Walking On Volume 6, Issue 9, September 2019 - Page 8

For the Health of It Using Progesterone as a Diagnostic Tool during Equine Pregnancy Reprinted with permission from Equine Disease Quarterly, Volume 25, Number 3 Progestogens are a class of steroid hormones largely responsi- ble for sustaining the embryo and maintaining uterine quiescence. In horses, at least 10 known progesto- gens are present in maternal cir- culation during gestation. To date, only a few of them are known to be biologically active. Progesterone, the most renowned of this class of steroid hormones, is the only one with clinical diagnostic application. During early pregnancy, progester- one is produced in the equine ovary by the corpus luteum (CL), and its concentrations remain elevated and peak between 60 and 120 days of gestation. From that point on, progesterone slowly decreases until it becomes nearly undetectable around 180 to 200 days of gesta- tion. During late gestation, other progestogens produced by the fe- to-placental unit are responsible for maintaining the pregnancy. These are first detectable by day 60 of ges- tation and are completely capable of maintaining pregnancy from around 120 to 140 days of gestation until term. Circulating progesterone has been used diagnostically to eval- uate luteal function during early 8 • Walking On pregnancy. When the circulating progesterone (P4) concentration is above 1 ng/mL, this is consid- ered consistent with the presence of luteal tissue, indicating that a follicle has ovulated, luteinized and is producing progesterone. When the circulating progesterone con- centration is above 4 ng/mL, this is considered adequate for the main- tenance of pregnancy. There are a number of reasons for monitoring and supplementing endogenous progesterone with progestins (syn- thetic progesterones) during preg- nancy, such as uterine infections, history of pregnancy loss, and luteal insufficiency. A few important issues regarding laboratory techniques and pro- gestogens require clarification. To date, all clinical veterinary diagnos- tic laboratories use immunoassays to measure circulating progester- one. The specificity of these tests is limited by the antibodies used in these assays. Due to the structural similarities among different pro- gestogens present in late gestation, after day 120 of gestation, anti- bodies are unable to differentiate between those different molecules and therefore can give false or in- accurate results. In addition, dif- ferent progesterone antibodies will result in disparate amounts of cross reactivity; therefore, each proges- terone assay will measure different amounts of progesterone, produc- ing varying results between labora- tories. It is important to emphasize that the best clinical interpretation for any progesterone result is the one provided by the clinical labora- tory that measured the progester- one, as they have reference ranges for their specific equine progester- one assay. The specificity lacking in immunoassays and the inter-labo- ratory variations can be overcome with the use of liquid chromatogra- phy-mass spectrometry (LC-MS). LC-MS has allowed researchers to evaluate changes in different progestogens during late gestation and further elucidate links between placental compromise during late gestation and the changes associ- ated with specific progestogens. It would be advantageous for clinical laboratories to switch to LC-MS to provide diagnostic panels of greater specificity and wider array of quan- tifiable progestogens. In summary, current tests for progesterone in the mare are useful to evaluate the presence of luteal tissue (P4>1ng/mL) and to ensure that levels of circulating progester- one are adequate for maintenance of early pregnancy (P4>4ng/mL) until about 120 days of gestation. From that point until term, current clinical tests are somewhat unreli- able due to the variety of progesto- gens present in maternal circu- lation. These limitations can be overcome with the use of LC-MS. CONTACT: Alejandro Esteller-Vico, DVM, PhD aestellervico@uky .edu • (859) 218-1098 Gluck Equine Research Center University of Kentucky • Lexington KY