Clinical significance of uric acid dihydrate in urinary stones.

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 of 33 patients and 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.