Metabolic evaluation of stone disease patients: a practical approach

Korean Society of Endourology and Robotics (KSER) recommendation on the diagnosis, treatment, and prevention of urolithiasis

This article provides evidence-based recommendations and expert opinions to aid urologists in making optimal decisions regarding managing urolithiasis in various clinical scenarios. The most frequently asked questions by urologists in their clinical practice have been collected and answered in the form of FAQs; based on the latest evidence and expert opinions. The natural history of urolithiasis is divided into active treatment and silent phases, with the active treatment stage divided into typical and special situations and peri-treatment management. The authors address 28 key questions, offering practical guidance for the proper diagnosis, treatment, and prevention of urolithiasis in clinical practice. This article is expected to be served as a valuable resource for urologists.

Efficacy of Theobromine and Its Metabolites in Reducing the Risk of Uric Acid Lithiasis

Uric acid lithiasis accounts for about 10% of all types of renal lithiasis. The most common causes of uric acid lithiasis are low urinary pH, followed by high concentration of urinary uric acid, and low diuresis. Treatment of patients consists of alkalinization of urine, reducing the consumption of purine-rich foods, and administration of xanthine oxidase inhibitors, because there are no established therapeutic inhibitors of uric acid crystallization. We recently found that theobromine inhibited uric acid crystallization in vitro, and that the increased urinary level of theobromine following its oral consumption was associated with the prevention of uric acid crystallization. In this study, we evaluated the inhibitory effects of theobromine metabolites and other methylxanthine-related compounds on uric acid crystallization. We also measured the urinary concentrations of theobromine and its metabolites in samples from healthy individuals and patients with uric acid stones and compared the extent of uric acid supersaturation and uric acid crystal formation in these different samples. Theobromine and other methylxanthines that lacked a substituent at position 1 inhibited uric acid crystallization, but other methylxanthines did not have this effect. Individuals with clinical parameters that favored uric acid crystallization did not develop uric acid crystals when theobromine and its metabolites were in the urine at high levels. Thus, theobromine and its metabolites reduced the risk of uric acid lithiasis.

Metabolic evaluation of first-time uncomplicated renal stone formers: A prospective study

Background

Nephrolithiasis is a global health problem. The recurrence rate after the first stone clearance is approximately 50% at 5 years. Metabolic abnormalities are an important factor responsible for stone recurrence. Our prevalidated study aimed to evaluate metabolic abnormalities associated with first-time uncomplicated renal stone formers (FTURSF).

Materials and methods

In this prospective, exploratory, time-bound, descriptive study, 30 first-time renal stone formers were evaluated for metabolic abnormalities. High-risk stone formers were excluded from the study. Data were collected in a predefined proforma, transferred to an Excel sheet, and analyzed using the Statistical Package for Social Sciences 20 and Epi Info 7. Fisher exact test, Mann-Whitney U test, paired t test, and Pearson correlation coefficient were used for statistical analyses.

Results

The mean age of the participants was 35.57 ± 11.07 years, with a male-to-female ratio of 1.72. The most common abnormality was a 24-hour urine volume of <2.5 L in 73.33% of the participants. One or more metabolic abnormalities were detected in 76.67% of the participants. Other common metabolic abnormalities detected were hypocitraturia (60%), hypercalciuria (16.67%), hyperoxaluria (13.33%), and hyperuricosuria (3.33%). Parathyroid adenoma was detected in one participant (3.33%).

Conclusions

Our study documented significant metabolic abnormalities in FTURSF. Therefore, a simplified metabolic evaluation protocol should be adopted while evaluating FTURSF. Detection of an underlying metabolic abnormality would enable the early institution of preventive measures to reduce stone recurrence and related complications.

Validation of a novel diagnostic test for assessing the risk of urinary uric acid crystallization

BACKGROUND AND AIMS

Uric acid (UA) kidney stones account for 10 to 11% of all kidney stones, and this percentage has increased over time. An accurate, rapid, simple, and low-cost test is needed to distinguish urine that is susceptible and resistant to the formation of UA crystals. The aim of this paper is to develop a test to assess the risk for UA crystallization (RUAC) and to validate its utility in routine clinical practice by analysis of urine samples of UA stone formers and healthy volunteers.

PATIENTS AND METHODS

Urine samples of 20 healthy adult volunteers and 54 active formers of UA stones were collected. Three samples were collected from each participant, with at least 7 days between each collection. The main lithogenic parameters for UA stones were determined, and an RUAC test was performed in all urine samples.

RESULTS

Our RUAC test reliably discriminated urine that was resistant and susceptible to the formation of UA crystals. This test had high specificity (94%) and a low percentage of false negatives.

CONCLUSION

The RUAC test described here had high diagnostic accuracy, low-cost, and a rapid assay time, that make this test an attractive screening tool for UA stone fomers follow-up.

Prevention and management of urinary stone

The prevalence of urolithiasis is increasing not only in South Korea but also around the world. Urolithiasis has a high recurrence rate, therefore, reducing it is very important in the quality of life for stone formers. For this purpose, dietary modifications and drug therapy can be performed through stone analysis and 24-hour urine collection. Stone analysis is recommended for all stone formers, and the 24-hour urine collection is usually recommended for recurrent stone formers or high-risk groups. A general dietary modification for all stone formers includes a sufficient fluid intake, low levels of sodium, sugar, and animal protein, a normal calcium diet, as well as a high amount of citrate intake. Drug therapy should be performed in cases such as the recurrence of stones or increase of the existing ones, even after the application of preservation therapy, such as dietary modification. The ideal drug therapy should prevent the occurrence of urolithiasis, have no side effects, and have a suitable patientsʼ compliance. Follow-up should be performed periodically, through 24-hour urine collections and imaging studies. For follow-up imaging studies, a lowdose non-enhanced computed tomography is recommended, and it can be performed once a year if the patient is in a stable state. To control various and complex metabolic abnormalities in recurrent stone formers, multiple approaches may be required through diet modifications, drug therapy, treatment of the metabolic syndrome, and lifestyle modifications.

Chronological Trends in Clinical and Urinary Metabolic Features over 20 Years in Korean Urolithiasis Patients

Urolithiasis is common and is becoming more prevalent worldwide. This study assessed the chronological trends in clinical and urinary metabolic features over 20 years in Korean urolithiasis patients. We performed a retrospective analysis of 4,076 patients treated at our clinic from 1996 to 2015. Urinary metabolic data and stone analysis data were available for 1,421 and 723 patients (34.9% and 17.7%), respectively. Patients were categorized into 4 groups according to the date of initial diagnosis: group 1 (1996-2000, n = 897), group 2 (2001-2005, n = 1,018), group 3 (2006-2010, n = 1,043), and group 4 (2011-2015, n = 1,118). Incidental detection of uric acid renal stones has become more prevalent in the past 10 years, accompanied by an increase in body mass index and age at diagnosis. Similarly, the prevalence of diabetes mellitus and of hypertension increased from one group to the next throughout the study period. Levels of 24-hour urinary excretion of sodium, calcium, uric acid, and oxalate have decreased significantly over the study period. The incidence of urinary metabolic abnormalities also showed an identical tendency. The proportion of stones composed of uric acid increased over the study period. In conclusion, incidental detection of uric acid renal stones has become more prevalent in Korea in the past 20 years. Urinary excretion of lithogenic constituents and the incidence of urinary metabolic abnormalities have decreased significantly over this period.

Effect of renal insufficiency on stone recurrence in patients with urolithiasis

The study was designed to assess the relationship between glomerular filtration rate (GFR) and urinary stone-forming constituents, and to assess the effect of renal insufficiency on stone recurrence risk in first stone formers (SF). Baseline serum creatinine levels were obtained, and renal insufficiency was defined as creatinine clearance ≤60 mL/min (Cockroft-Gault). This retrospective case-control study consists of 342 first SF; 171 SF with normal renal function were selected with 1:1 propensity scores matched to 171 SF with renal insufficiency. Urinary metabolic evaluation was compared to renal function. GFR was positively correlated with urinary calcium, uric acid, and citrate excretion. Subjects with renal insufficiency had significantly lower urinary calcium, uric acid, and citrate excretion than those with normal renal function, but not urine volume. With regard to urinary metabolic abnormalities, similar results were obtained. SF with renal insufficiency had lower calcium oxalate supersaturation indexes and stone recurrence rates than SF with normal renal function. Kaplan-Meier curves showed similar results. In conclusion, GFR correlates positively with urinary excretion of stone-forming constituents in SF. This finding implies that renal insufficiency is not a risk factor for stone recurrence.

Distinct metabolic characteristics and risk of stone recurrence in patients with multiple stones at the first-time presentation

OBJECTIVE

To characterize the clinical and metabolic abnormalities of patients presenting with multiple stones and determine their risk of new stone formation.

MATERIALS AND METHODS

This retrospective case-controlled study consisted of 911 patients who had ureter stones for the first time and 107 age- and sex-matched patients without stones. The patients were classified into 2 groups: those with a single ureter stone (n = 690) and those with 1 or more additional stones somewhere in the ureter or kidney (n = 221). All patients underwent 24-hour urinary metabolic evaluation. The 240 patients (26.3%) who were followed for >12 months (median follow-up, 35.0 months) were included in recurrence analyses. Stone recurrence was defined as "new stone formation," namely, the radiographic appearance of stones that had not been present in previous examinations.

RESULTS

The multiple-stone group had significantly lower urinary citrate excretion than the single-stone (P = .011) and control (P = .003) groups. Compared with the single-stone group, it also had a higher incidence of hypocitraturia (P = .011) and stone recurrence (27 of 84 [32.1%] vs 29 of 156 [18.6%] patients; P = .025). Multivariate Cox regression analyses revealed that stone multiplicity (hazard ratio, 2.343; 95% confidence interval, 1.302-4.220; P = .005) was an independent predictor of recurrent stone formation. Kaplan-Meier curves showed identical results.

CONCLUSION

The patients with multiple stones had distinct metabolic characteristics, particularly hypocitraturia and a significantly higher risk of recurrence than patients with 1 stone. Patients with multiple stones, even if it is their first stone episode, should undergo metabolic evaluation and possibly also potassium citrate therapy to prevent future stones.

A comparative study of experimental rat models of renal calcium oxalate stone formation

PURPOSE

The aim of this study was to design a simple and convenient rat model with significant calcium oxalate crystal deposition in the kidney.

MATERIALS AND METHODS

Sprague-Dawley rats were divided into seven groups of 30 rats each. One group of rats was untreated to serve as controls. Two of the groups of rats were provided with drinking water supplemented with 1% ethylene glycol (EG), and of these two groups, one group was also administered vitamin D. Two groups of rats received intraperitoneal injections of EG, and of these two groups, one group also received vitamin D. Two groups of rats received intraperitoneal injections of glyoxylate, and of these two groups, one group also received vitamin D. We analyzed 24-hour urine samples for urinary constituents for all experimental groups, including calcium, oxalate, citrate, uric acid, pH, and urine volume. The kidneys were examined for crystal deposition using histologic examination and for osteopontin expression using immunohistochemical staining.

RESULTS

Calcium-oxalate crystals were found in all rats injected with glyoxylate after 1 week. The degree of crystal deposition in rats injected with glyoxylate for 1 week was significantly increased compared with rats fed EG for 4 weeks.

CONCLUSIONS

Intraperitoneal injection of glyoxylate is a faster, more exact, and more reliable method to produce calcium oxalate crystal deposition in the kidney than previous urolithiasis animal models.

Adequacy of a single 24-hour urine collection for metabolic evaluation of recurrent nephrolithiasis

PURPOSE

There is much debate about whether 1 or 2, 24-hour urinalyses are adequate for metabolic evaluation of stone formers. We determined whether repeat 24-hour urine collection provides information similar to that of the initial 24-hour urine collection and whether repeat collection is necessary.

MATERIALS AND METHODS

We analyzed 2, 24-hour urine collections in 777 patients obtained from 2001 to 2005. Samples were collected 3 days or less apart before pharmacological intervention and analyzed elsewhere for routine stone risk profiles of urine calcium, oxalate, citrate, uric acid, sodium, potassium, magnesium, phosphorus, ammonium, chloride, urea nitrogen and creatinine.

RESULTS

No parameters showed a statistically significant difference between 24-hour urine samples 1 and 2 when mean values were compared (pairwise t test each p >0.05, range 0.06 to 0.87). Using Pearson's correlation all parameters showed positive correlation coefficients (r = 0.68 to 0.89, each p <0.0001). The mean of individual patient differences in samples 1 and 2 were compared to 0 and 6 of 12 showed no difference (p >0.05) while for the remaining 6 p value was <0.05. The percent difference was 0.5% to 4.19% for all urinary parameters.

CONCLUSIONS

One 24-hour urine sample is sufficient for metabolic evaluation of recurrent stone disease. There is no significant difference in 12 urinary parameters between 24-hour urine samples collected within 3 days of each other. This information is useful to providers and may decrease patient inconvenience and the overall cost of metabolic stone evaluation.

Phosphaturia as a promising predictor of recurrent stone formation in patients with urolithiasis

Purpose

Recent studies have suggested that renal phosphate leakage and the associated phosphaturia are significant underlying causes of calcium urolithiasis. The aims of this study were to assess whether phosphaturia relates to urinary metabolic abnormalities and recurrent stone formation.

Materials and Methods

A database of patient histories and urine chemistries was analyzed for 1,068 consecutive stone formers (SFs) and 106 normal controls. Urine values for phosphaturia that were higher than 95% of the normal control values were defined as indicating hyperphosphaturia, and the effect of phosphaturia on urinary metabolites and stone recurrence was determined. Of these patients, 247 patients (23.1%) who had been followed up for more than 36 months or had a recurrence of stones during follow-up (median, 46.0 months; range, 5-151) were included in the analyses for stone recurrence.

Results

Of the SFs, 19.9% (212/1,068) had increased urinary phosphate excretion. SFs with hyperphosphaturia had a greater urinary volume and higher levels of calcium, uric acid, oxalate, and citrate than did SFs with normophosphaturia. A multivariate Cox regression model, stratified by stone episodes, revealed that hyperphosphaturia was an independent predictor of recurrent stone formation in first-time SFs (hazard ratio [HR]: 2.122; 95% confidence interval [CI]: 1.100-4.097; p=0.025). No association was detected between hyperphosphaturia and recurrent stone formation in recurrent SFs. Kaplan-Meier curves showed identical results.

Conclusions

This study demonstrates that hyperphosphaturia is closely associated with urinary metabolic abnormalities. Furthermore, hyperphosphaturia is a significant risk factor for stone recurrence in first-time SFs.

Comparison of Metabolic Risk Factors in Urolithiasis Patients according to Family History

Purpose

Urolithiasis develops more frequently in patients with a family history (FHx). However, little is known about risk factors in stone formers with a FHx. The aim of this study was to examine the clinico-metabolic characteristics of urinary stone formers according to FHx.

Materials and Methods

A database of 1,068 stone formers who underwent a complete metabolic evaluation was reviewed. The patients were divided into two groups on the basis of the presence of a FHx. Clinical factors and metabolic parameters were compared between the two groups.

Results

There were no significant differences in clinical characteristics, such as gender, age, body mass index, stone episodes, or multiple stones, between the two groups (p>0.05, respectively). Compared with stone formers without a FHx, however, serum calcium concentrations were more elevated in stone formers with a FHx. Also, the urinary excretion of calcium was higher in stone formers with a FHx than in those without a FHx. Other urinary metabolites showed no significant differences between the two groups (p>0.05, respectively).

Conclusions

Our study revealed that stone formers with a FHx had increased urinary calcium excretion as well as elevated concentrations of serum calcium. This finding suggests that urolithiasis in stone formers with a FHx may be associated with calcium metabolic abnormalities.

Impact of obesity in patients with urolithiasis and its prognostic usefulness in stone recurrence

PURPOSE

Recent studies have suggested an increased prevalence of urolithiasis and recurrence associated with obesity. We assessed the influence of obesity on stone risk factors as well as on stone recurrence.

MATERIALS AND METHODS

A database of patient history, body mass index, and serum and urine chemistry was analyzed for 704 consecutive stone formers (467 first time stone formers and 247 recurrent stone formers). Obesity was defined as body mass index greater than 25 kg/m(2). The effect of obesity on stone risk factors and recurrence were stratified according to stone episodes. Of these, 163 (23.2%) patients who had been followed for more than 36 months (median 54, range 5 to 148) were included in recurrence analysis.

RESULTS

Obesity was significantly associated with stone episodes (p = 0.043). Obese stone formers excreted increased amounts of sodium, calcium, uric acid and citrate, while the urinary pH in a 24-hour urine sample was decreased compared to nonobese stone formers (p <0.05, respectively). Stone analysis revealed that uric acid stone was significantly more commonly found in the obese patients (p = 0.046). Multivariate Cox regression model stratified by stone episodes revealed that obesity (HR 2.572, 95% CI 1.376-4.807, p = 0.003) was the only strong predictor of stone recurrence in first time stone formers. No association between obesity and stone recurrence was detected in recurrent stone formers. Kaplan-Meier curves showed identical results.

CONCLUSIONS

This study reveals that obesity is associated with metabolic alterations and urinary stone recurrence. Weight control may be considered one of the preventive modalities against recurrent stone formation, especially in first time stone formers.

Changing composition of renal calculi in patients with neurogenic bladder

PURPOSE

Renal calculi are a significant source of morbidity for patients with neurogenic bladder. Calculi from patients with NB have traditionally been composed primarily of struvite and carbonate apatite secondary to chronic urea-splitting bacteriuria. In the current era there have been great improvements in the urological rehabilitation of patients with NB. We defined the composition of renal calculi in a contemporary cohort of patients with NB due to spinal cord injury or myelomeningocele who underwent percutaneous nephrolithotomy.

MATERIALS AND METHODS

We performed a retrospective evaluation of all patients with NB due to SCI or MM who underwent PNL between January 2002 and January 2005.

RESULTS

A total of 32 patients with NB (14 with SCI, 18 with MM) underwent PNL in this period. Stones were infectious in etiology in 37.5% (12 struvite/carbonate apatite) and metabolic in 62.5% (1 uric acid, 2 calcium oxalate monohydrate, 2 brushite, 6 hydroxyapatite, 9 mixed hydroxyapatite/calcium oxalate). All patients with struvite calculi were infected with urea-splitting bacteria on preoperative urine culture.

CONCLUSIONS

Patients with neurogenic bladder are traditionally thought to harbor infection related calculi. These data demonstrate that many contemporary patients will be found to have calculi of a metabolic etiology. Although patients with NB still have renal calculi, advances in urological treatment may have affected the composition of their calculi, as metabolic stones are becoming more commonly identified. When metabolic components are identified, stone activity may be attenuated with appropriate metabolic evaluation, pharmacological therapies and dietary modifications.

Metabolic evaluation and therapy

The recurrent formation of stones in the urinary tract is a well-recognized clinical problem. Unfortunately, progress in the medical care of these patients has not paralleled that of the surgical care. There are, however, several steps that can be taken to reduce the recurrence rate. In this regard it is desirable to identify patients at risk and to find risk factors that need to be eliminated. The extent of biochemical evaluation and the form of stone preventive treatment in patients with calcium stone formation is presently a matter of debate. This review focuses on some current aspects on the analysis of risk factors for calcium stone formation. The present situation on recurrence prevention using dietary and pharmacological measures is also summarized.