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

Nephrocalcinosis (NC) is a complication of the treatment of X-linked hypoph osphatemic rickets (XLHR). Some stud;es have found that treated patients ha ve enteric hyperoxaluria caused by phosphate therapy and have implicated ca lcium oxalate, whereas others have found only calcium phosphate in renal bi opsy tissue. Aim and methods: We aimed to study the urinary supersaturation of calcium o xalate and calcium phosphate and to determine whether these measures are ri sk factors for NC. We collected 24-hour urine samples from 20 patients (12 girls) with XLHR, mean +/- SD age 8.2 +/- 4.7years, and from 79 age-matched members of a healthy control group prospectively. Results: The median 24-hour urine excretions of oxalate, phosphate, and cit rate (mmol/1.73 m(2) per day) were significantly increased in patients comp ared 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 v olume in patients compared with the normal group (826 mL/m(2) 24-hour vs 59 7 mL/m2 24-hour; P <.005) was found. Twelve patients had NC at the time of investigation and although the oxalate excretion was significantly higher i n these patients, no significant difference was seen in the relative supers aturation of calcium oxalate monohydrate (CaC2O4. H2O) compared with the 8 without NC. Conclusions: Although 24-hour urine oxalate and phosphate excretion are inc reased in treated patients with XLHR, there is no increase in the supersatu ration of either calcium oxalate or, phosphate. Determination of the supers aturation of calcium oxalate or calcium phosphate does not predict the deve lopment of NC in XLHR.