There are two types of tests. The SNAP test that they do in the office for both FIV and FeLV is a good screening test, but it should always be confirmed with the IFA test since there are many false-positives with the SNAP test. I’m copying this from an ealier post of mine
excerpted from http://vetlab.ro/analiti/FeLV.htm
“FeLV Positive Results Interpretation
Positive ELISA results detect viral antigen in blood. Positive results should be confirmed by immediate IFA testing.
Positive IFA results detect viral antigen in cells and generally indicates infection of bone marrow.
The majority of IFA-positive cats remain persistently infected and viremic for life.
ELISA positive/IFA negative: discordant result.
Possibly due to early infection or false positive ELlSA.
Recommend recheck of ELISA in 30–60 days.
Positive results of any test should be correlated with signalment, history, and physical findings.”
Basically the ELISA/SNAP detects early-stage (the cat may or may not fight off the virus) and late-stage infection and is more likely to give false-positives. The IFA detects late-stage disease. Unlike the FIV test, the FeLV tests for viral proteins not antibodies, so vaccination for FeLV will not affect the results.
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