Liquid Chromatography and Mass Spectrometry assay for estrogens in mares’ pregnancy

This study aimed to define new estrogens reference values for healthy pregnancies in mares and to investigate the effect of mare's age, parity and breed on these concentration. The study utilized Liquid Chromatography coupled with Mass Spectrometer (LC-MS) method, which provides more sensitive and accurate measurements of sexual steroids. The study was a double-blind prospective study and included 30 mares (17 Spanish Pure-Breed and 13 Showjumping mares). The mares were scanned monthly by transrectal ultrasound and blood samples were collected. If placentitis signs were observed, the mares were not further used. The results showed that there was a peak in E1 and E1S concentration at 5 months of pregnancy, followed by a progressive decline until parturition, with a quicker decline for E1S. The study also observed a breed effect for non-sulfonated estrogens, with higher concentrations in Showjumping mares than in Spanish Pure-Breed mares. The quicker decrease in E1S concentration is likely due to a decrease in sulfonation activity, and the study suggests that LC-MS measurement of estrogen concentrations can be an effective diagnostic tool for placentitis in mares, particularly given the observed breed effect for non-sulfonated estrogens. As a result, E1S remains a promising marker for the diagnosis of placentitis in mares.