Evaluation of the relationship between ionized and total calcium concentrations in blood during the first week of lactation in dairy cows

BACKGROUND: Diagnosis of subclinical hypocalcemia in cattle is based on concentration of total Ca ([TCa]) in serum or plasma below the reference range, with [TCa] serving as proxy for the concentration of ionized Ca ([iCa]). HYPOTHESIS/OBJECTIVES: To investigate the relation between [iCa] and [TCa] in periparturient cows over time, and its association with various metabolites. ANIMALS: Thirty periparturient dairy cows. METHODS: Prospective observational study. Blood from periparturient cows was obtained between 4 days before and 7 days after calving. Samples underwent blood gas analysis and blood biochemical analysis. The ratio of [iCa] : [TCa] was computed. Repeated measures linear regression analyses, spearman correlation‐, Deming regression‐ and receiver operating characteristic (ROC) analyses were conducted. RESULTS: Correlations between [iCa] and [TCa] ranged from r = 0.55 to r = 0.84. The diagnostic value of [TCa] to identify cows with [iCa] below the arbitrary cut‐off of 1.1 mmol/L was weak in particular during the first 24 hours of lactation, but continuously improved toward the end of the study with areas under the ROC curve increasing from 0.64 at d 0 to 0.93 at d +4 after calving. Strongest association with [iCa] : [TCa] were found for albumin (r (2) = 0.58, P < .0001), pCO(2) (r (2) = 0.45, P = .0003), the standard [HCO(3)] (r (2) = 0.22, P = .01), lactate (r (2) = 0.16, P = .04) and [NEFA] (r (2) = 0.15, P = .05). CONCLUSION AND CLINICAL IMPORTANCE: The [TCa] is of limited value to identify cows with subnormal [iCa] in the first hours and days of lactation, a finding apparently attributable to the increased variation of a number of metabolic variables that affect the ratio of [iCa] : [TCa].