A solid-phase radioimmunoassay for progesterone and its application to pregnancy diagnosis in the cow

A solid-phase radioimmunoassay using antibody-coated tubes and ¹²⁵I-progesterone label was developed to provide an alternative to the conventional aqueous method of assaying progesterone in milk for early pregnancy diagnosis. The accuracy of diagnoses made using the solid-phase assay of progesterone in milk was assessed by comparison with diagnoses made using an aqueous assay of serum progesterone. Both methods agreed in the thirteen cows that were diagnosed. When “fat-free” milk was assayed by both aqueous (x) and solid-phase (y) methods, the progesterone values which resulted showed a high correlation (r = .94) and a linear relationship of y = 1.67x ? 0.68. Milk samples, in which the fat concentration ranged from 0.20 to 4.04%, were assayed by the solid-phase method and no relationship between milk fat and progesterone concentration was observed. Pregnancy diagnosis from solid-phase assay of milk samples collected on the day of breeding, 21 and 23 days following breeding was performed on 62 Holstein cattle. The accuracy of “non-pregnant” diagnoses was 95% to 100% and the accuracy of “pregnant” diagnoses was 80% if breeding day values were used and 72% if these values were excluded.The accuracy of this assay in diagnosing pregnancy was equal to that of previously published assays and provided the advantages of requiring less technical time and equipment. In addition, foremilk, composite or stripping samples can be accomodated in this assay since the estimates of progesterone are not affected by the concentration of milk fat.