Reliability of results of milk urea analysis by various methods using artificial milk control samples

The milk urea concentration (MUC) is a respected indicator of the health and nutrition status of dairy cows. It is in relation to their reproduction performance, longevity and technological milk indicators. The accuracy of the interpretation of results depends on their reliability, which is so important. There are a lot of principles of MUC analyses. Their results can be affected by a number of interferential factors. Many disproportions were noticed for the above-mentioned reasons in laboratory practice. That is the reason why relevant result variation sources are studied. The goal of this paper was to search the relationships between different methods of MUC determination with the use of specifically modified samples on a milk basis with the absence of dissolved components such as lactose. The results of two methods (photometric BI with diacetylmonoxime and FT-MIR (mid infrared)) were disqualified for a large shift and variance of values, unsatisfactory recovery and paralysed relation to other methods (BI r = from 0.184 to 0.213; P > 0.05). Therefore the second BI method was retained in the evaluation, and it was probably a local defect in the performance at disqualification. Nevertheless, the procedure showed poorer recovery (75.5 ± 14.3%) and necessity for methodical modifications for support of result reliability such as increase in the number of calibration points as compared to the contemporary procedure. The results of FT-MIR method were strongly systematically displaced due to lactose absence in particular (by 33.824 ± 3.794 mg/100 ml). Nevertheless, the correlations with results of other relevant methods were tight (from 0.991 to 0.999; P < 0.001). The photometrical method with Ehrlich's agent (para-dimethylaminobenzaldehyde, EH) showed acceptable values of all the evaluated indicators of reliability. The specific Ureakvant method (UR; with conductivity difference measurement) showed the most proper results in combination with all the reliability indicators (recovery as much as 93.2 ± 10.2%; correlation from 0.989 to 1.0; P < 0.001; acceptable ratio of systematic and random error components). It is possible to use the tested specific standard samples for the control or calibration of all methods (BI, EH and UR) with the exception of FT-MIR.