- enzyme-linked immunosorbent assay (ELISA)
- immunofluorescence assay (IFA) also called the Hardy, or slide test
The main difference between these tests is that ELISA detects antigens in the blood serum, and IFA detects them in the white blood cells. ELISA can detect FeLV antigens early in the course of the infection, while the virus is in the blood and before it invades the bone marrow and white blood cells. Once FeLV reaches the bone marrow, both ELISA and IFA can detect it.
ELISA results in more false positive results. ELISA can also be used to test for antigens in a cat's saliva and tears, but the results are not reliable. Saliva and tear tests are used to screen a large number of cats, and to test cats from whom it is difficult to obtain sufficient blood samples.
Kittens that test positive by ELISA should be retested when they're older than 16 weeks. Uninfected kittens can test positive, if they are carrying their mother's antigens to FeLV. By 16 weeks of age, the mother's antigens should be out of a kitten's system.
Cats that test positive by IFA are generally positive for life. Negative results do not necessarily mean that the cat is uninfected. Negative test results can occur in infected cats that have been exposed only recently to the virus and are not producing antibodies yet.