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Uric acid crystallizes as an anhydrous compound (UAA), a dihydrate (UAD) or a mixture of both. A monohydrate form is very rare. About 20% of uric acid stones contain a significant amount (≥20%) UAD. It is believed that UAD crystallizes under highly acidic conditions (urine pH ≤ 5.0). Up to now, metabolic data on patients with UAD stones have not been reported in the literature. One hundred and fifty patients with pure uric acid calculi were studied. Stone analysis was performed using X-ray diffraction. According to the stone analysis, they were divided in two groups: 1. UAD (≥20% UAD), 2. UAA (<20% UAD). In all patients the following parameters were examined: age, sex, number of recurrences, body mass index (BMI); blood: creatinine, uric acid, calcium, sodium, and potassium; urine: pH-profiles, volume, calcium, uric acid, citrate, ammonia, and urea. Group 1 (≥20% UAD) consisted of33patientsand group 2 (<20% UAD) of 117 patients. Between these groups, there was a significant difference concerning the number of recurrences, the urine volume, and the urinary excretion of calcium. Patients with ≥20% dihydrate had a mean BMI of 31.6 ± 7.5, a mean number of recurrences of 0.24 ± 0.44, an urine volume of 2.6 ± 0.8 l/24 h, and a calcium excretion of 4.5 ± 2.2 mmol/24 h, whereas those with <20% dihydrate had BMI of 29.9 ± 5.0, 1.10 ± 1.42 recurrences, urine volume of 2.3 ± 1.2 l/24 h, and calcium excretion of 3.2 ± 2.4 mmol/24 h. All the other parameters tested were not significantly different. For the first time, our study shows metabolic data in uric acid patients with a significant amount of UAD. The comparison between this group and those patients with <20% UAD revealed that the first group is less prone to develop recurrences. This is a relevant difference concerning the necessity of metaphylactic measures. We could not confirm in patients with dihydrate if the urinary pH is more acid than in those with insignificant amounts of dihydrate. The higher 24-h urine volume, the higher excretion of calcium, and the higher BMI in the UAD group may be of pathophysiological relevance and requires further attention. [ABSTRACT FROM AUTHOR]

An abnormal urinary pH (UpH) represents an important risk factor for nephrolithiasis. In some stone formers, a fasting urine specimen is obtained instead of a 24-h urine collection for stone risk evaluation. We examined the relationship between 24-h and fasting UpH in non-stone forming individuals and stone formers with various etiologies and a wide range of urine pH to test the validity of fasting UpH. Data from 159 subjects was examined in this retrospective study. We included non-stone forming subjects and stone formers with hypercalciuria, distal renal tubular acidosis, idiopathic uric acid nephrolithiasis, or chronic diarrhea. Participants collected a 24-h urine followed by a 2-h fasting urine. For the entire cohort, a significant correlation was seen between fasting and 24-h UpH ( r = 0.49, p < 0.001). Fasting pH was significantly higher than 24-h UpH for the entire cohort (6.02 ± 0.63 vs. 5.89 ± 0.51; p < 0.001), and in the subgroups of non-stone formers and stone formers with hypercalciuria or distal renal tubular acidosis. Fasting UpH was >0.2 pH units different from 24-h UpH in 58% of participants. The difference between fasting and 24-h UpH did not correlate with net gastrointestinal alkali absorption or urine sulfate, suggesting that dietary factors alone cannot explain this difference in UpH. Fasting urine pH correlates moderately with 24-h urine pH in a large cohort of individuals. Significant variability between these two parameters is seen in individual patients, emphasizing the cardinal role of 24-h urine collection for evaluating UpH in nephrolithiasis. [ABSTRACT FROM AUTHOR]

Increase in body size increases the risk of renal stone formation. The mechanism explaining this relationship remains unclear. Urine pH is one of the important factors for urinary stone formation. The purpose of this study was to determine whether there is an association between urine pH and body mass index (BMI) in patients with urolithiasis. Medical charts review that included 342 urinary stone formers (248 men and 94 women). Data obtained included patient sex, age, BMI, urine pH at diagnosis, and stone composition. The patients were classified as normal weight (18.5 ≤ BMI < 24), overweight (24 ≤ BMI < 27), or obese (BMI ≥ 27). The mean urine pH of the normal body weight, overweight, and obese groups was 6.25, 6.14, and 6.00, respectively ( P < 0.05). Urine pH is inversely related to BMI among patients with urolithiasis. Among patients with urolithiasis, higher BMI will have lower urine pH. This may explain why obesity is associated with an increased risk of nephrolithiasis. Weight loss should be explored as a potential treatment to prevent kidney stone formation. The prevention of urinary stone disease gives clinicians an additional reason to encourage weight reduction through diet. [ABSTRACT FROM AUTHOR]

It is reasonable to assume that the rate of pH increase in urine induced by urease-producing microorganisms is one of the factors which determine whether crystallisation with subsequent stone formation will occur or not. To evaluate how the time needed to increase urine pH varies between different urine samples and how it depends on urine composition, a standardised amount of urease was added to different human urine samples. The incubations were performed in a pH-stat. This allowed simultaneous study of how urease enzymatic activity depends on urine pH and how it varies between different urines. The enzymatic activity was found to be negatively correlated to urine pH and to vary between different urines. The rate of the pH increase varied markedly between different urines. Small pH increases depended on the native urine pH and urease enzymatic activity. Higher pH increases up to the levels of phosphate crystallisation depended more on urine phosphate, the major urine buffer. The results presented show that urine composition influences the urease-induced pH increase. This might have clinical implications. [ABSTRACT FROM AUTHOR]