A comparison of progesterone assays for determination of peripheral pregnane concentrations in the late pregnant mare

Michelle A.A. Wynn, Alejandro Esteller-Vico, Erin L. Legacki, Alan J. Conley, Shavahn C. Loux, Scott D. Stanley, Thomas E. Curry, Edward L. Squires, Mats H. Troedsson, Barry A. Ball

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


During the latter half of gestation in mares, there is a complex milieu of pregnanes in peripheral blood. Progesterone concentrations are often assessed by immunoassay during late gestation as a measure of pregnancy well-being; however, interpretation of results is complicated by the numerous cross-reacting pregnanes present in high concentrations during late gestation. Further, many mares are supplemented with an exogenous progestin, altrenogest, which may also cross-react with existing assays and further confound interpretation. The objectives of this study were: 1) to compare differences in pregnane concentrations determined with four immunoassays compared to LC-MS/MS and 2) to assess cross-reactivity observed with the same immunoassays, specifically considering pregnenolone (P5), progesterone (P4), 5α-dihydroprogesterone (DHP), allopregnanolone, and altrenogest. Blood samples from four healthy mares in late gestation were evaluated by immunoassay and by LC-MS/MS. Measured immuno-reactive progesterone (ir-progesterone) concentrations differed (p < 0.0001) between immunoassays, although results were highly correlated (r = 0.85–1.0; p < 0.001). Measured ir-progesterone concentrations by immunoassay were linearly associated (r2 = 0.68–0.76; p < 0.001) with concentrations of P5, P4, DHP, and allopregnanolone determined by LC-MS/MS. There was no detectable cross-reaction of altrenogest in any immunoassay, but varying degrees of cross-reactivity was observed with other pregnanes analyzed. These data confirm ir-progesterone concentrations during late gestation vary depending upon the assay used and the cross-reactivity to other pregnanes present in late gestation, although the synthetic progestin altrenogest did not affect the results of any immunoassay tested.

Original languageEnglish
Pages (from-to)127-133
Number of pages7
StatePublished - Jan 15 2018


  • Cross-reactivity
  • Equine
  • Immunoassay
  • LC-MS/MS
  • Pregnancy
  • Pregnanes
  • Progesterone

ASJC Scopus subject areas

  • Small Animals
  • Food Animals
  • Animal Science and Zoology
  • Equine


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