This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Your vet will do a blood test; there are two types of blood tests which can be performed. Some vets will automatically do one of the tests before vaccinating your cat to make sure it is not already positive for the virus. The first (ELISA test) is where the vet takes some of your cat's blood, mixes it with a chemical and watches for a color change. If the blood changes color then your cat has tested positive for the virus. False positives are not uncommon in this form of test, so if your cat tests positive it may be a good idea to have it retested. "Light positives" are where the treated blood only changes color slightly. This means your cat is infected with the virus, but the virus is not very active in its system. The second type (IFA test) involves sending the blood sample to a special lab. This lab tests to see if the virus is being produced in the bone marrow. If this second test is positive, it is unlikely that your cat will ever test negative. Below is a flow chart (from a lecture by Dr. William Hardy, U of Penn. Vet School) which depicts what you should do if your cat tests positive on the ELISA test: * If positive:
+ whether healthy or sick, confirm by Indirect Fluorescent Antibody test (IFA)
* If negative:
+ if healthy and not exposed to a positive cat then no need to retest
+ if healthy but exposed to a positive cat then retest in 3 months because the healthy cat may be incubating the virus.
+ if sick then confirm the ELISA test with an IFA test