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 (r2 = 0.58, P <.0001), pCO2 (r2 = 0.45, P =.0003), the standard [HCO3] (r2 = 0.22, P =.01), lactate (r2 = 0.16, P =.04) and [NEFA] (r2 = 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]. [ABSTRACT FROM AUTHOR]