Serum calcium determination with atomic absorption spectrophotometry and colorimetric method: association and predictive equations between them in cattle and sheep samples

The objectives of the present study were to compare the serum calcium (Ca) concentrations in cattle and sheep measured by the flame atomic absorption spectrophotometry (AAS) and those obtained with a colorimetric method (CM) and to generate equations that allow their prediction. Serum Ca concentration was determined in 50 dairy cattle and 50 dairy sheep non-hemolyzed blood samples with AAS and CM. The data were analyzed with paired sample t test and Bland–Altman plots were generated. Using a random number generator, 25 samples from each species were selected to create predictive equations between the two methods using Passing-Bablok regression analysis. The predictive equations were validated at the remaining 25 samples for each species using Bland–Altman plots. The average bias between the two methods was 5.5% for cattle and 18.1% for sheep samples. Mean Ca values determined with CM were significantly lower than those measured with AAS (P < 0.05) in both species. The use of the generated equations y = 0.706 + 0.727x for cattle and y = 0.744 + 0.833x for sheep (mmol/l) allows the prediction of AAS determined Ca (y) using CM determined Ca values (x) with average bias 1.8% for cattle and 1.9% for sheep. The two methods for calcium determination evaluated cannot be used interchangeably and the respective reference intervals and cut-off values should be taken into account to make clinical decisions in cattle and sheep. The use of the proposed equation for cattle is considered as acceptable for the calculation of bias corrected reference ranges and cut-off values.