FeLV tests (p27 antigen and provirus PCR) in cats

What it shows

FeLV is a retrovirus: after infection it can insert a copy of its genetic material, the provirus, into the DNA of the cat’s cells. The two lines on a form look at different things:

• FeLV antigen (p27) — a test for a protein of the virus circulating free in the blood. It is the usual screening test, often done in the clinic. Most cats test positive within 30 days of exposure, some later. Because it looks for the virus and not for antibodies, it is not affected by vaccination or by antibodies a kitten got from its mother.
• FeLV provirus PCR — a test for viral DNA built into the cat’s cells. It also finds cats in whom the virus is held in check; a quantitative PCR estimates how much provirus there is.

Guidelines describe several outcomes of infection. In progressive infection the immune system does not contain the virus: antigen stays in the blood and the cat sheds virus. In regressive infection the immune system holds the virus back without removing it: the antigen test turns negative, while the provirus remains and PCR stays positive. In abortive infection the cat clears the virus early, and both tests are negative.

Higher than reference — what it may be a sign of

• Antigen positive, PCR positive — fits progressive infection. It is considered confirmed when antigen stays positive on repeat testing; retesting antigen-positive cats without signs after 6–12 weeks is advised.
• Antigen negative, PCR positive — fits regressive infection. Such cats are usually healthy and do not shed virus. The virus can reactivate, particularly if the cat’s immunity is suppressed, even many years later. They can still pass the infection on through a blood transfusion.
• Antigen positive, PCR negative — rare. It may reflect a focal infection confined to certain tissues; a false positive antigen test is another possibility. A positive in-clinic test is confirmed by another method, most of all in low-risk cats, where false positives are more likely.
• Quantitative PCR — Merck gives a cut-off: fewer than 1 million provirus copies per mL points more to regressive infection, 1 million or more to progressive infection.

Until the status is clear, cats with discordant results are regarded as possibly infected and a possible source of infection for other cats.

Lower than reference

Both tests negative is the expected result. It does not rule out a very recent infection, because antigen may not be detectable yet. After a possible exposure the test is repeated: ABCD suggests a follow-up test six weeks after the first one. In regressive infection antigen and provirus levels are low and can fall below a test’s detection limit, so one negative result does not settle every case.

Look at together with

• Repeat testing — guidelines stress that FeLV status rests on repeated tests over time, not on one result.
• FIV — usually tested at the same time, often in one combined rapid test.
• Complete blood count — haemoglobin, white cells and platelets: viral DNA in the cells can later lead to bone marrow suppression.

Units and conversion

Both results are qualitative and have no unit. The antigen test is reported as positive or negative. PCR is reported as detected or not detected. A quantitative PCR may also print the number of provirus copies per mL, and the lab’s comment says how it reads that number. In the app these results are stored as text.

Reference range

The reference range is on your lab’s form; for both tests it is “negative” or “not detected”. A positive result is read together with the other test, the cat’s history and signs, and a repeat test.

Cats and dogs

Sources

1. AAFP — 2020 Feline Retrovirus Testing and Management Guidelines (J Feline Med Surg)
2. ABCD (European Advisory Board on Cat Diseases) — Guideline for feline leukaemia virus infection
3. ABCD — Feline leukaemia virus infection: fact sheet (2024)
4. Merck Veterinary Manual — Feline leukemia virus disease
5. Cornell Feline Health Center — Feline leukemia virus