Oestrone sulphate testing is the most reliablemeans of pregnancy diagnosis in the mare from approx 100 to 310 days. Increased concentrations of oestrone sulphate indicate a viable foetus and concentrations will drop immediately if foetal death occurs. Oestrone sulphate concentrations can decrease in late pregnancy and can give a negative result in mares in late pregnancy. A non-pregnant mare has an oestrone sulphate concentration of < 5 ng/ml. The concentration in a pregnant mare is typically > 20 ng/ml and generally around 100 ng/ml.
Cryptorchid (rig) identification
The equine testis produces ten times the quantity of oestrogen compared with testosterone and it is therefore much easier to differentiate a cryptorchid from a gelding by measuring serum oestrone sulphate rather than testosterone. This test is not recommended in horses less than 3 years of age and donkeys, which will require testosterone assay after stimulation with hCG.