Serum total calcium (tCa) exists in 3 fractions: protein-bound (primarily albumin), ionized (iCa), and complexed. iCa is the most clinically relevant fraction of tCa for evaluating calcium homeostasis; however, methods for measuring iCa in clinical practice are not always readily available. Although formulas to adjust tCa to correct for serum protein concentration have been evaluated, these formulas are not designed for and do not predict iCa in the general population. These formulas were hypothesized to be predictive of iCa in patients with hypoalbuminemia but not hyperphosphatemia, which can affect the complexed fraction of calcium. A retrospective review of 262 dogs with serum albumin concentration ≤2.5 g/L and serum phosphorus concentration ≤5 mg/dL calculated adjusted calcium concentration (aCa) using the formula aCa = tCa (mg/dL) – serum albumin concentration (g/dL) + 3.5 (g/dL). Results demonstrated that low aCa was useful for accurate detection of ionized hypocalcemia in this population of dogs.
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