Assessment of symmetric dimethylarginine as a biomarker of renal function in hyperthyroid cats treated with radioiodine

Comparison of serum creatinine, point-of-care symmetric dimethylarginine and renal imaging with glomerular filtration rate measured by renal scintigraphy in healthy and early chronic kidney diseased cats

The aim of this study was to evaluate routinely used tests to diagnose cats in early stages of chronic kidney disease (CKD) and to describe a model for evaluating these variables simultaneously. Apparently healthy cats were screened using serum creatinine (sCr), point-of-care symmetric dimethylarginine (POC SDMA), urinalysis, urine protein/creatinine ratio (UPC) and imaging evaluation. Those parameters were compared to glomerular filtration rate (GFR) assessed by renal scintigraphy. Forty-four cats were included and consisted of 14 (31.8%) healthy cats (absence of abnormalities in renal morphology and sCr less than 1.6 mg/dL), 20 (45.5%) cats classified as CKD I (presence of abnormalities in renal morphology and sCr less than 1.6 mg/dL) and ten (22.7%) as CKD II (sCr equal to or greater than 1.6 mg/dL, with or without abnormalities in renal morphology). A large number (40.9%) of apparently healthy cats presented reduction in GFR, which included half of CKD I patients. Point-of-care SDMA was not a good predictor for decreased GFR, nor was it correlated with the variables GFR and sCr. Glomerular filtration rate was significantly lower in CKD I and II groups in comparison with healthy cats, but there was no significant difference between the CKD I and II groups. Multivariate logistic regression model identified three variables that affected the odds of a cat having decreased GFR (< 2.5 mL/min/kg): sCr (OR = 18.3; p = 0.019; CI = 1.6-207.2), and the ultrasonographic findings 'reduced corticomedullary definition' (OR = 19.9; p = 0.022; CI = 1.6-254.0) and 'irregular contour' (OR = 65.6; p = 0.003; CI = 4.2-1038.2). Renal ultrasonography evaluation should always be considered for screening early CKD in apparently healthy cats.

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.

Changes in symmetric dimethylarginine and glomerular filtration rates in hyperthyroid cats undergoing radioiodine therapy

OBJECTIVES

The aims of this study were to evaluate concentrations of symmetric dimethylarginine (SDMA) in hyperthyroid cats before and after radioiodine treatment, and to compare results with other variables used to assess kidney function in cats (creatinine, urine specific gravity [USG] and glomerular filtration rate [GFR] measured by renal scintigraphy).

METHODS

Thirteen cats diagnosed with hyperthyroidism based on clinical signs and increased serum total thyroxine (TT4) were included in this prospective study. Study design included physical examination, complete blood count, serum chemistry, TT4, urinalysis and SDMA before treatment (T0) and at 1 month (T1) and 3 months post-treatment (T3). GFR was quantified by renal scintigraphy at T0 and T3.

RESULTS

Median GFR decreased significantly from baseline (3.18 ml/kg/min; range 1.35-4.87) at T3 (2.22 ml/kg/min; range 1.81-3.42 [P = 0.005]). While median creatinine and serum urea nitrogen increased post-treatment (creatinine: T0 = 0.8 mg/dl [range 0.4-1.1], T1 = 1.3 mg/dl [range 0.9-2]; T3 = 1.65 mg/dl [range 0.8-2.8]; P <0.001; serum urea nitrogen: T0 = 23 mg/dl [range 15-26]; T1 = 27 mg/dl [range 20-40]; T3 = 27.5 mg/dl [range 20-36]; P <0.001), SDMA and USG did not change significantly (SDMA: T0 = 11 µg/dl [range 7-15]; T1 = 12 µg/dl [range 6-16]; T3 = 10.5 µg/dl [range 8-21]; P = 0.789; USG: T0 = 1.030 [range 1.011-1.059]; T1 = 1.035 [range 1.012-1.044]; T3 = 1.030 [range 1.007-1.055]; P = 0.792).

CONCLUSIONS AND RELEVANCE

Our data suggest that factors other than GFR may affect serum SDMA in hyperthyroid cats and that SDMA does not offer an advantage over other biomarkers traditionally used to predict changes in renal function following radioiodine therapy.

Hyperthyroid cats and their kidneys: a literature review

Hyperthyroidism and chronic kidney disease (CKD) are common diseases of geriatric cats, and often occur concurrently. Thus, a thorough understanding of the influence of thyroid function on renal function is of significant value for all feline practitioners. Among other effects, hyperthyroidism causes protein catabolism and increases renal blood flow and glomerular filtration rate (GFR). These effects render traditional renal markers insensitive for the detection of CKD in cats with uncontrolled hyperthyroidism. Furthermore, the development of iatrogenic hypothyroidism with over treatment of hyperthyroidism can be detrimental to renal function and may negatively affect long-term survival. This review discusses important diagnostic considerations of feline hyperthyroidism, as well as key treatment modalities, with an emphasis on the use of radioiodine and the importance of post treatment monitoring of thyroid and renal parameters. In Australia, a common curative treatment for cats with benign hyperthyroidism (i.e. thyroid hyperplasia or adenoma) is a fixed dose of orally administered radioiodine, regardless of the serum total thyroxine concentration at the time of diagnosis. This review discusses the long term outcomes of this standard of care in comparison with current, relevant research literature from around the world. Finally, this review explores the use of symmetric dimethylarginine (SDMA) in assessing renal function before and after treatment in hyperthyroid cats. SDMA correlates well with GFR and creatinine in non-hyperthyroid cats, but our understanding of its performance in hyperthyroid cats remains in its infancy.

Feline Comorbidities: Balancing hyperthyroidism and concurrent chronic kidney disease

PRACTICAL RELEVANCE

Both hyperthyroidism and chronic kidney disease (CKD) are common long-term conditions in older cats, which might be diagnosed concurrently or develop at different times. Hyperthyroidism may mask the presence of CKD, and vice versa, by various mechanisms that are described in this review. Hyperthyroidism treatment options should be carefully considered when CKD has also been diagnosed.

CLINICAL CHALLENGES

Although it can be difficult to diagnose hyperthyroidism and CKD simultaneously, given that one condition may mask the other, it is important to consider the presence of both diseases when examining an older cat presenting with vomiting, weight loss, polyuria/ polydipsia, anorexia or sarcopenia. The concurrent presence of hyperthyroidism and CKD requires careful monitoring of glomerular filtration rate biomarkers, and adequate and prompt support of kidney function when normal thyroid function is re-established. Iatrogenic hypothyroidism is a recognised complication of all of the treatment options for hyperthyroidism, and increases the risk of azotaemia. Therapy with levothyroxine is recommended for cats that are hypothyroid and azotaemic.

EVIDENCE BASE

The information in this review draws on current literature and guidelines related to the pathophysiology, diagnosis and treatment recommendations for feline hyperthyroidism and CKD.

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.

Liver-type fatty acid-binding protein and neutrophil gelatinase-associated lipocalin in cats with chronic kidney disease and hyperthyroidism

BACKGROUND

Liver-type fatty acid-binding protein (L-FABP) and neutrophil gelatinase-associated lipocalin (NGAL) are candidate biomarkers for the detection of early chronic kidney disease (CKD) in cats.

OBJECTIVE

To evaluate urinary and serum L-FABP and NGAL concentrations in CKD cats and in hyperthyroid cats before and after radioiodine (¹³¹ I) treatment.

ANIMALS

Nine CKD cats, 45 healthy cats and hyperthyroid cats at 3 time points including before (T0, n = 49), 1 month (T1, n = 49), and 11 to 29 months after (T2, n = 26) ¹³¹ I treatment.

METHODS

Cross-sectional and longitudinal study. Serum L-FABP (sL-FABP), serum NGAL (sNGAL), urinary L-FABP (uL-FABP), and urinary NGAL (uNGAL) were compared between the 3 groups and between hyperthyroid cats before and after treatment. Data are reported as median (min-max).

RESULTS

CKD cats had significantly higher sL-FABP (13.50 [3.40-75.60] ng/ml) and uL-FABP/Cr (4.90 [0.97-2139.44] µg/g) than healthy cats (4.25 [1.34-23.25] ng/ml; P = .01 and 0.46 [0.18-9.13] µg/g; P < .001, respectively). Hyperthyroid cats at T0 had significantly higher uL-FABP/Cr (0.94 [0.15-896.00] µg/g) than healthy cats (P < .001), thereafter uL-FABP/Cr significantly decreased at T2 (0.54 [0.10-76.41] µg/g, P = .002). For the detection of CKD, uL-FABP/Cr had 100% (95% confidence interval [CI], 66.4-100.0) sensitivity and 93.2% (95% CI, 81.3-98.6) specificity. There were no significant differences in sNGAL and uNGAL/Cr between the 3 groups.

CONCLUSIONS AND CLINICAL IMPORTANCE

L-FABP, but not NGAL, is a potential biomarker for the detection of early CKD in cats. Utility of uL-FABP to predict azotemia after treatment in hyperthyroid cats remains unknown.

Plasma symmetric dimethylarginine and creatinine concentrations and glomerular filtration rate in cats with normal and decreased renal function

Background

Glomerular filtration rate (GFR) is the gold standard in assessing renal function but is impractical. Serum creatinine (sCr) has limited sensitivity in identifying early chronic kidney disease (CKD), whereas symmetric dimethylarginine (SDMA) has been commercialized as more accurate biomarker. Studies comparing SDMA and sCr with GFR in cats are limited.

Objectives

To further investigate the diagnostic performance of SDMA in nonazotemic and azotemic cats.

Animals

Forty‐nine client‐owned cats: 17 cats with CKD, 15 cats with diabetes mellitus (DM), and 17 healthy cats.

Methods

Retrospective study using spare blood samples from cats with documented sCr and GFR results for SDMA analysis. Diagnostic performances of SDMA and sCr were evaluated using correlation coefficients, sensitivities, specificities, and receiver operator characteristic curves.

Results

Compared to healthy cats and cats with DM, CKD cats had significantly higher SDMA_plasma (26.7 ± 9.9 μg/dL) and sCr (249.7 ± 71.6 μmol/L [2.8 ± 0.8 mg/dL]; both P < .001) values. SDMA_plasma (τ_B = −0.57; P < .001) and sCr (τ_B = −0.56; P < .001) were significantly correlated with GFR. SDMA_plasma (τ_B = 0.52; P < .001) had a significant relationship with sCr. SDMA_plasma and sCr had similar sensitivity (76%‐94% and 71%‐88%, respectively) in detecting reduced renal function. Creatinine had higher specificity (94%‐96%) than SDMA_plasma (75%‐76%) (P < .05).

Conclusion and Clinical Importance

In this study of azotemic and nonazotemic cats, SDMA was a reliable marker to identify decreased GFR. However, superiority of SDMA over sCr could not be confirmed.

The new age of renal biomarkers: does SDMA solve all of our problems?

Within clinical small animal practice, diagnosis of both chronic kidney disease and acute kidney injury is common. To assess renal function, measurement of glomerular filtration rate is considered the gold standard. Currently, routine tests of kidney function include surrogate markers of glomerular filtration rate such as serum creatinine, and urea, each with their own limitations, whilst urine protein to creatinine ratio gives an indication of glomerular and tubular handling of protein, and urine specific gravity information about urine concentrating ability by the kidney. These parameters are used together with historical and physical examination data to give a diagnosis of kidney disease following which creatinine, proteinuria and blood pressure are used to stage chronic kidney disease and, together with urine output, grade acute kidney injury according to the International Renal Interest Society. However, there has been much concern that creatinine is insensitive when used to indicate early decline in renal function and this has highlighted the need for additional methods of diagnosing and monitoring these patients, with the potential to allow earlier therapeutic intervention. Symmetric dimethylarginine is a novel biomarker, which has been shown to perform as a surrogate marker of glomerular filtration rate in small animals. This article will review current research on symmetric dimethylarginine and the ways in which it may be utilised in small animal practice; current research supports the use of symmetric dimethylarginine as a screening test for detection of early chronic kidney disease according to International Renal Interest Society guidelines, but further research is required in to the usefulness of symmetric dimethylarginine as a tool for monitoring disease and the effect of non-renal influences.

Assessment of serum symmetric dimethylarginine and creatinine concentrations in hyperthyroid cats before and after a fixed dose of orally administered radioiodine

Background

Serum symmetric dimethylarginine (SDMA) is a sensitive renal biomarker for detecting early chronic kidney disease (CKD) in nonhyperthyroid cats, but knowledge regarding its performance in hyperthyroid cats remains limited.

Objectives

To determine the relationship between serum SDMA, creatinine and total thyroxine (TT4) concentrations in hyperthyroid cats before (T0) and 3 months after (T1) receiving a PO fixed dose of radioiodine.

Animals

Eighty client‐owned hyperthyroid cats.

Methods

Prospective cohort study. Serum TT4, and SDMA, creatinine concentrations, and urine specific gravity were measured at T0 and T1. Nonparametric tests were used to determine the relationship among SDMA, and creatinine and TT4 concentrations. Agreement between SDMA and creatinine regarding CKD staging at both time points was assessed using Goodman and Kruskal's gamma statistic.

Results

Mean serum SDMA concentration increased after treatment of hyperthyroidism. However, 21 of 75 cats experienced a decrease in SDMA between T0 and T1, whereas creatinine decreased in only 2 cats. A moderate correlation between SDMA and creatinine was seen at T1 (r = 0.53; P < .001) but not at T0 (r = 0.13; P = .25). Where assessable at T1, poor agreement was observed between SDMA and creatinine and CKD stage (Goodman and Kruskal's gamma 0.20; P = .29).

Conclusions and clinical importance

Discordant outcomes between SDMA and creatinine after radioiodine treatment in cats with hyperthyroidism suggest extrarenal factors may interfere with the reliability of SDMA to adequately reflect renal function. As a result, SDMA should not be interpreted in isolation in hyperthyroid cats treated with radioiodine.

Symmetric dimethylarginine in hyperthyroid cats before and after treatment with radioactive iodine

OBJECTIVES

The purpose of this study was to evaluate symmetric dimethylarginine (SDMA) in hyperthyroid cats before and after treatment with radioactive iodine and to determine how pretreatment SDMA relates to the development of post-treatment azotemia.

METHODS

Eighty-four non-azotemic hyperthyroid cats had serum SDMA and creatinine evaluated before and 1, 3 and 6 months after treatment with radioiodine therapy.

RESULTS

Baseline SDMA was increased in 7% (n = 6/84) of cats, whereas SDMA was increased in 19% (n = 15/81), 20% (n = 16/80) and 32% (n = 26/81) at 1 month, 3 months and 6 months after treatment, respectively. Creatinine was not elevated in any of the cats at baseline because of the study design, and was elevated in 6% (n = 5/81), 15% (n = 12/80) and 15% (n = 12/81) of cats at 1, 3 and 6 months after treatment, respectively. SDMA (median 11 μg/dl, range 1-22 μg/dl) was significantly higher at 3 (12 μg/dl, range 6-45 μg/dl; P = 0.005) and 6 months (11 μg/dl, 6-25 μg/dl; P <0.001) compared with baseline (11 μg /dl, range 1-21 μg/dl). The median baseline SDMA was significantly higher in the azotemic group (13 μg/dl, range 11-22 μg/dl) compared with the non-azotemic group (10 μg/dl, range 1-21 μg/dl, P = 0.002). The sensitivity of SDMA for detecting azotemia after treatment was 15.4%, with a specificity of 94.4%. Baseline serum SDMA concentration had a moderately positive association with baseline creatinine concentration (P <0.001, r = 0.437). At 6 months, there was a strong positive correlation between SDMA and creatinine concentrations (P <0.001, r = 0.721). There was no significant correlation with SDMA and thyroxine at baseline (P = 0.772, r = -0.034) or 6 months (P = 0.492, r = -0.078).

CONCLUSIONS AND RELEVANCE

SDMA increases in cats treated for hyperthyroidism with radioactive iodine and likely reflects associated changes in glomerular filtration rate. An increased SDMA concentration above the reference interval prior to treatment has a high specificity but poor sensitivity for the prediction of post-treatment azotemia.

New Tests in Feline Veterinary Medicine: When to Use Them and When to Stick with Tried-and-True Tests

The focus of this article is on how interpretations of laboratory data can utilize both population and individual reference intervals, while making the most of routine testing procedures coupled with some of the newer laboratory tests, which can complement the existing tests in diagnosing disease.

Assessment of symmetric dimethylarginine as a biomarker of renal function in hyperthyroid cats treated with radioiodine

Background

Measurement of serum creatinine (sCr) and urea nitrogen fail to detect decreased renal function in many hyperthyroid cats because of low muscle mass and glomerular hyperfiltration of affected cats. Serum symmetric dimethylarginine (sSDMA) is an earlier and more sensitive renal biomarker than sCr.

Objective

Evaluate sSDMA as a biomarker of renal function in hyperthyroid cats before (T0) and 1 month after (T1) radioiodine (¹³¹I) treatment.

Animals

Forty‐seven client‐owned hyperthyroid nonazotemic cats were evaluated at T0 and T1.

Methods

A prospective study in which sCr and sSDMA concentrations were determined in 47 hyperthyroid cats at T0 and at T1. Glomerular filtration rate (GFR) was estimated at T0 and T1 in 10 of these 47 cats using plasma exogenous creatinine clearance test.

Results

Serum SDMA was elevated (>14 μg/dL) in 6 of 47 cats at T0 and normalized after treatment in 4 of those cats. All cats remained nonazotemic after treatment. In 10 cats in which GFR was measured, correlation between GFR and sSDMA was low and not significant (τ_b = −0.35, P = .17 at T0 and τ_b = −.22, P = .41 at T1), whereas correlation between GFR and sCr was moderate and significant (τ_b = −0.52, P < .05 at T0 and τ_b = −.53, P = <.05 at T1).

Conclusions and Clinical Importance

Careful interpretation of mildly increased sSDMA with normal sCr in hyperthyroid cats is warranted as sSDMA values might normalize after resolution of hyperthyroidism in some cats. In this population of hyperthyroid cats, sSDMA was poorly correlated with GFR.