Prediction of serum ionized calcium concentration by use of serum total calcium concentration in dogs

OBJECTIVE: To determine whether total serum calcium (tCa) or adjusted tCa concentrations accurately predict ionized calcium (iCa) status in dogs. SAMPLE POPULATION: 1,633 canine serum samples. PROCEDURE: The tCa concentration was adjusted for total protein (TP) or albumin concentration by use of published equations. Correlations between iCa and tCa or adjusted tCa, tCa and TP, and tCa and albumin were calculated. Diagnostic discordance between tCa or adjusted tCa and iCa was determined. Diagnostic discordance in predicting iCa was also determined for 490 dogs with chronic renal failure (CRF). Sensitivity, specificity, positive and negative predictive values, and positive and negative diagnostic likelihood ratios were calculated for tCa, tCa adjusted forTP, and tCa adjusted for albumin. RESULTS: Diagnostic discordance was 27% when tCa concentration was used to predict iCa status. Use of adjusted tCa increased diagnostic discordance to approximately 37% for all dogs and 55% for dogs with CRF. Positive predictive value and positive diagnostic likelihood ratios were poor when tCa concentration was used to predict iCa status. The tCa concentration overestimated normocalcemia and underestimated hypocalcemia. Adjusted tCa overestimated hypercalcemia and underestimated hypocalcemia. CONCLUSIONS AND CLINICAL RELEVANCE: Adjusted tCa or tCa concentrations are unacceptable for predicting iCa status in dogs. Use of adjustment equations is not recommended. Direct measurement of iCa concentration is necessary for accurate assessment of calcium status. Use of tCa or adjusted tCa concentrations to predict iCa status in dogs could cause serious mistakes in diagnosis and case management, especially in dogs with CRF.