The urinary calcium–magnesium product as a potential indicator of nephrolithiasis in primary hyperparathyroidism

Nephrolithiasis is a common complication of primary hyperparathyroidism (PHPT), but the mechanisms underlying stone formation remain incompletely understood. Calcium-sensing receptor (CaSR) activity, indirectly assessed by serum calcitriol levels and urinary excretion of calcium and magnesium, may influence the risk of nephrolithiasis. Current diagnostic methods are cumbersome, prompting the need for more practical biomarkers. This study aimed to evaluate a novel parameter - calcium and magnesium fractional excretion (CAMFE) - as a predictor of nephrolithiasis risk in patients with PHPT. A retrospective analysis was conducted on 109 patients with PHPT. CAMFE was calculated from 24-hour urine collection under a standard diet. Associations with nephrolithiasis were analyzed using logistic regression and receiver operating characteristic (ROC) curve analysis. Nephrolithiasis was present in 40% of patients. The CAMFE index was correlated significantly with kidney stone formation. Calcitriol levels were higher in stone formers, supporting its role in enhanced intestinal calcium absorption. CAMFE showed good predictive power with an optimal cut-off value of 6.18, offering a simpler alternative to dual urine collection protocols. Low CAMFE (< 6.18) may be connected with a higher risk of nephrolithiasis, potentially serving as a useful marker for assessing the risk of renal complications in patients with PHPT.