Hypocalciuria as a Predictor of Reduced Intestinal Calcium Absorption

Impaired intestinal calcium absorption contributes to osteoporosis, but its measurement is limited to research settings. We hypothesized that 24-hour urine calcium (24HUC) values could diagnose low fractional calcium absorption (FCA). We performed a post hoc analysis of clinical trial data to determine whether 24HUC predicted low FCA compared with the gold standard dual calcium isotope method. Two hundred thirty postmenopausal women <75 years old without osteoporosis underwent 445 FCA measurements using calcium isotopes (8 mg of oral ⁴⁴Ca, 3 mg of intravenous ⁴²Ca) and a 24-hour inpatient urine collection at 0 and 12 months. We determined subjects' total calcium intake via review of food diaries and supplements. Net calcium absorption (NCA) was total calcium intake × FCA. NCA and 24HUC values demonstrated a positive correlation (r = 0.34; 95% confidence interval, 0.25 to 0.42; P < 0.001). We calculated sensitivity, specificity, positive (PPV) and negative predictive value (NPV) for the ability of 24HUC thresholds to predict calcium malabsorption. When low calcium absorption was defined as <120 mg/d, a 24HUC value <150 mg demonstrated 65% sensitivity, 67% specificity, 31% PPV, and 89% NPV. When calcium malabsorption was defined as <100 mg/d, a 24HUC value <150 mg demonstrated 72% sensitivity, 65% specificity, 22% PPV, and 94% NPV. A 24HUC value <150 mg demonstrated a high NPV for calcium malabsorption. We suggest that 24HUC levels can exclude calcium malabsorption in postmenopausal women.