Clinical and biochemical differences in patients with pure calcium oxalate monohydrate and calcium oxalate dihydrate kidney stones.

To examine the factors and patient characteristics predisposing to formation of calcium oxalate monohydrate or calcium oxalate dihydrate kidney stones, we compared blood and 24-hour urine tests, gender distribution and patient age in 2 groups of patients with pure calcium oxalate monohydrate (422) and calcium oxalate dihydrate (68) stones treated at the lithotripsy unit of the Wellesley Hospital and University of Toronto during 4 years. The calcium oxalate monohydrate group included relatively more women (31% versus 16% in the calcium oxalate dihydrate group, chi-square 7.89, p = 0.005). Patients were older in the calcium oxalate monohydrate group (59 +/- 13 versus 51 +/- 13 years, p = 0.001). The calcium oxalate monohydrate group had lower urinary calcium (4.19 +/- 2.34 versus 7.19 +/- 3.38 mmol. per day, p < 0.0001), calcium oxalate relative saturation rate (6.9 +/- 3.9 versus 8.9 +/- 3.3, p = 0.001), brushite relative saturation rate (0.7 +/- 0.8 versus 1.2 +/- 0.9, p = 0.0001) and urinary pH (5.72 +/- 0.75 versus 5.93 +/- 0.72). When corrected for patient age and gender, the calcium oxalate dihydrate group still had higher urine calcium levels. Higher urine pH in the calcium oxalate dihydrate group was age-related. In summary, we present evidence that calcium oxalate dihydrate stones are relatively more common among younger male patients with higher urine calcium levels and higher urine pH.