Idiopathic calcium nephrolithiasis with pure calcium oxalate composition: clinical correlates of the calcium oxalate dihydrate/monohydrate (COD/COM) stone ratio

Pure calcium oxalate is the most frequent type of idiopathic kidney stone composition. Fourier transform infrared spectroscopy (FT-IR) allows to detect the ratio of calcium oxalate dihydrate (COD) and monohydrate (COM) crystals in stones, but the clinical significance of this parameter remains uncertain. The objective of this observational study was to verify the association of clinical and laboratory parameters of kidney stone disease with COD/COM ratio in a group of 465 (322 M, age 46 ± 14) patients suffering from idiopathic calcium nephrolithiasis with pure calcium oxalate stones (≥ 97%). Each participant underwent a complete clinical examination, serum chemistry, 24-h urine collection for the determination of the profile of lithogenic risk, and had stones analyzed by FT-IR. Most (62%) of the stones had a COD/COM ratio ≤ 0.25, and the urine chemistry of the corresponding patients showed a low prevalence of urinary metabolic abnormalities. With increasing COD/COM ratio intervals (0–0.25, 0.26–0.50, 0.51–0.75, 0.76–1), a significant association was observed for the number of urological procedures, serum calcium, 24-h urinary calcium excretion, prevalence of hypercalciuria and relative calcium oxalate supersaturation, and a negative trend was detected for the age of the first stone episode (all p values < 0.05). A linear regression model showed that the only parameters significantly associated with COD/COM ratio were 24-h urinary calcium excretion (standardized β = 0.464, p < 0.001) and urine pH (standardized β = 0.103, p = 0.013). In pure calcium oxalate idiopathic stones, COD/COM ratio may reflect the presence of urinary metabolic risk factors, and represent a guide for the prescription of urinary analyses.