, or receiving renal diets were excluded. FGF-23 concentrations were measured with the FGF-23 ELISA Kit. RIs were calculated using the reference interval advisor software 2.1 (Microsoft Excel). FGF-23 concentrations were correlated with sex and age. The RI for FGF-23 concentrations spanned 85.8 to 387.0 pg/mL (90% confidence interval: lower limit 40.5 to 103.9 pg/mL, upper limit: 354.6 to 425.0 pg/mL). No significant relationships (r2 = 0.044) were detected with age (p = 0.081) or sex (p = 0.191). Other studies of the same diagnostic assay calculated RIs of 56 to 700 pg/mL in 79 cats and <336 pg/mL in 108 cats, and in concordance with the present study, did not detect any correlation with sex or age.

Blood fibroblast growth factor 23 concentration in cats with and without chronic kidney disease: a scoping review

OBJECTIVES

This study undertook a scoping review of research on blood fibroblast growth factor 23 (FGF-23) concentrations in healthy non-azotemic cats and cats with chronic kidney disease (CKD) to describe the volume and nature of existing literature, to determine whether published studies provide adequate evidence to support the use of FGF-23 as a biomarker in clinical practice and to identify any existing gaps in knowledge.

METHODS

PRISMA Extension for Scoping Reviews guidelines were used to design and perform the scoping review. Online databases were used to identify observational and clinical studies of blood FGF-23 concentrations in healthy cats and cats with CKD published before December 2022. Study and population characteristics and descriptive data on FGF-23 concentrations were extracted.

RESULTS

A total of 205 publications were reviewed; 17 were retained for inclusion. Most studies were retrospective. Most studies included cats with International Renal Interest Society stage 2-4 CKD, with some variation. Key concepts explored in the literature include FGF-23 concentrations by CKD stage, effect of dietary phosphate restriction on FGF-23 concentrations, relationship between FGF-23 concentrations and blood phosphorus, calcium and magnesium concentrations, and FGF-23 concentrations in cats with progressive CKD. FGF-23 concentrations tended to be higher in cats with CKD compared with healthy cats, with an overlap between healthy and CKD populations, and there was significant variation within stages of CKD.

CONCLUSIONS AND RELEVANCE

FGF-23 is a biomarker of interest for the management and monitoring of phosphate overload in cats. Studies support several potential clinical applications for measuring FGF-23 concentration in practice; however, evidence is limited. Research on FGF-23 in cats with CKD would benefit from longitudinal, prospective studies that standardize CKD diagnosis and categorize cats by stage using current guidelines. Studies should include cats with early-stage, non-azotemic CKD and use commercially available assays so such results are comparable across studies.

Fibroblast Growth Factor-23 (FGF-23) in Dogs-Reference Interval and Correlation with Hematological and Biochemical Parameters

Fibroblast growth factor-23 (FGF-23) is a phosphaturic hormone used to monitor chronic kidney disease (CKD) in humans. The aims of this study were (1) to determine the intra- and interassay precision of the FGF-23 concentrations in dogs as measured via the Kainos ELISA FGF-23 kit, (2) to calculate a reference interval, and (3) to assess the correlation of the FGF-23 concentration with the hematological and biochemical parameters. The coefficient of variation was below 15% for both the intra- and interassay precision, indicating good reproducibility. The reference interval ranged between 95.8 (90% confidence interval: 44.6; 139.2) and 695.1 pg/mL (598.7; 799.1) based on 136 clinically healthy dogs, classified as such according to the information of treating veterinarians as well as the unremarkable results of hematology and biochemistry. The FGF-23 concentration differed significantly between dogs aged <9 and ≥9 years (p = 0.045). Four groups of 10 dogs each were retrospectively formed based on the creatinine concentration classification according to the IRIS staging. Correlation was the strongest for the renal parameters. Statistically significant differences in the FGF-23 concentration were demonstrated between the study groups I and III (p < 0.001), I and IV (p < 0.001), and II and IV (p = 0.005). There was a trend for a rising FGF-23 concentration in older dogs. Due to the wide reference interval, diagnostic cut-offs and/or subject-based FGF-23 reference values in each dog are needed for monitoring and clinical interpretation.

Comparison of Three Different Diagnostic Assays for Fibroblast Growth Factor-23 (FGF-23) Measurements in Cats: A Pilot Study

Fibroblast growth factor-23 (FGF-23) is a phosphaturic hormone used to monitor chronic kidney disease (CKD) in humans. The aim of this pilot study was to compare three diagnostic assays and to assess how the results correlate with parameters of renal dysfunction in cats. Four groups of 10 cats each were formed retrospectively according to creatinine, based on IRIS staging. FGF-23 was measured using two different ELISAs (MyBioSource and Kainos ELISA FGF-23 Kit) and an automated assay on the DiaSorin Liaison platform. Measurements were performed in 40 cats. Spearman's rank correlation coefficient showed a strong correlation between the Kainos and DiaSorin assays (ρ = 0.742/p < 0.001) and a low correlation (ρ = 0.443/p = 0.005) between the Kainos and MyBioSource assays. The measurements with the Kainos assay strongly correlated with urea (ρ = 0.835/p < 0.001) and creatinine (ρ = 0.764/p < 0.001), and moderately correlated with SDMA (ρ = 0.580/p < 0.001) and phosphorus (ρ = 0.532/p < 0.001). The results of the MyBioSource and DiaSorin assays only showed a moderate correlation with urea (ρ = 0.624/0.572) and creatinine (ρ = 0.622/0.510) concentrations (p = 0.001 each). The Kainos assay showed the strongest correlation (ρ = 0.806) with the various creatinine concentrations according to the IRIS, followed by the MyBioSource (ρ = 0.663/p < 0.001) and DiaSorin assays (ρ = 0.580/p < 0.001). Overall, the Kainos assay demonstrated the best correlations with both biomarkers and various creatinine concentrations according to the IRIS. Individual assay-based reference values should be established to make a reliable interpretation of FGF-23 levels possible to diagnose or monitor feline CKD.

Relationship between FGF 23, SDMA, Urea, Creatinine and Phosphate in Relation to Feline Chronic Kidney Disease

Chronic kidney disease (CKD) is a common diagnosis in older cats, and its prevalence increases with age. Conventional indirect biomarkers of glomerular filtration rate (GFR) have their limitations, and are not efficient in detecting early decreases in glomerular filtration rate. Recently, symmetric dimethylarginine (SDMA) concentrations have been proposed as a novel biomarker of GFR for the early detection of CKD. This study discusses the relationship between SDMA, FGF 23 and previously used indicators of kidney function, mainly creatinine, urea and phosphate. Ninety-nine cats were included in this study. Based on their SDMA values, 48 cats had CKD and the remaining 51 cats were used as a healthy control group. Serum of these cats was assayed for creatinine, urea and phosphate concentrations as well as FGF 23 values, and correlations between them were evaluated. Cats with CKD had higher FGF 23 concentrations than healthy cats, and no correlation was found between FGF 23 and SDMA, nor between FGF 23 and phosphate. On the other hand, phosphate strongly correlated with SDMA, urea and creatinine, making it a possible independent factor of CKD progression.

Serum fibroblast growth factor-23 concentrations in young and mature adult cats with chronic kidney disease

OBJECTIVES

The aim of this study was to investigate serum fibroblast growth factor (FGF)-23 concentrations in young and mature adult cats with chronic kidney disease (CKD).

METHODS

The present study retrospectively investigated the serum samples and medical records of 1-8-year-old clinically healthy cats (control group, n = 7) and cats with CKD (n = 54). Cats with CKD were divided into four stages based on serum creatinine concentrations, according to the International Renal Interest Society (IRIS) CKD guidelines. Serum FGF-23 concentrations were compared between cats in the control group, IRIS stages 1, 2 and 3-4 CKD. Continuous variables were analysed using correlations and multiple linear regression.

RESULTS

Serum FGF-23 concentrations were significantly higher in cats with IRIS stages 1, 2 and 3-4 CKD, compared with those in the control group (P = 0.02, P = 0.002 and P = 0.002, respectively). Additionally, serum FGF-23 concentrations in cats with IRIS stages 3-4 CKD had higher serum FGF-23 concentrations than those with IRIS stages 1 and 2 CKD (P = 0.04 and P = 0.02, respectively). In the multiple linear regression analysis, serum urea nitrogen concentration, serum phosphorus concentration and blood ionised calcium concentration were independent variables predicting serum FGF-23 concentration.

CONCLUSIONS AND RELEVANCE

Serum FGF-23 concentrations in younger cats with CKD increased in early-stage CKD and were associated with mineral metabolic markers, as also occurs in geriatric cats.

New concepts in phosphorus homeostasis and its impact on renal health with particular reference to the cat

New discoveries relating to phosphorus homeostasis include the hormones fibroblast growth factor-23 and klotho produced by bone and kidney. These hormones, together with novel understanding of how calcium and phosphate ions are carried in colloidal form as calciprotein particles, have changed our view of how phosphorus is regulated. Recognition that high dietary intake of inorganic forms of phosphorus in humans is a risk factor for both cardiovascular and renal diseases have led to re-examination of the impact of inorganic sources of phosphorus in prepared cat foods on renal health. Data suggest that when homeostatic mechanisms lead to proximal tubular (S3 segment) phosphate concentrations exceeding 3.25mmol/L for a significant part of the day, tubular stress and structural kidney damage ensues. Recent experimental rodent studies support the concept that calciprotein particles form in the proximal tubule at these prevailing phosphate concentrations and trigger proximal tubular damage. Long-term feeding studies in cats suggest that carefully formulated prepared diets containing 1g/Mcal of inorganic phosphorus (in the form of sodium tripolyphosphate or potassium monophosphate and pyrophosphate), resulting in estimated tubular phosphate concentrations <2.5mmol/L can be fed to healthy adult cats without detectable adverse effects on renal health.

Renal biomarkers in cats: A review of the current status in chronic kidney disease

Serum creatinine concentration, the classical biomarker of chronic kidney disease (CKD) in cats, has important limitations that decrease its value as a biomarker of early CKD. Recently, serum symmetric dimethylarginine concentration was introduced as a novel glomerular filtration rate biomarker for the early detection of CKD in cats. However, data on its specificity are still limited. The limitations of conventional biomarkers and the desire for early therapeutic intervention in cats with CKD to improve outcomes have prompted the discovery and validation of novel renal biomarkers to detect glomerular or tubular dysfunction. Changes in the serum or urinary concentrations of these biomarkers may indicate early kidney damage or predict the progression of kidney before changes in conventional biomarkers are detectable. This review summarizes current knowledge on renal biomarkers in CKD in cats, a field that has progressed substantially over the last 5 years.

First genome-wide association study investigating blood pressure and renal traits in domestic cats

Hypertension (HTN) and chronic kidney disease (CKD) are common in ageing cats. In humans, blood pressure (BP) and renal function are complex heritable traits. We performed the first feline genome-wide association study (GWAS) of quantitative traits systolic BP and creatinine and binary outcomes HTN and CKD, testing 1022 domestic cats with a discovery, replication and meta-analysis design. No variants reached experimental significance level in the discovery stage for any phenotype. Follow up of the top 9 variants for creatinine and 5 for systolic BP, one SNP reached experimental-wide significance for association with creatinine in the combined meta-analysis (chrD1.10258177; P = 1.34 × 10^–6). Exploratory genetic risk score (GRS) analyses were performed. Within the discovery sample, GRS of top SNPs from the BP and creatinine GWAS show strong association with HTN and CKD but did not validate in independent replication samples. A GRS including SNPs corresponding to human CKD genes was not significant in an independent subset of cats. Gene-set enrichment and pathway-based analysis (GSEA) was performed for both quantitative phenotypes, with 30 enriched pathways with creatinine. Our results support the utility of GWASs and GSEA for genetic discovery of complex traits in cats, with the caveat of our findings requiring validation.

Risk factors and implications associated with renal mineralization in chronic kidney disease in cats

Background

Nephrocalcinosis is a pathological feature of chronic kidney disease (CKD). Its pathophysiological implications for cats with CKD are unexplored.

Objectives

Identify nephrocalcinosis risk factors and evaluate its influence on CKD progression and all‐cause mortality.

Animals

Fifty‐one euthyroid client‐owned cats with International Renal Interest Society (IRIS) stages 2‐3 azotemic CKD.

Methods

Retrospective cohort study. Histopathological kidney sections were assessed for nephrocalcinosis (von Kossa stain). Nephrocalcinosis severity was determined by image analysis (ImageJ). Ordinal logistic regressions were performed to identify nephrocalcinosis risk factors. The influence of nephrocalcinosis on CKD progression and mortality risk were assessed using linear mixed model and Cox regression, respectively. Cats were categorized by their owner‐reported time‐averaged phosphate‐restricted diet (PRD) intake, where PRD comprised ≥50%, 10‐50%, or none of food intake.

Results

Nephrocalcinosis was rated as mild‐to‐severe in 78.4% and absent‐to‐minimal in 21.6% of cases. Higher baseline plasma total calcium concentration (tCa; odds ratio [OR] = 3.07 per 1 mg/dL; P = .02) and eating a PRD (10%‐50%: OR = 8.35; P = .01; ≥50%: OR = 5.47; P = .01) were independent nephrocalcinosis risk factors. Cats with absent‐to‐minimal nephrocalcinosis had increasing plasma creatinine (0.250 ± 0.074 mg/dL/month; P = .002), urea (5.06 ± 1.82 mg/dL/month; P = .01), and phosphate (0.233 ± 0.115 mg/dL/month; P = .05) concentrations over a 1‐year period, and had shorter median survival times than cats with mild‐to‐severe nephrocalcinosis.

Conclusion and Clinical Importance

Higher plasma tCa at CKD diagnosis and PRD intake are independently associated with nephrocalcinosis. However, nephrocalcinosis is not associated with rapid CKD progression in cats.

Analytical Validation of a New Immunoenzymatic Method for the Measurement of Feline Parathyroid Hormone in Cats with Chronic Kidney Disease

Simple Summary

Parathyroid hormone (PTH) is involved in many metabolic diseases, such as chronic kidney disease (CKD) and calcium disorders, and its measurement could be of clinical utility. However, available methods for the measurement of feline PTH are limited and not widely accessible. The aim of this study was to perform the analytical validation of a new method for PTH measurement in cats. Thirty-eight cats affected with CKD were included. The analytical protocol provided an evaluation of the precision, accuracy, and storage stability at different temperatures. The method investigated showed good precision and accuracy and good stability for 1 week of storage at freezing temperatures. The method was validated in cats, allowing its future use in diagnostic procedures.

Abstract

The determination of parathyroid hormone (PTH) in cats could be of clinical utility in many metabolic disorders, such as renal diseases, hypercalcemia, or nutritional imbalances. However, the available methods for the measurement of feline PTH are limited, not widely available, and need radioimmunoassays. The aim of this study was to perform the analytical validation of a new immunoenzymatic method for the measurement of feline PTH. Thirty-eight cats affected with chronic kidney disease (CKD) were included. PTH was measured using a two-site immunoenzymatic method validated in humans and dogs (ST AIA-PACK^® Intact PTH, Tosoh Bioscience, Tessenderlo, Belgium). The analytical validation provided the evaluation of precision (intra-assay and inter-assay), accuracy (linearity under dilution (LUD) and spike recovery test (SRT)), and the storage stability of serum samples at 20 °C, 4 °C, and −20 °C. The method showed good precision (intra-assay CVs (coefficient of variations) 3.19–9.61%; inter-assay CVs 9.26–15.28%). In both the intra- and inter-assays, the highest imprecision was found with the low concentration pool (9.61% and 15.28%) and accuracy (LUD and SRT r² = 0.99, p < 0.001), while the stability was optimal up until 7 days at −20 °C (−7.7%). The method was successfully validated in cats, allowing its future use in diagnostic procedures.

Clinical progression of cats with early-stage chronic kidney disease fed diets with varying protein and phosphorus contents and calcium to phosphorus ratios

Background

Dietary protein and phosphorus (P) restriction is the mainstay for nutritional management of chronic kidney disease (CKD). However, adequate restriction levels for cats with early CKD remain unclear.

Objectives

To investigate responses in cats with early CKD to varying dietary protein, P, and calcium (Ca) : P ratio.

Animals

Nineteen research colony cats with International Renal Interest Society stages 1‐2 CKD.

Methods

In an opportunistic longitudinal case study, cats were fed a low protein (59 g/Mcal), low P (0.84 g/Mcal) dry diet (LP‐LP; Ca : P = 1.9) for 18 months and later transitioned onto a moderate protein (76‐98 g/Mcal), moderate P (1.4‐1.6 g/Mcal) dry‐wet diet regimen (MP‐MP; Ca : P = 1.4‐1.6) for 22 months. Fold‐changes in serum creatinine, total Ca (tCa) and P (primary outcomes) and fibroblast growth factor 23 (FGF23) were assessed by linear‐mixed models.

Results

While feeding LP‐LP, mean serum creatinine decreased (0.87‐fold, 95% confidence interval [CI] 0.81, 0.93, P < .001) to within reference range after 6 months, while increases in total Ca (tCa; 1.16‐fold, 95% CI 1.11, 1.22, P < .001) and FGF23 (2.72‐fold, 95% CI 1.72, 4.31, P < .001), but not in P (1.03‐fold, 95% CI 0.945, 1.124, P = .94), were observed after 17 months. On MP‐MP, mean creatinine, tCa and P remained within reference ranges and did not significantly change (P = .11, P = .98, and P = 1, respectively), while FGF23 significantly decreased (0.58‐fold, 95% CI 0.36, 0.95, P = .02) after 22 months.

Conclusions and Clinical Importance

Cats with early CKD developed hypercalcemia after long‐term feeding of a highly P‐restricted diet. Increasing dietary P and reducing Ca : P ratio maintained renal markers, while improving Ca‐P balance. Cats with early CKD could benefit from moderately protein‐ and P‐restricted diets.

A feline-focused review of chronic kidney disease-mineral and bone disorders - Part 2: Pathophysiology of calcium disorder and extraosseous calcification

Derangements in mineral metabolism are one of the main entities in chronic kidney disease-mineral and bone disorder (CKD-MBD). This is the second of a two-part review of the physiology and pathophysiology of calcium homeostasis in feline CKD-MBD. While dysregulation in calcium homeostasis is known to contribute to the development of vascular calcification in CKD, evidence characterising the relationship between serum calcium concentration and nephrocalcinosis and nephrolithiasis is limited. Recently, fibroblast growth factor 23 (FGF23) and α-Klotho have gained increased research interest and been shown to be important biomarkers for the prediction of CKD progression in human patients. However, conflicting evidence exists on their role in calcium homeostasis and vascular and soft tissue calcification. This review details the pathophysiology of calcium disorders associated with CKD-MBD and its implications on vascular and soft tissue mineralisation in human and feline patients. Further prospective studies investigating the clinical consequences of calcium disturbances in cats with CKD are warranted and this may provide additional insight into the pathophysiology of feline CKD-MBD.

Serum fibroblast growth factor 23 (FGF-23): associations with hyperphosphatemia and clinical staging of feline chronic kidney disease

Fibroblast growth factor 23 (FGF-23) is an independent monitor of the progression of chronic kidney disease (CKD) in human medicine, and FGF-23 may have value as a biomarker in feline CKD. We evaluated the relationship between serum FGF-23 and CKD stages, and the effect of age on FGF-23 in normal cats. We measured FGF-23 and intact parathyroid hormone (iPTH) concentrations by ELISA, with intra- and inter-assay CVs ≤ 15%. The percentage recovery of FGF-23 and iPTH remained stable for up to 7 d in samples stored at -20°C and -80°C. We measured FGF-23 in 304 cats, among which 196 were diagnosed with CKD. The 108 clinically healthy cats were divided into 5 subgroups based on growth stage (0-2 y, 3-6 y, 7-10 y, 11-14 y, ≥ 15 y). No statistical difference was found in FGF-23 among age groups (p = 0.15) or by sex in healthy subjects. Using the International Renal Interest Society guideline, 34 cats were defined as CKD stage 1, 74 stage 2, 51 stage 3, and 37 stage 4. FGF-23 was higher in cats in all CKD stages than in controls. Higher serum phosphorus was observed in stage 3 (p = 0.04) and 4 (p < 0.01) compared to controls. iPTH increased as CKD progressed. Pearson analysis indicated a positive linear relationship between FGF-23 and iPTH (control: r = 0.70, p < 0.01; CKD: r = 0.46, p = 0.02). FGF-23 may be a useful biomarker of feline CKD and may precede hyperphosphatemia in advanced feline CKD.

Vitamin D Metabolism and Its Role in Mineral and Bone Disorders in Chronic Kidney Disease in Humans, Dogs and Cats

Some differences regarding Vitamin D metabolism are described in dogs and cats in comparison with humans, which may be explained by an evolutionary drive among these species. Similarly, vitamin D is one of the most important regulators of mineral metabolism in dogs and cats, as well as in humans. Mineral metabolism is intrinsically related to bone metabolism, thus disturbances in vitamin D have been implicated in the development of chronic kidney disease mineral and bone disorders (CKD-MBD) in people, in addition to dogs and cats. Vitamin D deficiency may be associated with Renal Secondary Hyperparathyroidism (RSHPT), which is the most common mineral disorder in later stages of CKD in dogs and cats. Herein, we review the peculiarities of vitamin D metabolism in these species in comparison with humans, and the role of vitamin D disturbances in the development of CKD-MBD among dogs, cats, and people. Comparative studies may offer some evidence to help further research about vitamin D metabolism and bone disorders in CKD.

A review of phosphorus homeostasis and the impact of different types and amounts of dietary phosphate on metabolism and renal health in cats

Elevated concentrations of serum phosphate are linked with progression and increased case fatality rate in animals and humans with chronic kidney disease. Elevated concentrations of serum phosphate can be a risk factor for development of renal and cardiovascular diseases or osteoporosis in previously healthy people. In rodents, an excess intake of dietary phosphorus combined with an inverse dietary calcium : phosphorus ratio (<1 : 1) contributes to renal calcification. Renal injury also has occured in cats fed experimental diets supplemented with highly soluble phosphate salts, especially in diets with inverse calcium : phosphorus ratios. However, not all phosphorus sources contribute similarly to this effect. This review, which focuses on cats, summarizes the published evidence regarding phosphorus metabolism and homeostasis, including the relative impact of different dietary phosphorus sources, and their impact on the kidneys. No data currently shows that commercial cat foods induce renal injury. However, some diets contain high amounts of phosphorus relative to recommendations and some have inverse Ca : P ratios and so could increase the risk for development of kidney disease. While limiting the use of highly soluble phosphates appears to be important, there are insufficient data to support a specific upper limit for phosphate intake. This review also proposes areas where additional research is needed in order to strengthen conclusions and recommendations regarding dietary phosphorus for cats.

Expression of Phosphatonin-Related Genes in Sheep, Dog and Horse Kidneys Using Quantitative Reverse Transcriptase PCR

Simple Summary

Traditionally, it has been thought that control of body phosphorus was secondary to the tighter control of calcium. However, over the last 20 years, an extensive system for control of body phosphorus by proteins called phosphatonins has been shown to exist. Most research on phosphatonins has been done in rat or mouse models. This paper looks at whether important proteins and phosphorus channels in the phosphatonin pathways are present in the kidneys of dogs, horses and sheep. The results showed that all of the components of the phosphatonin system are present in these species, but that there are species differences in which protein or channel is most common, and in the relationships between the proteins and channels. This research is important because the phosphatonin system is involved in the progression of chronic kidney disease in humans and animals, and differences in the systems between animal species may affect treatment of chronic kidney disease.

Abstract

The aim of this preliminary study was to determine the relative expression of phosphatonin pathway-related genes in normal dog, sheep and horse kidneys and to explore the relationships between the different genes. Kidneys were collected post-mortem from 10 sheep, 10 horses and 8 dogs. RNA was extracted, followed by reverse transcriptase quantitative polymerase chain reaction for fibroblast growth factor receptor 1 IIIc (FGFR1IIIC), sodium-phosphate co-transporter (NPT) 1 (SLC17A1), NPT2a (SLC34A1), NPT2c (SLC34A3), parathyroid hormone 1 receptor (PTH1R), klotho (KL), vitamin D receptor (VDR), 1a-hydroxylase (CYP27B1) and 24-hydroxylase (CYP24A1). NPT2a was highly expressed in the dog kidneys, compared with those of the horses and sheep. NPT1 had greatest expression in horses and sheep, although the three different NPTs all had relatively similar expression in sheep. There was little variability in FGFR1IIIc expression, particularly in the dogs and horses. FGFR1IIIc expression was negatively correlated with NPT genes (except NPT2a in sheep), while NPT genes were all positively correlated with each other. Unexpectedly, klotho was positively correlated with NPT genes in all three species. These results provide the basis for further research into this important regulatory system. In particular, species differences in phosphatonin gene expression should be considered when considering the pathogenesis of chronic kidney disease.

An artificial neural network-based model to predict chronic kidney disease in aged cats

BACKGROUND

Chronic kidney disease (CKD) frequently causes death in older cats; its early detection is challenging.

OBJECTIVES

To build a sensitive and specific model for early prediction of CKD in cats using artificial neural network (ANN) techniques applied to routine health screening data.

ANIMALS

Data from 218 healthy cats ≥7 years of age screened at the Royal Veterinary College (RVC) were used for model building. Performance was tested using data from 3546 cats in the Banfield Pet Hospital records and an additional 60 RCV cats-all initially without a CKD diagnosis.

METHODS

Artificial neural network (ANN) modeling used a multilayer feed-forward neural network incorporating a back-propagation algorithm. Clinical variables from single cat visits were selected using factorial discriminant analysis. Independent submodels were built for different prediction time frames. Two decision threshold strategies were investigated.

RESULTS

Input variables retained were plasma creatinine and blood urea concentrations, and urine specific gravity. For prediction of CKD within 12 months, the model had accuracy, sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of 88%, 87%, 70%, 53%, and 92%, respectively. An alternative decision threshold increased specificity and PPV to 98% and 87%, but decreased sensitivity and NPV to 42% and 79%, respectively.

CONCLUSIONS AND CLINICAL IMPORTANCE

A model was generated that identified cats in the general population ≥7 years of age that are at risk of developing CKD within 12 months. These individuals can be recommended for further investigation and monitoring more frequently than annually. Predictions were based on single visits using common clinical variables.

Understanding the Nutritional Needs of Healthy Cats and Those with Diet-Sensitive Conditions

Diets for cats must provide complete nutrition and meet the needs of the individual patient. There is no single diet that is perfect for all cats, and veterinarians must consider the needs of the cat as well as the preferences of the owners when making dietary recommendations. This article focuses on the interface between animal factors and nutritional needs in cats and is divided into 3 sections. Section 1 addresses the dietary needs of healthy cats, including differences among life stages. Section 2 addresses common myths regarding feline nutrition. Section 3 addresses common nutrient-sensitive conditions in cats, including sarcopenia of aging.

Evaluation of phosphorus, calcium, and magnesium content in commercially available foods formulated for healthy cats

Background

High dietary phosphorus (P) and low calcium‐to‐phosphorus ratio (Ca:P) are associated with kidney damage in cats. There are no established guidelines for dietary P maximum for cats.

Objectives

To quantify crude protein, P, Ca, and magnesium (Mg) concentrations in cat foods and compare among food formats (dry, canned, raw), primary protein ingredients, protein concentrations (low, moderate, high), grain‐free versus grain‐containing foods, foods intended for adult maintenance versus all life stages, and cost.

Samples

Eighty‐two commercial nonprescription cat foods.

Methods

Descriptive study. Mineral concentrations were measured using inductively coupled argon plasma‐optical emission spectroscopy. Crude protein was measured using the Dumas nitrogen combustion method. Mineral and crude protein concentrations were compared among food categories.

Results

Twenty‐seven foods contained ≥3.6 g P/1000 kcal metabolizable energy (ME), of which 7 exceeded 4.8 g/1000 kcal ME. Thirteen foods had low Ca:P ratio (≤1.0). The low‐protein diet group had no products ≥3.6 g P/1000 kcal ME, which was significantly different compared to the high‐protein diet group (52% of products had ≥3.6 g P/1000 kcal ME; P = .01). No significant differences in P content and Ca:P ratio were found among other diet categories. Canned foods had significantly lower Mg compared to dry (P < .001) and raw (P = .007) foods. Declared minimum P and Ca were significantly lower than analyzed concentrations (P = .0005 and P = .003, respectively).

Conclusions and Clinical Importance

The high number of foods with high P and low Ca suggest that pet food regulatory reform should be considered.

Evaluation of Electrolyte Concentration and Pro-Inflammatory and Oxidative Status in Dogs with Advanced Chronic Kidney Disease under Dietary Treatment

An integrated study on the effect of renal diet on mineral metabolism, fibroblast growth factor 23 (FGF-23), total antioxidant capacity, and inflammatory markers has not been performed previously. In this study, we evaluated the effects of renal diet on mineral metabolism, oxidative stress and inflammation in dogs with stage 3 or 4 of chronic kidney disease (CKD). Body condition score (BCS), muscle condition score (MCS), serum biochemical profile, ionized calcium (i-Ca), total calcium (t-Ca), phosphorus (P), urea, creatinine, parathyroid hormone (PTH), FGF-23, interleukin 6 (IL-6), interleukin 10 (IL-10), tumor necrosis factor alpha (TNF-α) and total antioxidant capacity (TAC) were measured at baseline (T0) and after 6 months of dietary treatment (T6). Serum urea, P, t-Ca, i-Ca, PTH, FGF-23, IL-6, IL-10, TNF-α and TAC measurements did not differ between T0 and T6. Serum creatinine (SCr) was increased at T6 and serum PTH concentrations were positively correlated with serum SCr and urea. i-Ca was negatively correlated with urea and serum phosphorus was positively correlated with FGF-23. Urea and creatinine were positively correlated. The combination of renal diet and support treatment over 6 months in dogs with CKD stage 3 or 4 was effective in controlling uremia, acid–base balance, blood pressure, total antioxidant capacity, and inflammatory cytokine levels and in maintaining BCS and MCS.

Total serum magnesium in cats with chronic kidney disease with nephrolithiasis

OBJECTIVES

Magnesium has been 'the forgotten ion' for many years. Over the past decade, however, the role of magnesium in essential physiological functions and several illness conditions have been elucidated. Nevertheless, the investigation of magnesium in cats with chronic kidney disease (CKD) and nephrolithiasis is yet to be determined. The purpose of this study was to investigate whether CKD cats with nephrolithiasis have changes in total serum magnesium concentrations, and whether magnesium disorders may be associated with other electrolyte disturbances, as well as with prognosis. We also aimed to evaluate whether total serum magnesium concentration differs between CKD cats with and without nephrolithiasis.

METHODS

Total serum magnesium concentrations were assessed in 42 cats with CKD with stage 1-4 nephrolithiasis. The correlation between magnesium and other electrolytes, as well as Kaplan-Meier survival analysis, were performed. We also selected 14 control cats with CKD without nephrolithiasis age-matched with 14 cats with CKD with nephrolithiasis.

RESULTS

Hypermagnesemia was observed in 16/42 (38.1%) and hypomagnesemia in 6/42 (14.3%) cats. Serum magnesium abnormalities were observed in cats of all stages, and marked hypermagnesemia was noted in cats with stage 4 CKD with nephrolithiasis (P <0.001). There was a negative correlation between total serum magnesium and ionized calcium (r = -0.64; P <0.01), and a positive correlation between total serum magnesium and serum phosphorus (r = 0.58, P = 0.01). Cats with CKD with nephrolithiasis and hypomagnesemia or hypermagnesemia had higher mortality than those with normal total serum magnesium concentration (P <0.01), regardless of CKD stage. There was no difference in total serum magnesium concentration between CKD cats with and without nephrolithiasis.

CONCLUSIONS AND RELEVANCE

Cats with CKD with nephrolithiasis have magnesium abnormalities. Hypomagnesemia and hypermagnesemia were associated with an increase in mortality, and thus total serum magnesium abnormalities may be used as prognostic factors in these cases.

Fibroblast growth factor 23 and symmetric dimethylarginine concentrations in geriatric cats

Background

Fibroblast growth factor 23 (FGF23) is a phosphaturic hormone that is increased in azotemic cats with chronic kidney disease (CKD) and predictive of the onset of azotemia in older cats. The introduction of symmetric dimethylarginine (SDMA) as a biomarker of glomerular filtration rate has led to the identification of cats in which SDMA is increased, but plasma creatinine concentrations remains within reference range. There is currently little understanding of the metabolic changes present in such cats.

Objectives

To examine the relationship between plasma FGF23 and SDMA concentrations in non‐azotemic geriatric cats.

Animals

Records of a cross section of client‐owned cats (n = 143) without azotemic CKD.

Methods

Clinicopathological information was obtained from cats (≥ 9 years) from records of 2 first opinion practices. The relationship between plasma SDMA and FGF23 concentrations was examined using Spearman's correlation and variables compared using the Mann‐Whitney U test.

Results

Cats with increased SDMA concentrations had significantly higher plasma FGF23 (P < .001) and creatinine (P < .001) concentrations compared to cats with SDMA concentrations within reference range. A weak positive relationship was demonstrated between plasma FGF23 and SDMA concentrations (r = .35, P < .001) and between plasma FGF23 and creatinine (r = .23, P = .005) concentrations.

Conclusions and Clinical Importance

More cats with increased SDMA concentrations had higher FGF23 concentrations than those with SDMA concentrations within the reference range, suggesting the presence of an alteration in phosphate homeostasis. Further studies are warranted to identify influencing factors and to explore the utility of FGF23 concentration to inform management of cats with early stage CKD.

Predicting early risk of chronic kidney disease in cats using routine clinical laboratory tests and machine learning

Background

Advanced machine learning methods combined with large sets of health screening data provide opportunities for diagnostic value in human and veterinary medicine.

Hypothesis/Objectives

To derive a model to predict the risk of cats developing chronic kidney disease (CKD) using data from electronic health records (EHRs) collected during routine veterinary practice.

Animals

A total of 106 251 cats that attended Banfield Pet Hospitals between January 1, 1995, and December 31, 2017.

Methods

Longitudinal EHRs from Banfield Pet Hospitals were extracted and randomly split into 2 parts. The first 67% of the data were used to build a prediction model, which included feature selection and identification of the optimal neural network type and architecture. The remaining unseen EHRs were used to evaluate the model performance.

Results

The final model was a recurrent neural network (RNN) with 4 features (creatinine, blood urea nitrogen, urine specific gravity, and age). When predicting CKD near the point of diagnosis, the model displayed a sensitivity of 90.7% and a specificity of 98.9%. Model sensitivity decreased when predicting the risk of CKD with a longer horizon, having 63.0% sensitivity 1 year before diagnosis and 44.2% 2 years before diagnosis, but with specificity remaining around 99%.

Conclusions and clinical importance

The use of models based on machine learning can support veterinary decision making by improving early identification of CKD.

Not all forms of dietary phosphorus are equal: an evaluation of postprandial phosphorus concentrations in the plasma of the cat

Phosphorus is present in diets as naturally occurring P from raw materials or added as an inorganic salt. However, little is known about postprandial kinetics of P absorption in cats. Here, we describe several studies quantifying postprandial kinetics following the ingestion of diets of varying composition. Briefly, cats were fed a meal consisting of 50 % of their metabolic energy requirement in a randomised crossover design. A pre-meal baseline blood sample was taken via cephalic catheter and repeated measurements taken regularly up to 6 h post-meal to assess the whole blood ionised Ca, plasma P and parathyroid hormone concentrations. A diet containing 4·8 g total P/4184 kJ (1000 kcal), 3·5 g P from sodium dihydrogen phosphate (NaH2PO4)/4184 kJ (1000 kcal) and Ca:P 0·6 caused a marked increase in plasma P from baseline to a peak of 1·976 (95% CI 1·724, 2·266) mmol/l (P <0·001), whereas a diet containing 3·38 g total P/4184 kJ (1000 kcal), no added inorganic P and Ca:P 1·55 resulted in a postprandial decrease in plasma P (P = 0·008). Subsequent data indicate that added inorganic P salts in the diet above 0·5 g P/4184 kJ (1000 kcal) cause an increase in plasma P in cats, while diets below this do not. The data presented here demonstrate that sources of added inorganic P salts cause a temporary postprandial increase in plasma P in a dose-dependent manner, prolonged in diets with Ca:P <1·0. Dietary P derived from natural food ingredients (e.g. meat or vegetable matter) does not appear to have any effect on postprandial plasma P.

Urinary heat shock protein-72: A novel marker of acute kidney injury and chronic kidney disease in cats

Acute kidney injury (AKI) in cats is associated with high mortality, partially attributed to late recognition of the disease when using currently available markers. Feline chronic kidney disease (CKD) has a variable progression rate. This study aimed to evaluate the sensitivity and specificity of urinary heat shock protein-72 to urinary creatinine ratio (uHSP72:uCr) as a diagnostic and prognostic marker in feline AKI, and as a prognostic indicator in feline CKD. The study included 63 cats, divided into five groups: healthy controls (n=10), urethral obstruction (UO; n=7), CKD (n=15), AKI (16 cats) and acute decompensating CKD (ACKD; n=15). Median uHSP72:uCr (ng/mg) of healthy, UO, CKD, AKI and ACKD cats were 0.44 (range, 0.13-1.1), 1.96 (range, 0.64-11.9), 4.2ng/mg (range, 0.57-22.16), 3.2 (range, 0.42-10.91) and 7.0 (range, 1.2-20.96), respectively, and differed (P<0.001) among groups. uHSP72:uCr was significantly lower in the controls vs. the CKD, AKI and ACKD groups. Receiver operator characteristic analysis of uHSP72:uCr, including the AKI and control groups, showed an area under the curve of 0.93 (95% confidence interval, 0.84-1.00), indicating an excellent predictive performance for diagnosing AKI. A 0.54ng/mg cutoff point corresponded to 94% sensitivity and 70% specificity for diagnosing AKI. The median survival time of cats with CKD with low uHSP72:uCr was longer (P=0.036) than in those with high uHSP72:uCr (561 vs. 112 days, respectively). uHSP72:uCr is a highly sensitive, moderately specific marker of AKI in cats, and is associated with the survival of cats with CKD.

Factors associated with survival in dogs with chronic kidney disease

Background: Chronic kidney disease (CKD) is associated with morbidity and mortality in dogs. Plasma fibroblast growth factor‐23 (FGF‐23) concentration is an independent predictor of CKD progression and survival in cats and people with CKD.

Objectives: To investigate the relationship among FGF‐23, parathyroid hormone (PTH), vitamin D metabolites, and other clinical variables with survival time in dogs with CKD.

Animals: Twenty‐seven azotemic CKD dogs.

Methods: Dogs were recruited prospectively into the study and followed until death or study conclusion. Dogs were International Renal Interest Society (IRIS) staged into stage 2 (n = 9), stage 3 (n = 12), and stage 4 (n = 6) CKD. Survival times were calculated from the date of study inclusion. Univariable Cox regression was used to assess variables associated with survival including body condition score (BCS), muscle condition score, hematocrit, creatinine, CKD stage, serum phosphorus, urine protein:creatinine ratio (UPC), calcium phosphorus product (CaPP), PTH, 25‐hydroxyvitamin D, 1,25‐‐dihydroxyvitamin D, and FGF‐23 concentrations.

Results: Significant hazard ratios (hazard ratio; 95% confidence interval; P value) were as follows: BCS < 4/9 (1.579; 1.003‐2.282; P = .05), muscle atrophy (2.334; 1.352‐4.030; P = .01), increased creatinine (1.383; 1.16‐1.64; .01), hyperphosphatemia (3.20; 1.357‐7.548; P = .005), increased UPC (3.191; 1.310‐7.773; P = .01), increased CaPP (4.092; 1.771‐9.454; P = .003), and increased FGF‐23 (2.609; 1.090‐6.240; P = .05). Survival times for each IRIS CKD stage were significantly different (P = .01).

Conclusions and Clinical Importance: Multiple variables, including FGF‐23, were associated with duration of survival in CKD dogs. FGF‐23 could be a prognostic marker in dogs with CKD.

Associations between single nucleotide polymorphisms in the calcium sensing receptor and chronic kidney disease-mineral and bone disorder in cats

Feline chronic kidney disease (CKD) is associated with high variability in severity of CKD-mineral and bone disorder (CKD-MBD). The calcium sensing receptor (CaSR) regulates circulating parathyroid hormone (PTH) and calcium concentrations. Single nucleotide polymorphisms (SNPs) in the CaSR are associated with severity of secondary renal hyperparathyroidism and total calcium concentrations in human patients receiving haemodialysis. The objective of this study was to explore associations between polymorphisms in the feline CaSR (fCaSR) and biochemical changes observed in CKD-MBD. Client owned cats (≥9years) were retrospectively included. SNP discovery was performed in 20 cats with azotaemic CKD and normal or dysregulated calcium concentrations. Non-pedigree cats (n=192) (125 with azotaemic CKD and 66 healthy), Persians (n=40) and Burmese (n=25) were genotyped for all identified SNPs using KASP. Biochemical parameters from the date of CKD diagnosis or from first visit to the clinic (healthy cats) were used. Associations between genotype and ionized calcium, total calcium, phosphate, PTH and FGF-23 were performed for non-pedigree cats using logistic regression. Sequence alignment against the fCaSR sequence revealed eight novel exonic SNPs. KASP genotyping had high accuracy (99.6%) and a low failure rate (<6%) for all SNPs. Allele frequencies varied between breeds. In non-pedigree cats, one synonymous SNP CaSR:c.1269G>A was associated with logPTH concentration (adjusted for plasma creatinine concentration), with a recessive model having the best fit (G/G vs A/A-G/A, P=0.031). Genetic variation in the fCaSR is unlikely to explain the majority of the variability in presence and severity of CKD-MBD in cats.

Serum fibroblast growth factor 23 concentrations in dogs with chronic kidney disease

The aim of this study was to determine if serum fibroblast growth factor (FGF23) concentrations were increased in dogs with chronic kidney disease (CKD). Serum samples submitted to a commercial laboratory were collected over a 15-month period, 14 samples were from dogs with a history of polyuria/polydipsia, azotaemia and low urine specific gravity, 20 samples were from non-azotaemic dogs. Serum FGF23, parathyroid hormone, total calcium and phosphorus, urea and creatinine were measured. Mann-Whitney test was used to determine differences between non-azotaemic and CKD groups; a one-way ANOVA with Tukey pairwise comparisons was used to determine any differences between International Renal Interest Society stages; and regression models were used to determine predictors of International Renal Interest Society stage, serum phosphorus and FGF23 concentrations. The median serum FGF23 concentration of dogs with CKD was 5194.6pg/mL, which was significantly greater (P<0.001) than the median serum FGF23 concentration of non-azotaemic dogs (259.2pg/mL). Log serum FGF23 and age were significantly associated with IRIS stage (P=0.027 and P=0.032 respectively), while log serum phosphorus concentration (P<0.001) was significantly associated with log serum FGF23 concentration. In summary, serum FGF23 concentration is increased in dogs with CKD, and is associated with serum phosphorus concentration. This phosphatonin pathway may be a useful target for the development of future treatments to control plasma phosphorus concentrations in chronic kidney disease.

Fibroblast Growth Factor-23 Concentration in Dogs with Chronic Kidney Disease

Background

Chronic kidney disease (CKD) is associated with hyperphosphatemia, decreased vitamin D metabolite concentrations, and hyperparathyroidism. This syndrome is known as CKD‐mineral bone disorder (CKD‐MBD). Recently, it has been shown that an increase in fibroblast growth factor‐23 (FGF‐23) concentration is an early biomarker of CKD in people. It is an independent risk factor for both progression of renal disease and survival time in humans and cats with CKD. Information about FGF‐23 in healthy dogs and those with CKD is lacking.

Objectives

To measure FGF‐23 concentration in dogs with different stages of CKD and determine its association with factors involved in CKD‐MBD, including serum phosphorus and parathyroid hormone (PTH) concentrations. A secondary aim was to validate an ELISA for measurement of plasma FGF‐23 concentration in dogs.

Animals

Thirty‐two client‐owned dogs with naturally occurring CKD and 10 healthy control dogs.

Methods

Prospective cross‐sectional study. An FGF‐23 ELISA was used to measure plasma FGF‐23 concentration in dogs and their association with serum creatinine, phosphorus, calcium, and PTH concentrations.

Results

Plasma FGF‐23 concentrations increased with severity of CKD and were significantly different between IRIS stages 1 and 2 versus stages 3 and 4 (P < .0001). Increases in FGF‐23 concentrations were more frequent than hyperparathyroidism or hyperphosphatemia in this cohort. Serum creatinine and phosphorus concentrations were the strongest independent predictors of FGF‐23 concentration.

Conclusions and clinical importance

Plasma FGF‐23 concentrations increase in dogs with CKD as disease progresses. Plasma FGF‐23 concentrations appear to be useful for further study of the pathophysiology of CKD‐MBD in dogs.

Controversies in Veterinary Nephrology: Renal Diets Are Indicated for Cats with International Renal Interest Society Chronic Kidney Disease Stages 2 to 4: The Con View

Renal diets typically incorporate protein and phosphorus restriction, supplement with potassium and Omega-3 fatty acids, and address metabolic acidosis. Compared to "maintenance" diets, these modifications appear to benefit cats with chronic kidney disease (CKD). However, there is limited data in cats justifying the specific amounts of the nutrients used in these diets, and there is little evidence supporting protein restriction in cats with CKD. Energy intake, maintenance of body weight, and muscle and body condition need to be addressed, and may take precedence over special diets. Further research is needed to better define optimum diets for cats with CKD.

The Effect of Moderate Dietary Protein and Phosphate Restriction on Calcium-Phosphate Homeostasis in Healthy Older Cats

Background

Dietary phosphate and protein restriction decreases plasma PTH and FGF‐23 concentrations and improves survival time in azotemic cats, but has not been examined in cats that are not azotemic.

Hypothesis

Feeding a moderately protein‐ and phosphate‐restricted diet decreases PTH and FGF‐23 in healthy older cats and thereby slows progression to azotemic CKD.

Animals

A total of 54 healthy, client‐owned cats (≥ 9 years).

Methods

Prospective double‐blinded randomized placebo‐controlled trial. Cats were assigned to test diet (protein 76 g/Mcal and phosphate 1.6 g/Mcal) or control diet (protein 86 g/Mcal and phosphate 2.6 g/Mcal) and monitored for 18 months. Changes in variables over time and effect of diet were assessed by linear mixed models.

Results

A total of 26 cats ate test diet and 28 cats ate control diet. There was a significant effect of diet on urinary fractional excretion of phosphate (P = 0.045), plasma PTH (P = 0.005), and ionized calcium concentrations (P = 0.018), but not plasma phosphate, FGF‐23, or creatinine concentrations. Plasma PTH concentrations did not significantly change in cats fed the test diet (P = 0.62) but increased over time in cats fed the control diet (P = 0.001). There was no significant treatment effect of the test diet on development of azotemic CKD (3 of 26 (12%) test versus 3 of 28 (11%) control, odds ratio 1.09 (95% CI 0.13–8.94), P = 0.92).

Conclusions and Clinical Importance

Feeding a moderately protein‐ and phosphate‐restricted diet has effects on calcium‐phosphate homeostasis in healthy older cats and is well tolerated. This might have an impact on renal function and could be useful in early chronic kidney disease.

Controversies in Veterinary Nephrology: Renal Diets Are Indicated for Cats with International Renal Interest Society Chronic Kidney Disease Stages 2 to 4: The Pro View

Renal diets have been the mainstay of therapy for cats with chronic kidney disease (CKD) for many decades. Clinical trials in cats with CKD have shown them to be effective in improving survival, reducing uremic crises, and improving serum urea nitrogen and phosphorous concentrations. It has shown that, when food intake is adequate, renal diets can maintain body weight and body condition scores for up to 2 years. Although some have questioned whether renal diets provide adequate protein and have advocated feeding higher-protein diets to cats with CKD, there is currently no convincing evidence in support of this proposal.

Current Understanding of the Pathogenesis of Progressive Chronic Kidney Disease in Cats

In cats with chronic kidney disease (CKD), the most common histopathologic finding is tubulointerstitial inflammation and fibrosis. However, these changes reflect a nonspecific response of the kidney to any inciting injury. The risk of developing CKD is likely to reflect the composite effects of genetic predisposition, aging, and environmental and individual factors that affect renal function over the course of a cat's life. However, there is still little information available to determine exactly which individual risk factors predispose a cat to develop CKD. Although many cats diagnosed with CKD have stable disease for years, some cats show overtly progressive disease.

Update on Mineral and Bone Disorders in Chronic Kidney Disease

The inappropriate phosphorus retention observed in chronic kidney disease is central to the pathophysiology of mineral and bone disorders observed in these patients. Subsequent derangements in serum fibroblast growth factor 23, parathyroid hormone, and calcitriol concentrations play contributory roles. Therapeutic intervention involves dietary phosphorus restriction and intestinal phosphate binders in order to correct phosphorus retention and maintain normocalcemia. Additional therapies may be considered to normalize serum fibroblast growth factor 23 and parathyroid hormone.

ISFM Consensus Guidelines on the Diagnosis and Management of Feline Chronic Kidney Disease

PRACTICAL RELEVANCE

Chronic kidney disease (CKD) is one of the most commonly diagnosed diseases in older cats. In most cats, CKD is also a progressive disease and can be accompanied by a wide range of clinical and clinicopathological changes. These ISFM Consensus Guidelines have been developed by an independent panel of clinicians and academics to provide practical advice on the diagnosis and management of this complex disease.

CLINICAL CHALLENGES

Although CKD is a common clinical problem in cats, the manifestations of disease vary between individuals. Thus there is a need for careful and repeat evaluation of cats with CKD and adjustment of therapy according to individual needs. In addition to addressing problems arising from CKD and improving quality of life (QoL) for the patient, therapy may also target slowing the underlying progression of disease and hence prolonging life. While maintaining QoL is of paramount importance in our patients, this can be challenging when multiple therapies are indicated. In some cases it is necessary to prioritise therapy, given an understanding of what is likely to most benefit the individual patient.

EVIDENCE BASE

In preparing these Guidelines, the Panel has carefully reviewed the existing published literature, and has also graded the quality of evidence for different interventions to help to provide practical recommendations on the therapeutic options for feline CKD. This is a field of veterinary medicine that has benefited from some excellent published clinical research and further research findings will undoubtedly modify the recommendations contained in these Guidelines in the future.

Impact of Increasing Dietary Calcium Levels on Calcium Excretion and Vitamin D Metabolites in the Blood of Healthy Adult Cats

Background

Dietary calcium (Ca) concentrations might affect regulatory pathways within the Ca and vitamin D metabolism and consequently excretory mechanisms. Considering large variations in Ca concentrations of feline diets, the physiological impact on Ca homeostasis has not been evaluated to date. In the present study, diets with increasing concentrations of dicalcium phosphate were offered to ten healthy adult cats (Ca/phosphorus (P): 6.23/6.02, 7.77/7.56, 15.0/12.7, 19.0/17.3, 22.2/19.9, 24.3/21.6 g/kg dry matter). Each feeding period was divided into a 10-day adaptation and an 8-day sampling period in order to collect urine and faeces. On the last day of each feeding period, blood samples were taken.

Results

Urinary Ca concentrations remained unaffected, but faecal Ca concentrations increased (P < 0.001) with increasing dietary Ca levels. No effect on whole and intact parathyroid hormone levels, fibroblast growth factor 23 and calcitriol concentrations in the blood of the cats were observed. However, the calcitriol precursors 25(OH)D₂ and 25(OH)D₃, which are considered the most useful indicators for the vitamin D status, decreased with higher dietary Ca levels (P = 0.013 and P = 0.033). Increasing dietary levels of dicalcium phosphate revealed an acidifying effect on urinary fasting pH (6.02) and postprandial pH (6.01) (P < 0.001), possibly mediated by an increase of urinary phosphorus (P) concentrations (P < 0.001).

Conclusions

In conclusion, calcitriol precursors were linearly affected by increasing dietary Ca concentrations. The increase in faecal Ca excretion indicates that Ca homeostasis of cats is mainly regulated in the intestine and not by the kidneys. Long-term studies should investigate the physiological relevance of the acidifying effect observed when feeding diets high in Ca and P.

Chronic Kidney Disease in Aged Cats: Clinical Features, Morphology, and Proposed Pathogeneses

Chronic kidney disease (CKD) is the most common metabolic disease of domesticated cats, with most affected cats being geriatric (>12 years of age). The prevalence of CKD in cats exceeds that observed in dogs, and the frequency of the diagnosis of CKD in cats has increased in recent decades. Typical histologic features include interstitial inflammation, tubular atrophy, and fibrosis with secondary glomerulosclerosis. In contrast to people and dogs, primary glomerulopathies with marked proteinuria are remarkably rare findings in cats. Although a variety of primary renal diseases have been implicated, the disease is idiopathic in most cats. Tubulointerstitial changes, including fibrosis, are present in the early stages of feline CKD and become more severe in advanced disease. A variety of factors-including aging, ischemia, comorbid conditions, phosphorus overload, and routine vaccinations-have been implicated as factors that could contribute to the initiation of this disease in affected cats. Factors that are related to progression of established CKD, which occurs in some but not all cats, include dietary phosphorus intake, magnitude of proteinuria, and anemia. Renal fibrosis, a common histologic feature of aged feline kidneys, interferes with the normal relationship between peritubular capillaries and renal tubules. Experimentally, renal ischemia results in morphologic changes similar to those observed in spontaneous CKD. Renal hypoxia, perhaps episodic, may play a role in the initiation and progression of this disease.

Oral Manifestations of Chronic Kidney Disease and Renal Secondary Hyperparathyroidism: A Comparative Review

Recent epidemiological studies have demonstrated that significant associations exist between oral disease and diseases involving non-oral tissues. Occasionally, the roles may be reversed and the oral cavity can be severely affected by systemic disease originating in another part of the body. Renal secondary hyperparathyroidism is a common endocrinopathy that occurs as a consequence of chronic azotemic kidney disease. Renal osteodystrophy, the most dramatic clinical consequence of renal secondary hyperparathyroidism is uncommon, but can result in demineralization of maxillofacial bones, loosening of teeth, and pathological jaw fractures. The purpose of this report is to update the current understanding of the pathophysiology of this endocrine disease and to compare the oral manifestations of renal secondary hyperparathyroidism in humans and companion animals. A 50-year review of the veterinary literature was undertaken to examine the clinical presentation of renal osteodystrophy in dogs, and to determine what clinical consequences of renal secondary hyperparathyroidism have been reported in domestic cats.

Relationship between Plasma Fibroblast Growth Factor-23 Concentration and Survival Time in Cats with Chronic Kidney Disease

BACKGROUND

Fibroblast growth factor-23 (FGF-23) and parathyroid hormone (PTH) are commonly increased in cats with azotemic chronic kidney disease (CKD). Both are predictors of survival time in human patients, but these relationships have not previously been examined in the cat.

OBJECTIVES

To investigate the relationship between plasma FGF-23 and PTH concentrations at diagnosis of CKD in cats with survival time and with disease progression over 12 months.

ANIMALS

214 azotemic, client-owned cats (≥9 years).

METHODS

Retrospective study: Biochemical and urinary variables at diagnosis of azotemic CKD, including plasma FGF-23 and PTH concentrations were assessed as predictors of survival time (all-cause mortality) using Cox regression, and as predictors of CKD progression over 12 months using logistic regression.

RESULTS

In the final multivariable Cox regression model, survival was negatively associated with plasma creatinine (P = .002) and FGF-23 concentrations (P = .014), urine protein-to-creatinine ratio (P < .001) and age (P < .001). Survival was positively associated with PCV (P = .004). In the final multivariable logistic regression model, independent predictors of CKD progression included logFGF-23 and age. Neither plasma phosphate nor PTH was found to be an independent predictor of survival time or of CKD progression.

CONCLUSIONS AND CLINICAL IMPORTANCE

Plasma FGF-23 concentration is a novel prognostic indicator in cats with CKD, independent of other factors including plasma creatinine and phosphate concentrations. Further work is required to assess if FGF-23 contributes directly to CKD progression, but regardless these findings may make FGF-23 a useful biomarker for predicting poorer outcomes in cats with CKD.

Fibroblast Growth Factor 23

Traditionally, control of phosphorus in the body has been considered secondary to the tighter control of calcium by parathyroid hormone and vitamin D. However, over the past decade, substantial advances have been made in understanding the control of phosphorus by the so-called phosphatonin system, the lynchpin of which is fibroblast growth factor 23 (FGF23). FGF23 binds to the klotho/FGFR1c receptor complex in renal tubular epithelial cells, leading to upregulation of Na/Pi cotransporters and subsequent excretion of phosphorus from the body. In addition, FGF23 inhibits parathyroid hormone and the renal 1α-hydroxylase enzyme, while it stimulates 24-hydroxylase, leading to decreased 1,25-dihydroxyvitamin D3. FGF23 is intimately involved in the pathogenesis of a number of diseases, particularly the hereditary hypophosphatemic rickets group and chronic kidney disease, and is a target for the development of new treatments in human medicine. Little work has been done on FGF23 or the other phosphatonins in veterinary medicine, but increases in FGF23 are seen with chronic kidney disease in cats, and increased FGF23 expression has been found in soft tissue sarcomas in dogs.

Feline hyperparathyroidism: pathophysiology, diagnosis and treatment of primary and secondary disease

PRACTICAL RELEVANCE

Hyperparathyroidism exists in primary and secondary forms. Primary hyperparathyroidism has typically been considered a disease that uncommonly affects cats, but this condition is more prevalent than previous diagnoses would suggest. Secondary hyperparathyroidism may be caused by either nutritional influences (ie, nutritional secondary hyperparathyroidism) or chronic kidney disease (ie, renal secondary hyperparathyroidism). Tertiary hyperparathyroidism has yet to be documented in veterinary medicine, but it is possible that this condition occurs in some cats following longstanding renal secondary hyperparathyroidism.

CLINICAL CHALLENGES

Diagnosis of this group of calcium metabolic disorders presents a number of challenges for the clinician. For example, clinical signs can be non-specific and, especially in the case of primary hyperparathyroidism, there is often a low index of suspicion for the disease; careful sample handling is required for testing of parathyroid hormone (PTH) and ionized calcium levels; and there is currently no feline-specific assay for PTH, which has implications for test sensitivity and interpretation of results.

AIMS

This article briefly outlines PTH and calcium physiology by way of introduction to a review of PTH measurement and interpretation. Various forms of feline hyperparathyroidism are then described, encompassing diagnosis and treatment options.

Comparison of serum concentrations of symmetric dimethylarginine and creatinine as kidney function biomarkers in cats with chronic kidney disease

Background

Symmetric dimethylarginine (SDMA) has been shown to be an accurate and precise biomarker for calculating estimated glomerular filtration rate (GFR) in humans, as well as a more sensitive biomarker than serum creatinine concentration (sCr) for assessing renal dysfunction.

Objectives

The purpose of this retrospective study was to report on the utility of measuring serum SDMA concentrations in cats for detection of chronic kidney disease (CKD) before diagnosis by conventional measurement of sCr.

Animals

Chronic kidney disease cats (n = 21) included those persistently azotemic for ≥3 months (n = 15), nonazotemic cats with GFR >30% decreased from median GFR of normal cats (n = 4), and nonazotemic cats with calcium oxalate kidney stones (n = 2). Healthy geriatric cats (n = 21) were selected from the same colony.

Methods

Symmetric dimethylarginine concentrations (liquid chromatography‐mass spectroscopy) and sCr (enzymatic colorimetry) were determined retrospectively from historical data or banked serum samples in azotemic cats or at the time GFR (iohexol clearance) was measured in nonazotemic cats.

Results

Serum SDMA (r = −0.79) and sCr (r = −0.77) concentrations were significantly correlated to GFR (both P < .0001). Symmetric dimethylarginine became increased before sCr in 17/21 cats (mean, 17.0 months; range, 1.5–48 months). Serum SDMA had higher sensitivity (100%) compared with sCr (17%), but lower specificity (91% versus 100%) and positive predictive value (86% versus 100%).

Conclusion and Clinical Importance

Using serum SDMA as a biomarker for CKD allows earlier detection of CKD in cats compared with sCr, which may be desirable for initiating renoprotective interventions that slow progression of CKD.

Expression of fibroblast growth factor 23 by canine soft tissue sarcomas

Tumour-induced osteomalacia (TIO) is a rare paraneoplastic syndrome of humans. Some mesenchymal tumours (often resembling haemangiopericytomas) express molecules that normally regulate phosphorus metabolism; most frequently, fibroblast growth factor 23. Patients develop renal phosphate wasting and inappropriately low serum concentrations of 1, 25 (OH)2 vitamin D3 , leading to osteomalacia. Surgical removal of the tumour is curative. The authors examined expression of canine fibroblast growth factor 23 in 49 soft tissue sarcomas, and control tissues from normal adult dogs. RNA extracted from bone or formalin-fixed, paraffin-embedded tissues was analysed by end point and quantitative reverse transcriptase-polymerase chain reaction. Fibroblast growth factor 23 expression was detected in bone, lung, kidney, lymph node and thymus. Fifteen of 49 sarcomas (31%) expressed fibroblast growth factor 23, three of these had high relative expression and some features resembling phosphatonin-expressing mesenchymal tumours of humans. Further work is required to determine whether TIO may occur in dogs.

Estudo da ocorrência de litíase renal e ureteral em gatos com doença renal crônica

Investigou-se a ocorrência de nefrolitíase e/ou ureterolitíase em 72 gatos portadores de doença renal crônica (DRC), classificados predominantemente no estágio II, segundo os critérios designados pela IRIS - International Renal Interest Society. Destes pacientes, 47 (65,27%) apresentaram litíase renal e ou ureteral. Não houve diferença estatística entre o grupo de estudo (DRC com cálculo) e o grupo controle (DRC sem cálculo) em relação à idade (p=0,274). Apesar disso, os pacientes portadores de nefrolitíase e/ou ureterolitíase apresentaram maiores indícios de lesão renal, caracterizados por diferenças estatisticamente relevantes da densidade urinária (p=0,013) e pelo menor tamanho dos rins direito (p=0,009) e esquerdo (p=0,048). Encontrou-se similaridade entre os grupos em relação a outros parâmetros, tais como as concentrações plasmáticas de cálcio total, cálcio ionizado, fósforo, sódio, potássio e paratormônio intacto (PTHi). Os valores das concentrações séricas de ureia e bicarbonato diferiram entre os grupos, com valores de p=0,039 e p=0,037, respectivamente. Além disso, foi mensurada a pressão arterial, que se manteve inalterada na comparação entre o grupo de estudo e o grupo controle. Os resultados obtidos reforçam a necessidade de acompanhamento ultrassonográfico de todos os pacientes portadores de DRC, mesmo daqueles assintomáticos ou em estágios iniciais da doença.