A simple, noninvasive urine test to diagnose cryptorchidism in horses that appear to have been castrated has been developed by a group of researchers from the Racing Laboratory at the Hong Kong Jockey Club.

Male horses that appear or are presumed to be castrated can in fact have one or even two retained testicles. Retained testes should be removed to help eliminate objectionable behavior and because horses with retained testes can develop spermatic cord torsion and tumors.

There are two commonly-employed tests that are currently available to diagnose ridglings, or "rigs," as these horses are sometimes called. The first involves comparing testosterone concentrations in blood samples before and after an intravenous injection of the hormone human chorionic gonadotrophin (hCG). The second method involves measuring the hormone estrone sulfate in blood.

Both of these tests are problematic. The hCG test is expensive, cumbersome, can be affected by season, and does not produce unequivocal results 6.7% of the time, while the estrone sulfate test can not be used in horses less than 3 years old and produces equivocal results approximately 2% of the time