Relaxin pregnancy test in dogs and cats

What it shows

In carnivores progesterone is high after ovulation whether or not a female is pregnant, so it cannot answer the question “is she pregnant?”. Relaxin can: it comes from the developing placenta, appears in the blood about 20–25 days after fertilisation, peaks at around days 40–50 of gestation, falls at birth and can stay detectable for up to 50 days of lactation.

Tests are mostly quick qualitative kits used in the clinic, and they are described as specific and sensitive. A positive result shows that placental tissue is there; it does not show whether the puppies or kittens are alive, how many there are or how far the pregnancy has gone. For those questions vets use ultrasound, which is also regarded as more reliable than an in-clinic relaxin kit; later in pregnancy X-rays help to count the litter.

Higher than reference — what it may be a sign of

• Positive — pregnancy is likely. The test has very good specificity, so a detectable level points to pregnancy.
• Positive, then negative on a later test — may mean that the pregnancy has been lost; the vet decides what to check next.
• Positive in the weeks after birth — relaxin can stay detectable during lactation, so a positive result soon after a litter does not mean a new pregnancy.

Lower than reference

• Too early. Before the placenta starts making relaxin the test is negative even in a pregnant female. A dog with a negative test at 22–27 days after breeding is usually retested a week later; two negatives a week apart confirm that she is not pregnant in about 80% of cases, and rarely a third test at 31–34 days is needed. Retesting matters most when breeding dates are unknown.
• A very small litter. Small and toy breeds carrying one or two puppies may not make enough relaxin for a positive result.
• False pregnancy. A female with a false pregnancy makes no relaxin, so the test is negative and helps tell this state from real pregnancy.

Look at together with

• Ultrasound — confirms pregnancy, shows whether the foetuses are alive and estimates their age; best done from about 25–35 days.
• Progesterone — tells the vet about ovulation and the corpus luteum, not about pregnancy itself.
• Date of mating — without it an early negative cannot be read at all.

Units and conversion

The result has no unit. The kits — microwell ELISA or immunomigration strip tests — are qualitative and are read as positive or negative. On a pet’s form relaxin appears in the same way. In the app this result is stored as text.

Reference range

The reference range is on your lab’s form; for a non-pregnant female it is “negative”. A negative is only meaningful when the test was done late enough after mating, and a positive is followed by ultrasound to check the pregnancy itself.

Cats and dogs

Sources

1. VCA Animal Hospitals — Pregnancy Testing in the Dog
2. Whitbread T. J. Serologic Test Kits (relaxin: pregnancy diagnosis). Merck Veterinary Manual, professional version
3. Merck Veterinary Manual — Pregnancy Determination in Bitches and Queens (professional version)
4. Prenatal Care of the Bitch and Queen (review, PMC)