Technical note: Direct enzyme immunoassay of progesterone in bovine milk whey

A simple extraction-free or direct quantitative ELISA for progesterone in bovine milk whey was developed. Whey samples are easy to collect, transport, and store. This method also allows for monitoring progesterone levels in cattle, which is important in reproductive management. The assay was designed to cover the concentration range 0.05 to 2 ng/mL, and the sensitivity of the method was 1.5 pg/mL. The intra- and interassay coefficients of variation were 8 and 12%, respectively. A high correlation (r = 0.90) between ELISA and radioimmunoassay measurements of progesterone in the same milk whey samples was obtained. The method can be easily applied in practice because samples can be stored at room temperature (22 to 26 degrees C) for 4 d. Moreover, because analysis requires milk coagulation, that process can be initiated during transport by standard mail services to the laboratory. Upon arrival at the laboratory, whey can be kept refrigerated for 1 wk before analysis. This tool is useful for monitoring luteal activity of dairy cows.