Comparing 24 h Urine and Spot Urine Calcium Measurements in Clinical Routine: Accuracy and Limitations

Background/Objectives: Urinary calcium excretion is a key parameter in assessing mineral metabolism and diagnosing conditions such as nephrolithiasis, osteoporosis, and hyperparathyroidism. The 24 h urine collection is the gold standard for evaluating calcium excretion, but it is often impractical due to patient non-compliance and logistical challenges. As an alternative, the calcium-to-creatinine ratio (CCR) in spot urine has been proposed, although its reliability remains debated. This study aims to systematically compare the calcium levels in spot urine samples with those obtained from 24 h urine collections to assess their agreement and clinical applicability. Methods: This retrospective, multi-center study analyzed data from 201 patients who provided both 24 h and spot urine samples during routine diagnostic work-up between 1 January 2019 and 31 December 2024. Calcium excretion was normalized using the calcium-to-creatinine ratio (CCR). The agreement between the two methods was assessed using Bland–Altman analysis, Pearson and Spearman correlation coefficients, and receiver operating characteristic (ROC) curve analysis. Results: Hypercalciuria, defined as ≥6.25 mmol/24 h in women and ≥7.5 mmol/24 h in men, was detected in 52.7% of cases based on 24 h urine. ROC analysis showed that spot urine CCR had moderate diagnostic accuracy (AUC = 0.76). The optimal cut-off for predicting hypercalciuria was 4.4 mmol/g (sensitivity 70.8%, specificity 72.4%). Overall agreement between spot urine CCR and 24 h urine CCR was moderate, with a Bland–Altman geometric mean ratio of 1.06 and multiplicative limits of agreement of 0.59 to 1.91. A low spot urine CCR below 2 mmol/g showed high sensitivity but low specificity and had a negative predictive value of 82%. Conclusions: Spot urine CCR cannot replace 24 h urine collection for accurately assessing urinary calcium excretion, but very low values may have limited utility as an initial rule-out tool in selected patients. Very low spot urine CCR values may help rule out hypercalciuria in a limited subgroup of patients and may therefore support triage decisions in selected clinical situations. Further prospective studies are needed to validate these findings.