Urinary supersaturation of calcium oxalate and phosphate in patients with X-linked hypophosphatemic rickets and in healthy schoolchildren

Abstract

Nephrocalcinosis (NC) is a complication of the treatment of X-linked hypophosphatemic rickets (XLHR). Some studies have found that treated patients have enteric hyperoxaluria caused by phosphate therapy and have implicated calcium oxalate, whereas others have found only calcium phosphate in renal biopsy tissue. Aim and methods: We aimed to study the urinary supersaturation of calcium oxalate and calcium phosphate and to determine whether these measures are risk factors for NC. We collected 24-hour urine samples from 20 patients (12 girls) with XLHR, mean ± SD age 8.2 ± 4.7 years, and from 79 age-matched members of a healthy control group prospectively. Results: The median 24-hour urine excretions of oxalate, phosphate, and citrate (mmol/1.73 m 2 per day) were significantly increased in patients compared with the control group (oxalate 0.38 vs 0.28, P = .0012; phosphate 63.1 vs 25.8, P < .0001; citrate 4.18 vs 2.7, P = .0002). However, no significant differences were seen in the calcium oxalate or calcium phosphate between patients and the control group. No significant differences were seen in 24-hour urine calcium or magnesium excretion between patients and the control group; however, 8 patients had hypercalciuria. A significant higher urine volume in patients compared with the normal group (826 mL/m 2 24-hour vs 597 mL/m 2 24-hour; P < .005) was found. Twelve patients had NC at the time of investigation, and although the oxalate excretion was significantly higher in these patients, no significant difference was seen in the relative supersaturation of calcium oxalate monohydrate (CaC 2O 4·H 2O) compared with the 8 without NC. Conclusions: Although 24-hour urine oxalate and phosphate excretion are increased in treated patients with XLHR, there is no increase in the supersaturation of either calcium oxalate or phosphate. Determination of the supersaturation of calcium oxalate or calcium phosphate does not predict the development of NC in XLHR. (J Pediatr 1999;135:611-7)