The diurnal variation in urine acidification differs between normal individuals and uric acid stone formers. Kidney Int. 2012;81:1123-1130. [PMID: 22297671 PMCID: PMC3352978 DOI: 10.1038/ki.2011.480]

Metabolic Evaluation and Recurrence Prevention for Urinary Stone Patients: An EAU Guidelines Update

BACKGROUND AND OBJECTIVE

The aim of this review was to define patients who are at high risk of recurrence of urolithiasis, to delineate diagnostic and therapeutic algorithms for each type of stone, and to clarify general guidelines and recommendations for prevention of recurrence.

METHODS

A professional research librarian carried out literature searches for all sections of the urolithiasis guidelines, covering the timeframe between 1976 and June 2023.

KEY FINDINGS AND LIMITATIONS

For every patient with urolithiasis, an attempt should be made to analyse the stone. Patients should be given general instructions on how to prevent recurrence, including adequate fluid and calcium intake, and low consumption of sodium and protein. Identifying and correcting the causative factors is a cornerstone in preventing the recurrence of urolithiasis. Diagnostic and therapeutic algorithms by stone composition are available. Every patient should undergo baseline metabolic screening, while patients with calcium stones, who are at high risk of relapse and complications, should undergo extensive metabolic screening with two 24-h urine collections and should receive targeted therapy. Patients with uric acid, infection, or cystine stones are at high risk of relapse. All patients at high risk of recurrence should be closely monitored, especially those not complying with therapy in the long term.

CONCLUSIONS AND CLINICAL IMPLICATIONS

Metabolic stone evaluation and patient follow-up are highly recommended to prevent urolithiasis recurrence.

Proton pump inhibitors use is associated with a higher prevalence of kidney stones: NHANES 2007-2018

BACKGROUND

Proton pump inhibitors (PPIs) are widely used throughout the world as an effective gastrointestinal drug. Nevertheless, according to the existing literature, PPIs can reduce the excretion of magnesium, calcium and other components in urine, which may promote the formation of kidney stones. We used the National Health and Nutrition Examination Survey (NHANES) database to further investigate the association between the use of PPIs and the prevalence of kidney stones.

METHODS

We performed a cross-sectional analysis using data from 2007 to 2018 NHANES. PPIs use information of 29,910 participants was obtained by using prescription medications in the preceding month, and kidney stones were presented by a standard questionnaire. Multiple regression analysis and stratified analysis were used to estimate the association between PPIs use and kidney stones after an adjustment for potential confounders.

RESULTS

The multiple logistic regression indicated that the PPIs exposure group (P1) had a significantly higher risk of nephrolithiasis than the PPIs non-exposure group (P0) in Model 3 (OR 1.24, 95% CI 1.10-1.39, P < 0.001). The stratified analyses indicated there were significant statistical differences between PPIs use and kidney stones among females (OR 1.36, 95% CI 1.15-1.62, P < 0.001), non-Hispanic whites (OR 1.27, 95% CI 1.09-1.48, P = 0.002), individuals with an education level than 11th grade (OR 1.41, 95% CI 1.13-1.76, P = 0.002) and individuals with an annual family income of $0 to $19,999 (OR 1.32, 95% CI 1.06-1.65, P = 0.014) and $20,000 to $44,999 (OR 1.25, 95% CI 1.02-1.54, P = 0.033) in Model 3.

CONCLUSIONS

Our study revealed that PPIs use is associated with a higher prevalence of kidney stones for the US population, primarily among women, non-Hispanic whites, individuals with low education levels and individuals with low household income levels. Further studies are required to confirm our findings.

Effects of two commercial diets and two supplements on urinary pH in dogs

BACKGROUND

Urinary pH manipulation by therapeutic foods or supplements is part of the treatment for urolithiasis. The effectiveness of these diets and supplements should be studied to determine which of these strategies is most effective.

HYPOTHESIS/OBJECTIVES

To assess the effect of the oral supplementation of potassium citrate, an ammonium chloride solution (Urical) and two dry therapeutic foods-Hill's® Prescription Diet® u/d® Canine (u/d diet) and Royal Canin® Urinary S/O dog (S/O diet)-on a dog's urinary pH at different time points over 8 h.

ANIMALS

Seven healthy adult male research beagle dogs.

METHODS

A prospective interventional study lasting 31 days. The dogs either received a supplement (potassium citrate or rical) with a dry adult maintenance diet (control diet) or the therapeutic diet (u/d diet or S/O diet). Each treatment had a duration of 2-5 days, with 2- to 4-day washout periods in between. Urinary pH measurements were performed every 2 h between 07h00 and 15h00, with the food being given at 07h00 and 15h00, right after urine collection. The pH measurements obtained in each of the four treatments were compared to control (same dogs fed the control diet exclusively).

RESULTS

When compared to the control diet at the same time points, biologically relevant changes in urinary pH (defined as ≥0.5) were: increase with potassium citrate at 7h00 and 13h00; increase with u/d diet at 9h00, 13h00, and 15h00; decrease with S/O diet at 9h00 and 11h00; Urical did not have a detectable effect on urinary pH.

CONCLUSIONS AND CLINICAL IMPORTANCE

The present study confirms that therapeutic foods S/O and u/d, and potassium citrate supplement affected acid-base balance in healthy adult male beagle dogs, with the tested diets being more effective than the administered doses of the tested supplements at influencing urinary pH.

[Diurnal differences in acute gout attacks: A clinical study of male gout patients]

OBJECTIVE

To observe the diurnal difference of acute gout attacks in men, and provide reference for accurate clinical prevention and treatment.

METHODS

Using a single-center, cross-sectional study design, the patients diagnosed with gout in the outpatient department of Rheumatology and Immuno-logy of PLA Joint Logistic Support Force No.980 Hospital from October 2021 to April 2022 were selected. The information about the patient's current/last acute gout attacks (less than 2 weeks from visit), date and time of attacks, joint symptoms and signs, medication use, and relevant biochemical tests on the day of visit was recorded. The diurnal time difference of acute gout attacks in male patients was analyzed, and univariate comparison and multivariate Logistic regression analyses were conducted to compare the diurnal difference of acute gout attacks with clinical characteristics and biochemical indicators.

RESULTS

A total of 100 male gout patients were included, and 100 acute attacks were recorded. Diurnal distribution of acute gout attacks: morning (6:00~11:59, 18, 18%), afternoon (12:00~17:59, 11, 11%), the first half of the night (18:00~23:59, 22, 22%), the second half of the night (0:00~05:59, 49, 49%); During the day (included morning and afternoon, 29, 29%) and at night (included the first half of the night and the second half of the night, 71, 71%). The rate of acute gout attack was significantly higher at night than in the day (about 2.5 ∶1). No matter the first or recurrent gout, no matter the duration of the disease, the number of acute gout attacks had the difference of less in the day and more in the night. Serum urate (SU) level was higher in the patients with nocturnal attack than in those with daytime attack (P=0.044). Comorbidities were significantly different in the day-night ratio of the number of acute gout attack (P=0.028). Multiple Logistic regression analysis showed that SU level (OR=1.005, 95%CI: 1.001-1.009) and comorbidities (OR=3.812, 95%CI: 1.443-10.144) were the correlative factors of nocturnal acute gout attacks.

CONCLUSION

No matter the first or recurrent gout, no matter the duration of the disease, it has a diurnal variation characterized by multiple attacks at night, increased SU level and comorbidities are correlative factors for nocturnal acute attack of gout.

Urine pH and Citrate as Predictors of Calcium Phosphate Stone Formation

Key Points

The occurrence of calcium phosphate stones has increased over the past five decades, and this is most notable in female stone formers. High urine pH and hypocitraturia are the most discriminatory urine parameters between calcium phosphate and calcium oxalate stone formers. High urine pH in calcium phosphate stone formers is independent of the effect of dietary alkali and acid.

Background

Urinary parameters, including urine pH and citrate, are recognized as critical in the pathophysiology of calcium-based stones. The factors contributing to variation in these parameters between calcium oxalate (CaOx) and calcium phosphate (CaP) stone formers (SFs) are, however, not well-understood. In this study, using readily available laboratory data, we explore these differences to delineate the odds of forming CaP versus CaOx stones.

Methods

In this single-center retrospective study, we compared serum and urinary parameters between adult CaP SFs, CaOx SFs, and non–stone formers.

Results

Urine pH was higher and urine citrate lower in CaP SFs compared with same-sex CaOx SFs and non–stone formers. In CaP SFs, higher urine pH and lower citrate were independent of markers of dietary acid intake and gastrointestinal alkali absorption, suggesting abnormal renal citrate handling and urinary alkali excretion. In a multivariable model, urine pH and urine citrate were most discriminatory between CaP SFs and CaOx SFs (receiver-operating characteristic area under the curve of 0.73 and 0.65, respectively). An increase in urine pH by 0.35, a decrease in urine citrate by 220 mg/d, a doubling of urine calcium, and female sex all independently doubled the risk of CaP stone formation compared with CaOx stones.

Conclusions

High urine pH and hypocitraturia are two clinical parameters that distinguish the urine phenotype of CaP SFs from CaOx SFs. Alkalinuria is due to intrinsic differences in the kidney independent of intestinal alkali absorption and is accentuated in the female sex.

Comparison potassium sodium hydrogen citrate with sodium bicarbonate in urine alkalization: a prospective crossover-controlled trial

PURPOSE

Excessive alkalization will increase the incidence of nephrolithiasis. Sodium bicarbonate (NaHCO₃) and potassium sodium hydrogen citrate (PSHC) are commonly used drugs for urinary alkalization. We designed a trial to compare PSHC with NaHCO₃ in the urine alkalization for the Chinese healthy participants and to explore the effects of PSHC and NaHCO₃ on circadian rhythms of urine pH value.

METHOD

This study was a prospective, crossover, randomized, controlled trial, in which a total of 34 healthy volunteers participated in two study phases and took PSHC and NaHCO₃ at the maintenance dose, respectively.

RESULT

The average level of urine pH of PSHC participants in 24 h was significantly higher than that of NaHCO₃ (P < 0.001). The urine pH value of participants taking PSHC and NaHCO₃ or under physiological conditions showed significant variation in 24 h (P < 0.05) and fitted to a mathematical model (Fourier series). Under physiological conditions, the average urine pH value in the daytime was higher than that in the night, and reached the peak at about 10:00, 16:00, and 22:00. The peak of urine pH at 24 h after taking PSHC and NaHCO₃ was both higher than the baseline. The peak time of urine pH and the curve trend were similar, but the peak value in PSHC group was significantly higher than that in NaHCO₃ group.

CONCLUSIONS

There was a circadian rhythm of urine pH value under physiological conditions. PSHC was more effective in urinary alkalization than NaHCO₃ at the current maintenance oral dose and administration time without changing the rhythm of urine pH value.

CLINICAL TRIAL REGISTRATION

NCT04352153.

Estimating 24-hour urinary excretion using spot urine measurements in kidney stone formers

BACKGROUND

One limitation of the use of 24-hour collection is impracticality. We analysed the performance of spot urine measurements to estimate 24-hour excretion in patients with kidney stones.

METHODS

A total of 74 adult patients from two centres performed a 24-hour urine collection. A sample of the last micturition was sent for spot urine analysis. Twenty patients were asked to collect two additional spot urine samples, one before dinner and the other after dinner. Urinary concentrations of creatinine, calcium, oxalate, uric acid, citrate and magnesium were measured in the 24-hour and each of the spot urine samples. Four approaches were used to estimate 24-hour urinary excretion, multiplying the ratio of the spot urinary analyte to creatinine concentration by (i) measured 24-hour urinary creatinine excretion (Prediction 1), (ii) estimated 24-hour urinary creatinine excretion (Prediction 2), (iii) assumed 1-g 24-hour urinary creatinine excretion (Prediction 3) or (iv) assumed 1.5-g 24-hour urinary creatinine excretion (Prediction 4). For each parameter we computed Lin's concordance correlation coefficients (CCCs), Bland-Altman plots and 95% limits of agreement.

RESULTS

The performance of estimates obtained with Prediction 1 and Prediction 2 was similar, except for citrate and uric acid, for which Prediction 2 performed worse. Both approaches performed moderately well: citrate CCC {0.82 [95% confidence interval (CI) 0.75-0.90]}, oxalate [0.66 (95% CI 0.55-0.78)], magnesium [0.66 (95% CI 0.54-0.77)], calcium [0.63 (95% CI 0.50-0.75)] and uric acid [0.52 (95% CI 0.36-0.68)]. The performance of Predictions 3 and 4 was worse.

CONCLUSIONS

Although spot urine samples may hold promise for clinical and population-based research, at present they have limited utility in clinical practice. Measuring or estimating 24-hour creatinine, rather than assuming a given creatinine excretion, will be necessary in future studies of spot urine samples.

Circadian clocks of the kidney: function, mechanism, and regulation

An intrinsic cellular circadian clock is located in nearly every cell of the body. The peripheral circadian clocks within the cells of the kidney contribute to the regulation of a variety of renal processes. In this review, we summarize what is currently known regarding the function, mechanism, and regulation of kidney clocks. Additionally, the effect of extrarenal physiological processes, such as endocrine and neuronal signals, on kidney function is also reviewed. Circadian rhythms in renal function are an integral part of kidney physiology, underscoring the importance of considering time of day as a key biological variable. The field of circadian renal physiology is of tremendous relevance, but with limited physiological and mechanistic information on the kidney clocks this is an area in need of extensive investigation.

Neglected analytes in the 24-h urine: ammonium and sulfate

PURPOSE OF REVIEW

Evaluation of the kidney stone patient includes measurement of 24 h urine chemistries. This review summarizes the application of physiologic principles to the interpretation of urine chemistries, using sulfate and ammonium to estimate diet acid load, and the renal response.

RECENT FINDINGS

There has been increased recognition of the need to measure urine ammonium excretion in the clinical setting in order to understand renal acid excretion. Some 24 h urine kidney stone panels include ammonium measurements, providing an opportunity to apply this measurement to clinical practice. In order to better interpret ammonium excretion, one needs an estimate of dietary acid load to understand the driving forces for ammonium excretion. Sulfate is also included in some kidney stone panels and functions as an estimate of diet acid load. Combining these analytes with urine pH, the clinician can quickly estimate dietary stone risk as well as potential bowel disease, acidification disorders, and the presence of urease producing bacteria; all of which can affect stone risk.

SUMMARY

Measurement of ammonium and sulfate excretion along with urine pH provide important insights into the acid/alkali content of diet, presence and severity of bowel disease, presence of renal acidification disorders, and urinary infection.

Alkaline Urine in the Emergency Department Predicts Nitrofurantoin Resistance

BACKGROUND

The Proteeae group (i.e., Proteus species, Morganella morganii, and Providencia species) frequently causes urinary tract infections (UTIs) and is generally resistant to nitrofurantoin. Proteeae species can produce urease, which can increase urine pH.

OBJECTIVE

Our aim was to determine whether higher urine pH in the emergency department is associated with nitrofurantoin resistance.

METHODS

A single health system database of emergency department patients aged 18 years and older who received urinalysis between April 18, 2014, and March 7, 2017, was examined using χ² test and multivariable regression analysis.

RESULTS

Of 67,271 urine samples analyzed, 13,456 samples grew a single bacterial species. Urine cultures growing the Proteeae group were associated with significantly more alkaline urine than other bacteriuria cultures (odds ratio [OR] 2.20, 95% confidence interval [CI] 2.06-2.36; p < 0.001). The Proteeae species represented 4.4% of urine samples at pH 5-7, 24.4% at pH 8-9, and 40.0% at pH 9. At urine pH 5-7, 80.4% of urine samples were sensitive to nitrofurantoin; however, this percentage decreased to 66.1% for urine pH 8-9 and 54.6% for urine pH 9. Nitrofurantoin had the highest OR (2.10, 95% CI 1.85-2.39) among cefazolin, ciprofloxacin, and trimethoprim/sulfamethoxazole for bacteriuria sensitive to those antibiotics at urine pH 5-7. At urine pH 8-9 and 9, nitrofurantoin had the lowest OR among the antibiotics: 0.48 (95% CI 0.42-0.54) and 0.31 (95% CI 0.24-0.40), respectively (p < 0.001 for both).

CONCLUSIONS

Urine pH of 8 or higher is associated with high rates of nitrofurantoin resistance.

Evaluation of urinary acidification in children: Clinical utility

The kidney plays a fundamental role in acid-base homeostasis by reabsorbing the filtered bicarbonate and by generating new bicarbonate, to replace that consumed in the buffering of non-volatile acids, a process that leads to the acidification of urine and the excretion of ammonium (NH₄ ⁺). Therefore, urine pH (UpH) and urinary NH₄ ⁺ (UNH₄ ⁺) are valuable parameters to assess urinary acidification. The adaptation of automated plasma NH₄ ⁺ quantification methods to measure UNH₄ ⁺ has proven to be an accurate and feasible technique, with diverse potential indications in clinical practice. Recently, reference values for spot urine NH₄ ⁺/creatinine ratio in children have been published. UpH and UNH₄ ⁺, aside from their classical application in the study of metabolic acidosis, have shown to be useful in the identification of incomplete distal renal tubular acidosis (dRTA), an acidification disorder, without overt metabolic acidosis, extensively described in adults, and barely known in children, in whom it has been found to be associated to hypocitraturia, congenital kidney abnormalities and growth impairment. In addition, a low UNH₄ ⁺ in chronic kidney disease (CKD) is a risk factor for glomerular filtration decay and mortality in adults, even in the absence of overt metabolic acidosis. We here emphasize on the need of measuring UpH and UNH₄ ⁺ in pediatric population, establishing reference values, as well as exploring their application in metabolic acidosis, CKD and disorders associated with incomplete dRTA, including growth retardation of unknown cause.

Crosstalk of physiological pH and chemical pKa under the umbrella of physiologically based pharmacokinetic modeling of drug absorption, distribution, metabolism, excretion, and toxicity

Introduction: Physiological pH and chemical pKa are two sides of the same coin in defining the ionization of a drug in the human body. The Henderson-Hasselbalch equation and pH-partition hypothesis form the theoretical base to define the impact of pH-pKa crosstalk on drug ionization and thence its absorption, distribution, metabolism, excretion, and toxicity (ADMET).Areas covered: Human physiological pH is not constant, but a diverse, dynamic state regulated by various biological mechanisms, while the chemical pKa is generally a constant defining the acidic dissociation of the drug at various environmental pH. Works on pH-pKa crosstalk are scattered in the literature, despite its significant contributions to drug pharmacokinetics, pharmacodynamics, safety, and toxicity. In particular, its impacts on drug ADMET have not been effectively linked to the physiologically based pharmacokinetic (PBPK) modeling and simulation, a powerful tool increasingly used in model-informed drug development (MIDD).Expert opinion: Lacking a full consideration of the interactions of physiological pH and chemical pKa in a PBPK model limits scientists' capability in mechanistically describing the drug ADMET. This mini-review compiled literature knowledge on pH-pKa crosstalk and its impacts on drug ADMET, from the viewpoint of PBPK modeling, to pave the way to a systematic incorporation of pH-pKa crosstalk into PBPK modeling and simulation.

Uric Acid and Urate in Urolithiasis: The Innocent Bystander, Instigator, and Perpetrator

Uric acid is an end product of purine metabolism in human beings. An unusual and still unexplained phenomenon is that higher primates have relatively high uric acid levels in body fluids owing to a combination of absence of degradation and renal retention. The physiologic purpose of high uric acid levels still is enigmatic, but the pathobiologic burden is a variety of crystallopathies owing to the low aqueous solubility of uric acid such as gouty arthritis and acute uric acid nephropathy. In the urinary space, three distinct conditions result from chronic uric acid and/or urate precipitation. The first and most common variety is uric acid urolithiasis. In this condition, urate is a victim of a systemic metabolic disease in which increased acid load to the kidney is coupled with diminished urinary buffer capacity owing to defective ammonium excretion, resulting in titration of urate to its sparingly soluble protonated counterpart, uric acid, and the formation of stones. Uric acid is the innocent bystander of the crime. The second variety is hyperuricosuric calcium urolithiasis, in which uric acid confers lithogenicity via promotion of calcium oxalate precipitation by multiple mechanisms involving soluble, colloidal, and crystalline urate salts. Uric acid is the instigator of the crime. The third and least common condition involves urate as an integral part of the urolith as an ammonium salt driven by high ammonium and high urate concentrations in urine. Here, uric acid is one of the perpetrators of the crime. Both known and postulated pathogenesis of these three types of urolithiasis are reviewed and summarized.

Investigation on the Mechanisms of Synchronous Interaction of K3Cit with Melamine and Uric Acid That Avoids the Formation of Large Clusters

Uric acid (UA) has an enormous competence to aggregate over melamine (Mel), producing large UA clusters that "drag" Mel toward them. Such a combination of donor-acceptor pairs provides a robust Mel-UA composite, thereby denoting a high complexity. Thus, a straightforward but pragmatic methodology might indeed require either destruction of the aggregation of UA or impediment of the hydrogen-bonded cluster of Mel and UA. Here, potassium citrate (K₃Cit) is used as a potent inhibitor for a significant decrease of large UA-Mel clusters. The underlying mechanisms of synchronous interactions between K₃Cit and the Mel-UA pair are examined by the classical molecular dynamics simulation coupled with the enhanced sampling method. K₃Cit binds to the Mel-UA pair profoundly to produce a Mel-UA-K₃Cit complex with favorable complexation energy (as indicated by the reckoning of pairwise ΔG_bind° employing the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) method). The strength of interaction follows the order UA-K₃Cit > Mel-K₃Cit > Mel-UA, thus clearly demonstrating the instability caused by upsetting the π-stacking of UA and hydrogen bonding of Mel-UA simultaneously. The comprehensive, strategically designed "direct approach" and "indirect approach" cluster structure analysis shows that K₃Cit reduces the direct approach Mel-UA cluster size significantly irrespective of ensemble variation. Furthermore, the estimation of potentials of mean force (PMFs) reveals that the (UA)_decamer-Mel interaction prevails over (UA)_tetramer-Mel. The dynamic property (dimer existence autocorrelation functions) proves the essence of dimerization between Mel and UA in the absence and presence of K₃Cit. Moreover, the calculation of the preferential interaction parameter provides the concentration at which Mel-K₃Cit and UA-K₃Cit interactions are predominant over the interaction of Mel and UA.

Innovative prolonged-release oral alkalising formulation allowing sustained urine pH increase with twice daily administration: randomised trial in healthy adults

A multi-particulate fixed-dose combination product, consisting of a combination of two alkalising salts formulated as prolonged-release granules, ADV7103, was developed to obtain a sustained and prolonged alkalising effect. The specific release of both types of granules was shown in vitro through their dissolution profiles, which indicated that potassium citrate was released within the first 2-3 h and potassium bicarbonate up to 10-12 h after administration. The long-lasting coverage of ADV7103 was confirmed through a randomised, placebo-controlled, double-blind, two-period study, measuring its effect on urine pH in healthy adults (n = 16) at doses of alkalising agent ranging between 0.98 and 2.88 meq/kg/day. A significant increase of urine pH with a positive dose-response in healthy adult subjects was shown. Urine pH above 7 was maintained during 24 h with a dosing equivalent to 1.44 meq/kg twice a day, while urine pH was below 6 most of the time with placebo. The effect observed was non-saturating within the range of doses evaluated and the formulation presented a good safety profile. ADV7103 provided an effective prolonged release of alkalising salts to cover a 12-h effect with adequate tolerability and could afford a twice a day (morning and evening) dosing in patients requiring long-term treatment.

Urinary supersaturation on fractioned urine collections: which urine sample can explain better the variability observed on 24-h urine? A proof-of-concept study

Due to the difficulty of collecting 24-h urines in the stone-forming patient, some authors have suggested other types of urine collection, but their usefulness is not yet well studied. The objective of this study is to evaluate the variation of urinary supersaturation (SS) throughout the day and to analyze whether timed urine collections offer accurate information. 48 urine samples were collected from 12 young adults. Each 24-h urine was collected on 7 2-h urine fractions and a 10-h overnight sample. Solute concentrations and SS for calcium oxalate (CaOx), calcium phosphate (CaP), and uric acid (UA) were determined. Linear regression and relative importance of predictors were used to determine the percentage of R² attributed to each timed collection (individual SS). 43 24-h urine samples were included in the study. The highest SS values were: for CaOx, night period and first morning urine; for CaP, between 2 and 6 pm and at night; for UA, between 8 am and 12 pm. For CaOx, the SS from the samples between 8 pm and 8 am accounted for more than 40% of the R²; for CaP, the results were more equally distributed throughout the day, and for UA, the SS values from 12 to 4 pm accounted for more than 45% of the observed variability. In conclusion, urinary SS varies throughout the day, being higher for CaOx and CaP at night, and in the early morning for UA. For CaOx and UA, the overnight and 12-4 pm urine samples, respectively, contribute most to the variability observed in the SS of 24-h urine.

Circadian rhythms and the kidney

Numerous physiological functions exhibit substantial circadian oscillations. In the kidneys, renal plasma flow, the glomerular filtration rate and tubular reabsorption and/or secretion processes have been shown to peak during the active phase and decline during the inactive phase. These functional rhythms are driven, at least in part, by a self-sustaining cellular mechanism termed the circadian clock. The circadian clock controls different cellular functions, including transcription, translation and protein post-translational modifications (such as phosphorylation, acetylation and ubiquitylation) and degradation. Disruption of the circadian clock in animal models results in the loss of blood pressure control and substantial changes in the circadian pattern of water and electrolyte excretion in the urine. Kidney-specific suppression of the circadian clock in animals implicates both the intrinsic renal and the extrarenal circadian clocks in these pathologies. Alterations in the circadian rhythm of renal functions are associated with the development of hypertension, chronic kidney disease, renal fibrosis and kidney stones. Furthermore, renal circadian clocks might interfere with the pharmacokinetics and/or pharmacodynamics of various drugs and are therefore an important consideration in the treatment of some renal diseases or disorders.

Anthropometric variables, physical activity and dietary intakes of patients with uric acid nephrolithiasis

To evaluate anthropometric variables, energy expenditure by physical activity and nutrient intake of uric acid stone formers (UA-RSFs) compared to non-forming subjects (C). The study included 33 consecutive male patients with a diagnosis of "pure" stones of anhydrous uric acid at infrared spectroscopy and 49 male control subjects with no history of urinary stones. A personal interview was conducted including questionnaires for physical activity and dietary intakes. Anthropometric parametric and blood pressure were measured. Mean age, weight, height, waist circumference, body mass index, systolic and diastolic blood pressure values, dietary energy, carbohydrate intake, lipid intake, dietary acid load, time spent for different physical activities and total energy expenditure for physical activity were not different in UA-RSFs with respect to C. Mean dietary protein (76.2 ± 19.6 vs 65.4 ± 14.7 g/day, P = 0.006) and ethanol intake (10.4 ± 8.8 vs 4.1 ± 8.6, P = 0.002) were higher in UA-RSFs than in C. History of renal disease, heart disease and treatment with thiazides or allopurinol were more frequent and mean serum glucose and triglycerides (104 ± 12 vs 97 ± 11 mg/dl, P = 0.043) (172 ± 77 vs 123 ± 52 mg/dl, P = 0.023) were higher in UA-RSFs. Metabolic syndrome was more frequent in UA-RSFs (57% vs 39%) but not significant (p = 0.09). Increased dietary animal protein (and ethanol) intake can act as co-factors for uric acid stone formation although a more complex and not fully elucidated metabolic background can have an even more crucial role in the pathogenesis of this disease even in the absence of overweight.

Rapid time-resolved luminescence based screening of bacteria in urine with luminescence modulating biosensing phages

Urinary tract infections (UTIs) are a common problem worldwide. The most prevalent causative pathogen of UTI is Escherichia coli, focus of this study. The current golden standard for detecting UTI is bacterial culture, creating a major workload for hospital laboratories - cost-effective and rapid mass screening of patient samples is needed. Here we present an alternative approach to screen patient samples with a single-step assay utilising time-resolved luminescence and luminescence modulating biosensing phages. Filamentous phage M13 was biopanned for binding luminescence quenching metal (copper) and further E. coli. The screening assay luminescence modulation was further enhanced by selecting right chemical environment for the functioning phage clones. Semi-specific interaction between phage, target bacteria and metal was detected by modulation in the signal of a weakly chelating, easily quenchable lanthanide complex. In the presence of the target pathogen, the phages collected quenching metal from solution to the bacterial surface changing the quenching effect on the lanthanide label and thus modulating the signal. Our method was compared with the bacterial culture data obtained from 70 patient samples. The developed proof-of-principle screening assay showed sensitivity and a specificity at the 90% mark when compared to culture method although some samples had high turbidity and even blood. The detection limit of E. coli was in the range of 1000-10 000 colony forming units/mL. Untreated urine sample was screened and time-resolved luminescence signal result was achieved within 10 min in a single incubation step.

Net Acid Excretion and Urinary Organic Anions in Idiopathic Uric Acid Nephrolithiasis

BACKGROUND AND OBJECTIVES

Idiopathic uric acid nephrolithiasis, which is closely associated with obesity and the metabolic syndrome, is increasing in prevalence. Unduly acidic urine pH, the quintessential pathophysiologic feature of this disease, is in part explained by inadequate excretion of the principal urinary buffer ammonium. The role of net acid excretion in the pathogenesis of uric acid nephrolithiasis is incompletely understood.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We compared acid-base parameters of patients with idiopathic uric acid nephrolithiasis with matched control subjects under controlled diets in an inpatient metabolic unit. Measurements included fasting blood and 24-hour urine chemistries and 24-hour urine metabolomic analysis. Comparisons between groups included analysis of covariance models controlling for urine pH or body mass index.

RESULTS

Subjects with idiopathic uric acid nephrolithiasis had lower urine pH (5.5 versus 5.9; P<0.001) and higher net acid excretion (60 versus 43 mEq/24 h; P<0.001), with the excess H+ carried by nonammonium buffers. In all subjects, there was a positive relationship of net acid excretion with higher body mass index in spite of strictly controlled equivalent dietary acid intake. This relationship was most evident among control subjects (r=0.36; P=0.03). It was attenuated in patients with idiopathic uric acid nephrolithiasis whose net acid excretion remained fixedly high and ammonium excretion remained low relative to net acid excretion, resulting in low urine pH over a wide body mass index range. Urinary metabolomics was performed to attempt to identify excess organic acids presented to the kidney in idiopathic uric acid nephrolithiasis. Among the tricarboxylic acid cycle intermediates and amino acid and lipid metabolites analyzed, 26 organic anions with acid dissociation constants values in the range of urine pH showed greater protonation. However, protons carried by the identified organic acids did not entirely account for the higher titratable acidity seen in idiopathic uric acid nephrolithiasis.

CONCLUSIONS

Higher acid load to the kidney, resulting in higher urinary net acid excretion, is an important factor in the pathogenesis of idiopathic uric acid nephrolithiasis.

Rate-controlled nano-layered assembly mechanism of melamine-induced melamine–uric acid stones and its inhibition and elimination methods

The formation of kidney stones induced by melamine is a rate-controlled nano-scale supramolecular layered assembly process.

Comparison of potential dietary and urinary risk factors for ammonium urate nephrolithiasis in two bottlenose dolphin ( Tursiops truncatus) populations

Dietary and urinary risk factors have been implicated in conditions favoring ammonium urate nephrolithiasis in managed dolphins compared with free-ranging dolphins. In this study, urine samples were collected from 16 dolphins (8 cases, 8 controls) from the U.S. Navy Marine Mammal Program for the purposes of assessing changes in urinary biomarkers after a large meal. Urinary biomarkers and nephrolithiasis presence were assessed opportunistically in 15 long-term resident free-ranging dolphins living in Sarasota Bay, Florida. Additionally, the total purine contents of fish commonly consumed by each dolphin population were measured to evaluate potential dietary risk factors. Populations were compared for total dietary purine composition, recently fed status, nephrolithiasis presence, and differences in urinary biochemical, acid-base, and physicochemical parameters via Wilcoxon rank sum analysis and least square means. Managed dolphins had higher urinary pH and ammonium ([Formula: see text]) in both pre- and postprandial conditions and higher urinary uric acid and saturation indices of NH₄U in the postprandial condition compared with free-ranging dolphins ( P < 0.05). The purine content was greater ( P < 0.0001) in the diet consumed by managed dolphins [7 mmol/Mcal metabolizable energy (ME)] than in the free-ranging dolphin diet (4 mmol/Mcal ME). Free-ranging dolphins did not show evidence of nephrolithiasis. Observed differences in urinary biomarkers and dietary purine content in these two dolphin populations suggest a pathophysiologic basis for the role of fish types on the risk of NH₄U stone formation. Future research should investigate fish type and feeding frequency, inhibitors and promoters, and alkalinizing therapy for reducing NH₄U nephrolithiasis in dolphins.

Impact of food quantity and quality on the biochemical risk of renal stone formation

OBJECTIVE

This study evaluated the role of body mass index (BMI) and dietary potential renal acid load (PRAL) with urinary saturation for calcium oxalate (US-CaOx), calcium phosphate (US-CaP) and uric acid (US-UA) in renal stone formers.

MATERIALS AND METHODS

A retrospective analysis was conducted of laboratory data collected on 442 renal stone-forming patients. Demographic information, BMI and 24 h urinary samples were collected from patients on their regular diets. PRAL was calculated as the Load of Acid to Kidney Evaluation (LAKE) score through a short questionnaire.

RESULTS

Urinary risk factors, but also inhibitors of calcium stone formation such as magnesium, tended to increase in relation to BMI (p = .000). Urinary pH (p = .002) and ammonium/sulfate ratio (p = .000) were negatively related to BMI. This resulted in a positive correlation between BMI and US-UA (p = .000), whereas US-CaOx and US-CaP were not influenced by BMI. LAKE score was positively correlated with US-CaOx (p = .022) and US-CaP (p = .000) as a consequence of the inverse relationship between LAKE score and citrate (p = .000). Multiple linear regression analysis identified BMI (p = .009) and male gender (p = .002) as independent predictors of US-UA, and LAKE score (p = .004) and age (p = .001) as independent predictors of US-CaP.

CONCLUSIONS

BMI, which depends on excessive intake of energy from food, is not related to an increased biochemical risk of calcium stone formation, which is more dependent on the renal acid load of the diet. In contrast, obesity is associated with an increased risk of uric acid stone formation due to insulin resistance, impaired ammoniagenesis and low urinary pH.

Optimization of urinary dipstick pH: Are multiple dipstick pH readings reliably comparable to commercial 24-hour urinary pH?

Purpose

Accurate measurement of pH is necessary to guide medical management of nephrolithiasis. Urinary dipsticks offer a convenient method to measure pH, but prior studies have only assessed the accuracy of a single, spot dipstick. Given the known diurnal variation in pH, a single dipstick pH is unlikely to reflect the average daily urinary pH. Our goal was to determine whether multiple dipstick pH readings would be reliably comparable to pH from a 24-hour urine analysis.

Materials and Methods

Kidney stone patients undergoing a 24-hour urine collection were enrolled and took images of dipsticks from their first 3 voids concurrently with the 24-hour collection. Images were sent to and read by a study investigator. The individual and mean pH from the dipsticks were compared to the 24-hour urine pH and considered to be accurate if the dipstick readings were within 0.5 of the 24-hour urine pH. The Bland-Altman test of agreement was used to further compare dipstick pH relative to 24-hour urine pH.

Results

Fifty-nine percent of patients had mean urinary pH values within 0.5 pH units of their 24-hour urine pH. Bland-Altman analysis showed a mean difference between dipstick pH and 24-hour urine pH of -0.22, with an upper limit of agreement of 1.02 (95% confidence interval [CI], 0.45–1.59) and a lower limit of agreement of -1.47 (95% CI, -2.04 to -0.90).

Conclusions

We concluded that urinary dipstick based pH measurement lacks the precision required to guide medical management of nephrolithiasis and physicians should use 24-hour urine analysis to base their metabolic therapy.

Biochemical and dietary factors of uric acid stone formation

The aim of this study was to compare the clinical characteristics of "pure" uric acid renal stone formers (UA-RSFs) with that of mixed uric acid/calcium oxalate stone formers (UC-RSFs) and to identify which urinary and dietary risk factors predispose to their formation. A total of 136 UA-RSFs and 115 UC-RSFs were extracted from our database of renal stone formers. A control group of 60 subjects without history of renal stones was considered for comparison. Data from serum chemistries, 24-h urine collections and 24-h dietary recalls were considered. UA-RSFs had a significantly (p = 0.001) higher body mass index (26.3 ± 3.6 kg/m²) than UC-RSFs, whereas body mass index of UA-RSFs was higher but not significantly than in controls (24.6 ± 4.7) (p = 0.108). The mean urinary pH was significantly lower in UA-RSFs (5.57 ± 0.58) and UC-RSFs (5.71 ± 0.56) compared with controls (5.83 ± 0.29) (p = 0.007). No difference of daily urinary uric acid excretion was observed in the three groups (p = 0.902). Daily urinary calcium excretion was significantly (p = 0.018) higher in UC-RSFs (224 ± 149 mg/day) than UA-RSFs (179 ± 115) whereas no significant difference was observed with controls (181 ± 89). UA-RSFs tend to have a lower uric acid fractional excretion (0.083 ± 0.045% vs 0.107+/-0.165; p = 0.120) and had significantly higher serum uric acid (5.33 ± 1.66 vs 4.78 ± 1.44 mg/dl; p = 0.007) than UC-RSFs. The mean energy, carbohydrate and vitamin C intakes were higher in UA-SFs (1987 ± 683 kcal, 272 ± 91 g, 112 ± 72 mg) and UC-SFs (1836 ± 74 kcal, 265 ± 117, 140 ± 118) with respect to controls (1474 ± 601, 188 ± 84, 76 ± 53) (p = 0.000). UA-RSFs should be differentiated from UC-RSFs as they present lower urinary pH, lower uric acid fractional excretion and higher serum uric acid. On the contrary, patients with UC-RSFs show urinary risk factors more similar to those for calcium oxalate stones. The dietary approach in patients forming uric acid stones should be reconsidered with more attention to the quantity and quality of carbohydrate intake.

Selective screening for distal renal tubular acidosis in recurrent kidney stone formers: initial experience and comparison of the simultaneous furosemide and fludrocortisone test with the short ammonium chloride test

BACKGROUND

Distal renal tubular acidosis (dRTA) is associated with renal stone disease, and it often needs to be considered and excluded in some recurrent calcium kidney stone formers (KSFs). However, a diagnosis of dRTA, especially when 'incomplete', can be missed and needs to be confirmed by a urinary acidification (UA) test. The gold standard reference test is still the short ammonium chloride (NH₄Cl) test, but it is limited by gastrointestinal side effects and occasionally failure to ingest sufficient NH₄Cl. For this reason, the furosemide plus fludrocortisone (F+F) test has been proposed as an easier and better-tolerated screening test. The aim of the present study was to assess the usefulness of the F+F test as a clinical screening tool for dRTA in a renal stone clinic.

METHODS

We studied 124 patients retrospectively in whom incomplete dRTA was suspected: 71 had kidney stones only, 9 had nephrocalcinosis only and 44 had both. A total of 158 UA tests were performed: 124 F+F and 34 NH₄Cl; both tests were completed in 34 patients.

RESULTS

The mean age was 45.4 ± 15 years, and 49% of patients were male. The prevalence of complete and incomplete dRTAs was 7 and 13.7%, respectively. Of the 34 patients tested using both tests, 17 (50%) were abnormal and 4 (12%) were normal. Thirteen (39%) patients were abnormal by F+F, but normal by NH₄Cl [sensitivity 100% (95% CI 80-100), specificity 24% (95% CI 7-50), positive predictive value 57% (95% CI 37-75), negative predictive value 100% (95% CI 40-100)].

CONCLUSIONS

The F+F test is characterized by an excellent sensitivity and negative predictive value, and the diagnosis of incomplete dRTA can be excluded reliably in a patient who acidifies their urine normally with this test. However, its lack of specificity is a drawback, and if there is any doubt, an abnormal F+F test may need to be confirmed by a follow-up NH₄Cl test. Ideally, a prospective blinded study in unselected KSFs is needed to accurately assess the reliability of the F+F test in diagnosing, rather than excluding, dRTA.

Role of insulin resistance in uric acid nephrolithiasis

Metabolic syndrome has been implicated in the pathogenesis of uric acid stones. Although not completely understood, its role is supported by many studies demonstrating increased prevalence of uric acid stones in patients with metabolic syndrome and in particular insulin resistance, a major component of metabolic syndrome. This review presents epidemiologic studies demonstrating the association between metabolic syndrome and nephrolithiasis in general as well as the relationship between insulin resistance and uric acid stone formation, in particular. We also review studies that explore the pathophysiologic relationship between insulin resistance and uric acid nephrolithiasis.

Metabolic disorders: stones as first clinical manifestation of significant diseases

INTRODUCTION

Metabolic syndrome, type 2 diabetes, and primary hyperparathyroidism are metabolic disorders that should deserve a special focus in renal stone patients as a pathogenic link is established with some stone components. Indeed, an acidic urinary pH due to a decreased ammonium bioavailability explains the high prevalence of uric acid stones in patients with metabolic syndrome or diabetes and, primary hyperparathyroidism induced hypercalciuria increases the risk of calcium phosphate stones.

MATERIALS AND METHODS

We report here four clinical cases of renal stone patients with metabolic disorders encountered in a daily practice. Clinical and metabolic findings altogether with stone analysis components presented here, illustrate relevant pathophysiological links.

CONCLUSION

24 hours urine evaluation and stone analysis which includes both morphological typing and infrared spectroscopy, are key diagnostic steps for early recognition of metabolic disorders. Metabolic screening allows diet related stone identification, points out stone risk factors and identifies patients'comorbidity. The occurrence of nephrocalcinosis with or without chronic renal failure should require a more detailed metabolic evaluation in order to identify uncommon etiologies such as renal tubular acidosis.

Pathophysiological and physicochemical basis of ammonium urate stone formation in dolphins

PURPOSE

Nephrolithiasis is increasingly reported in bottle-nosed dolphins. All cases to date have been ammonium urate nephrolithiasis.

MATERIALS AND METHODS

A case-control study was performed in dolphins with and without evidence of nephrolithiasis to identify biomarkers and risk factors associated with stone formation in a managed population. Dolphins were sampled in fasting and postprandial states to study the effect of dietary factors on serum and urinary biochemistry. Urine was continuously collected for 6 hours via catheter and divided into 3, 2-hour collections with a bolus fish meal given after completing the first collection. Blood was sampled at the beginning of the fasting period and the end of the postprandial period.

RESULTS

There were no significant differences in serum and urine chemistry or acid-base profiles between dolphins with vs without stones at baseline or postprandially. This suggests that cases and controls represent a continuum of stone risk. On analysis combining cases and controls in a single cohort we noted significant postprandial increases in urinary uric acid, sulfate and net acid excretion accompanied by increased urinary ammonium excretion and a commensurate increase in urine pH. The supersaturation index of ammonium urate increased more than twofold postprandially.

CONCLUSIONS

These findings suggest that dolphins are susceptible to ammonium urate nephrolithiasis at least in part because a high dietary load of acid and purines results in a transient but marked increase in the urinary supersaturation of the sparingly soluble ammonium urate salt.

Renal ammonium excretion after an acute acid load: blunted response in uric acid stone formers but not in patients with type 2 diabetes

Idiopathic uric acid nephrolithiasis is characterized by elevated urinary net acid excretion and insufficient buffering by ammonium, resulting in excessively acidic urine and titration of the relatively soluble urate anion to insoluble uric acid. Patients with type 2 diabetes have similar changes in urinary pH, net acid excretion, and ammonium in 24-h urine collections at baseline, even after controlling for dietary factors, and are at increased risk for uric acid nephrolithiasis. However, not all patients with type 2 diabetes develop kidney stones, suggesting that uric acid stone formers may have additional urinary defects, perhaps not apparent at baseline. We performed a metabolic study of 14 patients with idiopathic uric acid nephrolithiasis, 13 patients with type 2 diabetes, and 8 healthy control subjects of similar body mass index. After equilibration on a fixed diet for 5 days, subjects were given a single oral acid load (50 meq ammonium chloride), and urine was collected hourly for 4 h. Uric acid stone formers had a lower ammonium excretory response to acute acid loading compared with diabetic and nondiabetic nonstone formers, suggesting that an ammonium excretory defect unique to uric acid stone formers was unmasked by the acid challenge. The Zucker diabetic fatty rat also did not show impaired urinary ammonium excretion in response to acute acid challenge. A blunted renal ammonium excretory response to dietary acid loads may contribute to the pathogenesis of idiopathic uric acid nephrolithiasis.

Demography and biochemistry of 2800 patients from a renal stones clinic

BACKGROUND

Because the causes of stones are uncertain, interventions to prevent recurrence have an insecure foundation. Progress depends on careful evaluation of stone formers.

METHODS

A descriptive retrospective database study of 1983 men and 816 women from the Southampton stones clinic from 1990 to March 2007. Anonymized data from the first attendance were analysed using non-parametric statistical tests.

RESULTS

Sex ratio (2.43:1), age (median 49 y, 2.5th-97.5th percentiles, 23-77 y men, 20-79 y women), recurrent stone formers (30%) and type of stone were similar to other centres. Women more often had a positive family history (24% versus 19% men), previous urinary infection (31% versus 5%) and structural urinary tract abnormality (14% versus 7%); more men had gout (5% versus 1%) and bladder outlet obstruction (3% versus <1%). Calcium, oxalate and uric acid excretion were increased in 43%, 17% and 22% respectively of men and 31%, 7% and 10% of women. Urinary calcium, oxalate and uric acid correlated significantly, r ranging from 0.149 to 0.311 for 24 h excretion and 0.510 to 0.695 for concentrations per litre. Twenty-two percent of men and 8% of women with normal parathyroid hormone had phosphaturia (excretion of phosphate corrected for glomerular filtration rate (TmPO4/GFR) < 0.70 mmol/L); 6% men and 1.6% women also had low plasma phosphate. Many variables correlated significantly but often weakly with age. Creatinine clearance, pH and (men) TmPO4/GFR decreased from 50 y, urine creatinine, calcium and citrate from 60 y.

CONCLUSIONS

Risk factors for stones differ between men and women, change with ageing and in some may have a genetic basis. The role of phosphaturia merits further exploration.

Uric acid stones and hyperuricosuria

Recent work has highlighted the strong relationships among obesity, diabetes, and the metabolic syndrome as causes of low urinary pH. Low urinary pH in turn is the major urinary risk factor for uric acid stones. Unlike calcium stones, uric acid stones can be dissolved and easily prevented with adequate urinary alkalinization. Recognizing the relevant risk factors should lead to increased identification of these radiolucent stones. The cornerstone of therapy is raising urinary pH; xanthine dehydrogenase inhibitors should be used only when urinary alkalinization cannot be achieved.