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

Abstract

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&#x2009;days before and 7&#x2009;days after calving. Samples underwent blood gas analysis and blood biochemical analysis. The ratio of [iCa]&#x2009;:&#x2009;[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&#x2009;=&#x2009;0.55 to r&#x2009;=&#x2009;0.84. The diagnostic value of [TCa] to identify cows with [iCa] below the arbitrary cut-off of 1.1&#x2009;mmol/L was weak in particular during the first 24&#x2009;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&#x2009;+4 after calving. Strongest association with [iCa]&#x2009;:&#x2009;[TCa] were found for albumin (r&#x2009;=&#x2009;0.58, P&#x2009;<&#x2009;.0001), pCO(r&#x2009;=&#x2009;0.45, P&#x2009;=&#x2009;.0003), the standard [HCO] (r&#x2009;=&#x2009;0.22, P&#x2009;=&#x2009;.01), lactate (r&#x2009;=&#x2009;0.16, P&#x2009;=&#x2009;.04) and [NEFA] (r&#x2009;=&#x2009;0.15, P&#x2009;=&#x2009;.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]&#x2009;:&#x2009;[TCa].