
Found this here:
http://www.nwlabs.co.uk/newsletter61.htmRelaxin - Canine Pregnancy Test
False negative results
The Relaxin test is used as a marker for canine pregnancy. The test is claimed to be sensitive from 21 days post fertilisation. However the test has been troubled by false negatives. This was thought to be related to confusion over the date of fertilisation. Because of delay in sperm transport and actual implantation, the date of fertilisation can be as much as 5 days later than the date of insemination.
Handleman et al in an evaluation of 40 bitches observed that the test was sensitive to litter size and noted that small litter sizes (1 - 4) may predispose to false negative results when tested at 25 days. The sensitivity of the test for pregnancy at 25 days was estimated to be 70%. They advised that testing should be delayed to 30 days which increased the sensitivity to 75%. They advise that animals should be tested at 30 days.
In a recent case a bitch with a litter of 3 pups had a negative Relaxin result at 30 days. It may be advisable to retest negative bitches again at 35 days. It should also be noted that the test may also give rise to false positive results due to resorption or abortion of the litter. It is important when submitting to give an accurate mating date as this can help with the interpretation of borderline results.
Reference
Evaluation of a test for plasma relaxin in pregnant and non-pregnant bitches. Handleman et al 2001 ACVIM proceedings.