Effects of pH and the plasma or serum concentrations of total calcium, chloride, magnesium, l‐lactate, and albumin on the plasma ionized calcium concentration in calves

BACKGROUND: The plasma ionized calcium concentration (cCa(2+)) represents the biologically active form of calcium and is the preferred method for evaluating calcium status in animals. Different pH‐corrective equations have been developed for human plasma, but the validity of the equations for bovine plasma is unknown. HYPOTHESIS: We hypothesized that pH‐corrective equations for bovine plasma would be similar to those used for human plasma; cCa(2+) was dependent on the plasma concentrations of total calcium (cTCa), chloride (cCl), L‐lactate (cLactate), and albumin (cAlbumin); and the in vitro and in vivo cCa(2+)‐pH relationships would differ. ANIMALS: Ten healthy calves (in vitro study), 1426 critically ill calves. METHODS: The in vitro plasma log(10)(cCa(2+))‐pH relationship was determined by CO(2) tonometry of 465 plasma samples. Plasma cCl was altered by equivolume dilution of plasma with 3 electrolyte solutions of different cCl. The in vivo plasma cCa(2+)‐pH relationship was investigated and validated using clinicopathologic data extracted from the medical records of 950 (model development) and 476 (model validation) critically ill calves. RESULTS: pH‐corrective equations for bovine plasma were similar to those used for human plasma. Plasma cCa(2+) increased in vitro with increases in plasma cCl. Plasma cCa(2+) in critically ill calves was associated with plasma cTCa, blood pH, plasma cCl, serum cMg, and cL‐lactate (R (2) = 0.69) but not plasma cAlbumin. CONCLUSIONS AND CLINICAL IMPORTANCE: Calculation of cCa(2+) from cTCa in calf plasma or serum requires adjustment for at least pH and cCl when 1 or both are outside the reference range.