Hypocalcemia is relatively common in dogs and cats, especially those living with critical illnesses (Holowaychuk MK, et al. J Vet Intern Med, 2009). Unfortunately, hypocalcemia is frequently underappreciated, as many assume total calcium (tCa) concentration is simply falsely low due to low albumin (ALB) or total protein (TP).

Assessment of a patient's calcium status has traditionally been based on evaluation of tCa that has been assumed to be directly proportional to ionized calcium (iCa), the biologically active form of calcium. Ionized calcium is the gold standard for determination of calcium status as it is not bound to ALB or complexed to other ions. Previously it has been suggested that tCa may be adjusted relative to ALB concentration to improve diagnostic interpretation. The two most common formulas utilized for "correcting" calcium are:

• Adjusting for ALB: tCa (mg/dL) - albumin (g/dL) + 3.5
• Adjusting for TP: tCa (mg/dL) - [0.4 x TP (g/dL)] + 3.3

Several previous studies have debunked the assumption of "correcting" calcium (Sharp CR, et al. J Vet Emerg Crit Care, 2009; Dickerson RN, et al. J Parenter Enteral Nutr, 2004; Slomp J, et al. Crit Care Med, 2003; Schenck PA, et al. Am J Vet Res, 2005). Indeed, calcium homeostasis in hypoalbunemic patients should be evaluated by iCa concentrations rather than the use of tCa or calcium adjusted for ALB or TP. Using tCa as an indicator of calcium status results in overestimation of hypocalcemia and underestimation of normocalcemia. Furthermore, using tCa adjusted for ALB or TP results in overestimation of normocalcemia and underestimation of hypocalcemia.

Ionized hypocalcemia has predicted a longer duration of both intensive care unit and hospital stay; this derangement was associated with increased mortality in dogs with acute kidney injury (Vaden SL, et al. J Vet Intern Med, 1997) and diabetic ketoacidosis (Hume DZ, et al. J Vet Intern Med, 2006), as well as in cats with pancreatitis (Kimmel SE, et al. J Am Vet Med Assoc, 2001). Thus early and accurate identification of hypocalcemia, as well as appropriate correction of this derangement, is essential for reducing morbidity and mortality associated with hypocalcemia.