Random Urinary Calcium/Creatinine Ratio as a Predictor of Dehydration in Hypercalcemic and Normocalcemic Patients

Background/Objectives: Dehydration is an under-recognized cause of hypercalcemia, and no validated bedside marker exists to distinguish it from other etiologies. We evaluated the urinary calcium-to-creatinine ratio (uCa/Cr) and the fractional excretion of calcium (FeCa) as predictors of dehydration across a spectrum of hypercalcemic and normocalcemic states. Methods: We conducted a retrospective cohort study of 96 patients admitted to an internal medicine ward with dehydration and/or hypercalcemia. Dehydration was identified using a combination of clinical criteria (presenting complaint, physical examination findings of volume depletion, and admitting physician assessment) and biochemical criteria (elevated plasma urea, creatinine, or sodium, or elevated urine osmolality). Patients were classified into three groups: non-dehydrated hypercalcemic (n = 37), dehydrated hypercalcemic (n = 24), and dehydrated normocalcemic (n = 35). Urinary analyses were performed on patients not receiving chronic diuretic therapy (n = 70 with complete data). Logistic regression, receiver operating characteristic (ROC) analysis, and CKD-stratified sensitivity analyses were performed. Results: uCa/Cr differed significantly across all three groups (medians 1.046, 0.484, and 0.056 mmol/mmol respectively; p < 0.001 for all pairwise comparisons). Hypocalciuria-defined as reduced urinary calcium excretion (uCa/Cr < 0.2 mmol/mmol)-was absent in non-dehydrated patients and present in 59.5% of dehydrated patients; this should not be confused with hypocalcemia (low serum calcium). ROC analysis yielded an AUC of 0.894 (95% CI: 0.819–0.969), significantly superior to FeCa (AUC 0.813; DeLong p = 0.012). At a threshold of 0.2 mmol/mmol, specificity and positive predictive value were both 100%. uCa/Cr remained independently associated with dehydration after adjustment for chronic kidney disease (CKD), age, and sex (OR 0.112, 95% CI: 0.030–0.307, p < 0.001). PTH suppression was significantly less frequent in dehydrated than non-dehydrated patients (24.4% vs. 51.5%, p = 0.018). Conclusions: Random uCa/Cr is a highly specific, CKD-independent predictor of dehydration. A uCa/Cr value below 0.2 mmol/mmol, particularly combined with non-suppressed PTH, should prompt consideration of dehydration as the primary cause of hypercalcemia.