Short communication: Relationship between methods for measurement of serum electrolytes and the relationship between ionized and total calcium and neutrophil oxidative burst activity in early postpartum dairy cows

Abstract

The objectives of this study were to (1) compare a test for serum measurement of total Ca (tCa), Mg, and P (VetTest Chemistry Analyzer, IDEXX Laboratories Inc., Westbrook, ME) to reference methods (spectrophotometric assays on a Beckman Coulter 640e automated clinical chemistry analyzer; Beckman Coulter, Brea, CA), (2) determine the relationship between ionized Ca (iCa) and reference method tCa in the immediate postpartum period, and (3) assess the relative value of these blood Ca indices as predictors of neutrophil oxidative burst activity. Samples were collected from multiparous Holstein cows (n = 33) over the first 5 d in milk. A total of 183 samples for objective 1 and 181 samples for objective 2 were available. Neutrophil oxidative burst activity was assessed once between 2 and 5 d in milk (n = 29). Linear regression demonstrated strong relationships between serum tCa, Mg, and P concentrations measured by the VetTest compared with the reference method. Bland Altman analysis indicated that the VetTest values were higher than the reference method by 0.22 mmol/L for tCa, 0.12 mmol/L for Mg, and 0.16 mmol/L for P. Compared with hypocalcemia categorized at ≤2.0 or ≤2.125 mmol/L with the reference method tCa, thresholds for the VetTest measured tCa of ≤2.23 mmol/L (sensitivity = 87%, specificity = 89%) or ≤2.30 mmol/L (sensitivity = 86%, specificity = 96%) could be used. The relationship between whole-blood iCa and reference method serum tCa differed by sampling time point after calving. Compared with identification of hypocalcemia with serum tCa measurements from the reference method (thresholds of ≤2.0 and 2.125 mmol/L), a whole-blood iCa threshold of ≤1.17 mmol/L resulted in the highest combined sensitivities (94 and 82%) and specificities (80 and 94%) at either threshold. Ionized Ca measurements were more consistently related to outcomes of neutrophil oxidative burst activity measured in vitro. The VetTest measurements of serum tCa reliably identified hypocalcemia when thresholds were adjusted to account for the bias of the test. The variation in the relationship between iCa and reference method tCa in the days following parturition suggest that these measures cannot be used interchangeably as indicators of Ca status. The more consistent associations between iCa and in vitro measures of neutrophil function, compared with tCa, indicated that this may be a more sensitive predictor of functional outcomes associated with postpartum Ca status.

Evaluation of the i‐STAT analyzer for determination of ionized calcium concentrations in bovine blood

“…It has been suggested that iCa 2+ can be a more sensitive predictor of functional outcomes associated with postpartum calcium status. That study showed a more consistent association between iCa 2+ and in vitro neutrophil function compared with total serum Ca 2 in early postpartum cows . Blood iCa 2+ concentration is generally measured using conventional blood gas analyzers in veterinary diagnostic laboratories, which might not be suitable for field practitioners and those in farm settings.…”

“…Moreover, iCa 2+ concentrations measured by the i‐STAT were regressed on iCa 2+ concentrations measured by the Radiometer ABL800 FLEX to determine curvilinearity. Lastly, receiver operating characteristic curves were developed at iCa 2+ cutoff concentrations of <1.00 and <1.18 mmol/L (subclinical hypocalcemia) to assess Se and Sp of the i‐STAT at the highest Youden index . A P of < 0 0.05 was considered significant.…”

A 100-Year Review: Metabolic health indicators and management of dairy cattle

“…Calcium status can be measured directly using either total Ca or ionized Ca concentrations in blood. Although ionized Ca is considered to be the more biologically active fraction and recent work (Leno et al, 2017) suggests that ionized Ca concentrations are more consistently related to neutrophil function in vitro than total Ca, research also suggests that the relationship between ionized Ca and total Ca varies in the days after calving (Ballantine and Herbein, 1991;Leno et al, 2017) and thus cannot be used interchangeably as indicators of Ca status. Notably, the large observational studies referenced above relating Ca status with postpartum outcomes related to disease, milk production, and reproduction all used total Ca as their indicator, and ionized Ca has received limited evaluation as a predictor of these outcomes.…”

Intravenous calcium infusion in a calving protocol disrupts calcium homeostasis compared with an oral calcium supplement

“…In clinical situations, the animals might not be able to maintain normal concentrations of these constituents in blood, thus substantially changing the correlation between tCa and iCa concentrations (Toffaletti, 1989). Leno et al (2017) demonstrated that the relationship between whole-blood iCa and serum tCa differed by sampling time point after calving. Additionally, these authors showed that iCa measurements were more consistently related to outcomes of neutrophil oxidative burst activity measured in vitro.…”

“…Additionally, these authors showed that iCa measurements were more consistently related to outcomes of neutrophil oxidative burst activity measured in vitro. Therefore, wholeblood iCa concentrations likely correlate with the clinical syndrome associated with abnormal Ca status (Lincoln and Lane, 1990;Leno et al, 2017).…”