FIV antibody test in cats

What it shows

FIV is a retrovirus of cats that slowly weakens the immune system. The usual test does not look for the virus itself but for the cat’s antibodies to it. Rapid in-clinic tests (ELISA and similar) detect antibodies to proteins of the virus.

A questionable result is checked in a reference laboratory with a confirmatory test, regarded as the most reliable, or with immunofluorescence (IFA). PCR, which looks for the virus’s genetic material (provirus), is another option, but how often it gives a correct answer differs a lot from one PCR test to another: some infected cats are missed because the virus varies or there is little of it.

Higher than reference — what it may be a sign of

• Infection — antibodies made in response to the virus. In high-risk cats (free-roaming, older, unneutered males) a positive result is more likely to be true.
• Vaccination — vaccinated cats make antibodies that some tests cannot tell apart from those after infection, and they can test positive for years. No FIV vaccine is licensed in Europe, and the US vaccine is no longer available: none has been sold in North America since 2016. So this matters mostly for cats vaccinated in the past or elsewhere, and the vaccination history then shapes how a positive result is read.
• Mother’s antibodies in kittens — kittens nursing from an infected or vaccinated queen receive antibodies and can test positive without being infected. ABCD advises retesting at about 16 weeks, when these antibodies are usually gone; rarely they last up to six months. A kitten still positive after six months is considered infected. PCR can give an earlier answer.
• False positive — no test is 100% specific. In low-risk groups, such as young indoor cats, a positive result is confirmed, for example with a laboratory confirmatory test.

Lower than reference

Negative is the expected result, and in low-risk cats it is generally reliable. There are two exceptions. The first is early infection: antibodies take time to appear — most cats make them within 60 days, though some sources give roughly two to six months — so after a possible exposure the test is repeated at least 60 days later. The second is late-stage disease, when, rarely, a cat no longer has detectable antibodies; the AAFP suggests PCR or a laboratory confirmatory test for high-risk cats with typical signs.

Look at together with

• FeLV — usually tested at the same time, often with one combined rapid test.
• Vaccination history — and, for kittens, the mother’s status.
• Complete blood count — white cells and lymphocytes give a general picture of the cat’s health; on its own the count neither confirms nor excludes FIV.
• Way of life — outdoor access and fights, since bites are the main route of spread.

Units and conversion

The result is qualitative and has no unit: positive or negative. A rapid test shows a spot or line next to a control spot. PCR is reported as detected or not detected. In the app the result is stored as text.

Reference range

The reference range is on your lab’s form; it is “negative”. A positive result is read together with the cat’s age, vaccination history and risk of exposure, and a questionable one is confirmed by another method. Blood from several cats should not be pooled for one rapid test, because that makes the test miss more infected cats.

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 immunodeficiency virus
3. Cornell Feline Health Center — Feline immunodeficiency virus (FIV)