Discriminatory performance of total calcium for ionized hypocalcemia and factors affecting ionized calcium in Assaf ewes during the pre- and post-lambing periods.

BACKGROUND: Total calcium concentration (tCa) poorly reflects ionized hypocalcemia across species, and multiple factors influence ionized calcium concentration (iCa2+). These relationships have not been evaluated in Assaf ewes. HYPOTHESIS/OBJECTIVES: Assess the diagnostic accuracy of tCa for detecting hypocalcemia and identify factors associated with iCa2+ during pre- and post-lambing periods. ANIMALS: A total of 105 clinically healthy multiparous Assaf ewes from a commercial flock. METHODS: Prospective observational study. Venous blood samples were collected during pre- and post-lambing periods. Blood gas-electrolytes (pH, pCO₂, pO₂, HCO₃-, TCO₂, Na+, K+, iCa2+) and biochemical analyses (total protein, albumin, tCa, magnesium, phosphorus, β-hydroxybutyrate [BHB], glucose, alkaline phosphatase) were performed. Discriminatory performance of tCa was evaluated using area under the receiver operating characteristic curve (AUC) analysis. Variables associated with iCa2+ were assessed using multiple linear regression. RESULTS: During the pre-lambing period, tCa showed moderate diagnostic performance (AUC, 0.73; 95% CI, 0.57-0.87; sensitivity, 67%; specificity, 80%) at a cutoff of ≤ 2.42 mmol/L, whereas performance was poor for the post-lambing period (AUC, 0.59; 95% CI, 0.47-0.70; sensitivity, 97%; specificity, 31%) at ≤ 2.77 mmol/L. In multiple regression, tCa, BHB, pCO₂, and pO₂ were associated with iCa2+ during pre-lambing (adjusted R2 = 0.27, P < .001), whereas tCa, BHB, phosphorus, albumin, and Na+ were associated with the post-lambing period (adjusted R2 = 0.25, P < .001). CONCLUSIONS AND CLINICAL IMPORTANCE: iCa2+ varied by production stage and was influenced by several physiologic variables. Direct measurement of iCa2+ should be prioritized to improve the accuracy of hypocalcemia assessment in Assaf ewes.