Prediction of ionized calcium concentration based on total calcium and protein levels in cattle and sheep

Despite being important, there are no equations for prediction of ionized calcium (iCa) in sheep and cattle. The objectives of this study were i) to create equations for the calculation of serum iCa concentration based on the serum concentrations of total calcium (tCa), albumin (Alb) and total proteins (TP) and ii) to investigate whether predicted serum iCa values are beneficial in clinical practice. Serum samples from 30 sheep and 30 dairy cattle were used. Serum tCa was determined colorimetrically, while serum iCa was determined with an ion selective electrode method. Serum Alb and TP concentration were determined using bromo-cresol green and biuret methods, respectively. Ionized calcium was also calculated based on serum tCa, using regression analysis, and with two equations based on Alb and TP concentration. Bland–Altman plots were plotted to evaluate the agreement between measured and predicted iCa; Passing and Bablok (P - B) regression analysis was used to assess their agreement. The initial equations were corrected using the P - B generated equation and Bland – Altman plots were run to evaluate the level of agreement between measured and predicted iCa using the final equations. Six equations were finally created for cattle and 6 for sheep. The total bias exceeded 10.00% in all of them indicating that they are clinically unacceptable for iCa prediction especially when the predicted result is very close to the cut-off point of < 1.00 mmol L(-1). So, it could be suggested that, when necessary, iCa concentration should be directly determined.