Gribbles Veterinary offers relaxin testing for the detection of pregnancy in both dogs and cats. This hormone is superior to progesterone measurement for pregnancy detection in both species.
Progesterone cannot reliably distinguish between pregnancy and pseudopregnancy in dogs or cats, and similar levels of progesterone are produced in pregnancy and dioestrus in dogs. Relaxin does not increase during pseudopregnancy in dogs1 or cats or in non-pregnant bitches in dioestrus.
Relaxin testing is most commonly used in dogs, but a 2010 study showed that the assay used by Gribbles also works well in cats. The study involved 162 female cats, including 24 queens from a breeding colony. The earliest date that pregnancy was detected in this study was gestational day 20. By gestational day 29, the sensitivity of the test was 100%. False positives occurred in three queens, two of which had large ovarian cysts. Two non-pregnant queens with smaller ovarian cysts were negative for relaxin, suggesting that not all cats with ovarian cysts will have false positive results.2
An older study in Labrador Retrievers and Beagles (using a different assay) looked at plasma relaxin levels in pregnant and lactating dogs. This study showed that plasma relaxin is first detectable in week three or four of gestation, peaks 2-3 weeks before whelping, declines significantly until whelping, and remains at a low level for several weeks into lactation.1
There are some test limitations and stability issues to be mindful of. Similar to the findings in the studies above, the manufacturer of the Relaxin kit reports that the test may detect pregnancy as early as gestational day 20, but the test is not 100% sensitive until day 31 in either species.3 Negative results before this date should be confirmed by retest or other diagnostic means (e.g. ultrasound).
In addition to timing of the test, accuracy may also be affected by size of the bitch and size of the litter (e.g. a large bitch with a small litter may have undetectable relaxin). Relaxin should remain positive through gestation.3 Note relaxin does decline near parturition, so results could be negative at this time with a small litter size (anecdotal experience), though data from the manufacturer and the JAVMA study2 do not bear this out.
Relaxin will degrade over time, and the test should be run within four hours of sample collection if the sample (serum or plasma) is kept at room temperature.4 Samples can be kept refrigerated for up to two days, and longer term storage requires freezing for sample stability (samples should ideally be separated prior to refrigerated storage, and samples MUST be separated for frozen storage).4 As mentioned above, ovarian cysts have caused false positives in rare documented cases in cats.2
A brief summary of our relaxin test can be found in the Gribbles Veterinary Handbook on our website here.
1. Steinetz BG, Goldsmith LT, Lust G. Plasma relaxin levels in pregnant and lactating dogs. Biol Reprod. 37:719-25, 1987.
2. DiGangi BA, Griffin B, Levy JK, Smith BF, Baker HJ. Use of a commercially available relaxin test for detection of pregnancy in cats. J Am Vet Med Assoc. 237:1267-74, 2010.
4. Zoetis Witness® Relaxin package insert