Determination of a serum total calcium concentration threshold for accurate prediction of ionized hypercalcemia in dogs with and without hyperphosphatemia

Abstract

Background: Total serum calcium (tCa) concentrations are poorly predictive of ionized calcium (iCa) status in dogs. Hypothesis: There is an optimal threshold of tCa concentration that is highly predictive of ionized hypercalcemia and this threshold is higher in hyperphosphatemic dogs as compared to nonhyperphosphatemic dogs. Animals: Nonhyperphosphatemic (n = 1593) and hyperphosphatemic (n = 250) adult dogs. Methods: Retrospective medical record review of paired tCa and iCa concentration measurements in dogs presented to a university teaching hospital over a 5-year period. Positive and negative predictive values, sensitivity, and specificity were calculated for tCa concentration thresholds of 11.0-15.0 mg/dL (upper limit of laboratory reference interval = 11.5 mg/dL) in nonhyperphosphatemic and hyperphosphatemic groups. Results: In nonhyperphosphatemic dogs, an optimal tCa concentration threshold of 12.0 mg/dL resulted in a positive predictive value of 93% (95% confidence interval [CI], 84%-98%) and sensitivity of 52% (95% CI, 43%-61%) for ionized hypercalcemia. An optimal tCa concentration threshold was not identified for hyperphosphatemic dogs. The nonhyperphosphatemic dogs had a higher prevalence of ionized hypercalcemia than the hyperphosphatemic dogs (7 versus 3%, P =.04) and a lower prevalence of ionized hypocalcemia (23 versus 62%, respectively; P