Biochemical profile of idiopathic uric acid nephrolithiasis

Background. The objective of this study was to elucidate a biochemical prof ile of patients with idiopathic uric acid nephrolithiasis, without secondar y causes (such as dehydration or diarrhea). Study subjects comprised 56 pat ients with idiopathic uric acid nephrolithiasis (UA stone group) who underw ent a full outpatient evaluation. The control group was composed of 54 with absorptive hypercalciuria and 2 normal subjects, matched with the UA stone group according to age, body mass index, and gender. Methods. Urinary pH and ammonium and serum and urinary uric acid were measu red. The fractional excretion of urate was calculated. Results. Compared with the control group, the UA stone group had a signific antly higher serum uric acid and significantly lower urinary uric acid, pH (5.34 +/- 0.23 vs. 6.17 +/- 0.36, P < 0.001), and fractional excretion of u rate (0.052 +/- 0.028 vs. 0.080 +/- 0.029, P < 0.001), but individual value s overlapped considerably between the two groups. Discriminant analysis of the relationship between urinary pH and fractional excretion of urate yield ed a "discriminant score," which provided a much better separation between the two groups, with a correct classification in 95.5% of subjects. In cont rast, urinary ammonium, citrate, sulfate, and potassium did not differ betw een two groups. Conclusions. In idiopathic uric acid nephrolithiasis, urinary pH and fracti onal excretion of urate are significantly lower than in control subjects, s uggestive of defects in urinary acidification and urate excretion. Since th ese impairments are believed to be associated with primary gout, the underl ying disturbance in idiopathic uric acid nephrolithiasis may be primary gou t.