Evaluation of colostrum quality in the Czech Republic using radial immunodiffusion and different types of refractometers

The objectives of this study were to determine the immunoglobulin G concentration of colostrum in Czech dairy cows, to compare refractometer results with results achieved using the radial immunodiffusion method and to evaluate the reliability of three types of refractometers and recommend the best solution for the evaluation of colostrum quality. Colostrum samples (n = 1522) were collected from 38 herds between 2015 and 2017. The immunological quality of colostrum was estimated using Brix refractometers (optical, simple digital, digital Misco) and compared with the immunoglobulin G concentration assessed using radial immunodiffusion. We found high variability in the quality of colostrum. The minimum, maximum and median of individual measurements were the following: radial immunodiffusion immunoglobulin G - 5.2, 199.1, 76.9 g/l; optical refractometer - 9.5, 32.0, 23.1% Brix; simple digital refractometer - 5.4, 35.0, 19.1% Brix; digital refractometer Misco - 9.8, 37.4, 23.2% Brix. On the basis of immunoglobulin G concentration assessed using radial immunodiffusion, 20.9% of colostrum samples were of low quality (immunoglobulin G < 50 g/l). The Spearman correlation coefficients between radial immunodiffusion and the Brix refractometer readings were 0.62-0.67 (P < 0.001) according to the type of refractometer. The cut-off evaluation of the readings from optical and Misco digital refractometers both showed 20% Brix, with sensitivities of 89.4% and 88.2%, specificities of 73.2% and 74.5% and accuracies of 86.0% and 85.4%, respectively. The cut-off level for the simple digital refractometer showed 17% Brix with a sensitivity of 77.5%, specificity of 80.4% and an insufficient accuracy of 78.1%. For optical and Misco refractometers we recommend the use of two cut-off levels for the evaluation of colostrum: 23% Brix for the selection of good quality colostrum suitable for freezing and 19% Brix to discard poor quality colostrum. The different cut-off levels obtained by measuring with different types of refractometers indicate the need to check the quality of the instruments prior to their use in practice and, where appropriate, to determine their cut-off levels by comparison with results obtained using the reference method.