A baseline sample for measurement of testosterone (20038) is usually sufficient to diagnose cryptorchidism in sexually mature dogs and cats. Serum is the required sample type. In neutered males, serum testosterone is expected to be less than 0.7 nmol/L. If there is a retained testicle, the testosterone concentration is typically above 1.0 nmol/L. In the infrequent instance of an equivocal baseline testosterone result, a human chorionic gonadotropin (hCG) stimulation test can be performed. The reference interval included on testosterone assay reports will be based on the reproductive status indicated for the patient on the submittal form.

A testosterone concentration of less than 0.7 nmol/L is considered appropriate for a gelding with the absence of functional testicular tissue. However, the gap between concentrations of testosterone between geldings and males with a retained testicle is narrower than that seen with other species. When the baseline testosterone concentration is below the limit of detection of the assay (< 0.7 nmol/L), and there are behavioral issues or continued concern for a retained testicle, there is an added benefit of diagnostic accuracy in performing a human chorionic gonadotropin (hCG) simulation test.

A diagnosis of ovarian remnant is made with integration of behavior, physical changes, histological examination of vaginal epithelium, and serum concentrations of anti-mullerian hormone (AMH). Note: the Michigan State University Veterinary Diagnostic Laboratory does not perform this test.

Measurement of progesterone may also be used when there is confidence in the stage of estrus (and induction of ovulation in feline patients) to indicate ovulation and development of luteal tissue. Serum is the required sample type for measurement of progesterone (20037). Please see the recommendations in the test catalog for special handling instructions for preparation and shipment of samples for progesterone.

Ovulation is induced by mating. If the female is pregnant, the corpus luteum (CL) will persist and progesterone will be increased until parturition. If the female is mated and ovulates but does not become pregnant, the corpus luteum will produce progesterone for two (2) to three (3) weeks and then regress. Thus, interpretation of a progesterone result is based on when the sample was collected in reference to known breeding dates or the time or previous exposure to a male. Progesterone concentrations at or above 6 nmol/L are consistent with the presence of functional luteal tissue. If a progesterone result is above 6 nmol/L in a sample collected a month or more after exposure to a male, the most likely reason for the result is pregnancy.

A reference interval for progesterone is not included on progesterone assay reports because “normal” will depend on reproductive status and stage within the cycle (for intact females). The rationale for use of serial progesterone assays is that increases of progesterone begin in close proximity to the preovulatory surge of gonadotropins in dogs and continue through ovulation and estrus. Estimates of the proximity of ovulation and time of optimal fertility are made from monitoring the increase of progesterone and the magnitude of the serum concentration. Listed below are guidelines used in interpretation of progesterone results. These guidelines should be utilized in conjunction with cytological changes of vaginal epithelium that are indicative of estrus.

In comparing progesterone results from different sources, be aware that progesterone concentrations may be reported in units of ‘ng/ml’ or ‘nmol/L’. The Michigan State University Veterinary Diagnostic Laboratory (MSU VDL) reporting unit for progesterone concentrations is nmol/L.

To convert results into similar units, use the following:

Progesterone in ng/ml x 3.18 = Progesterone in nmol/L

Progesterone in nmol/L ÷ 3.18 = Progesterone in ng/ml

The use of serial monitoring of progesterone in bitches has proven to be a valuable diagnostic tool for management of breeding. There is great variation among bitches as to the time interval from onset of proestrus until the initial rise of progesterone and then variation in the rate of increase. To be confident that ovulation is impending or has just occurred, sampling should be repeated until a result equal to or greater than 15 nmol/L has been obtained. Optimal fertility is anticipated when serum progesterone is between 30 – 65 nmol/L at the time of breeding.