Discordant test results occur when results of ELISA and IFA testing do not agree and may make it difficult to determine the
true FeLV status of a cat. Most typically, this is an ELISA-positive and IFA-negative cat. Discordant results may be due to
the stage of infection, the variability of host responses, or technical problems with testing. The status of the cat with
discordant results may eventually become clear by repeating both tests in 60 days and yearly thereafter until the test results
agree. Unfortunately, a significant number of these patients have persistently discordant test results and the cat's true
status may not be known. Cats with discordant test results are best considered as potential sources of infection for other
cats until their status is clarified.
PCR is offered by a number of commercial laboratories for the diagnosis of FeLV. PCR detects viral nucleic acid sequences
(RNA or DNA) and can be performed on blood, bone marrow and tissues. PCR tests for FeLV are usually positive within one week
of FeLV exposure. When performed by a well-equipped and well-trained laboratory, PCR can be the most sensitive test methodology
for FeLV and could help resolve cases with discordant test results and detect regressive infections. Independent evaluation
of commercial PCR testing for FeLV (or FIV) is not routinely performed nor are labs required to be licensed or regulated,
so that veterinarians may not be able to ascertain the diagnostic efficacy of a test offered by a particular laboratory.
Recently, a real-time quantitative PCR assay was used to screen 597 Swiss cats for FeLV. Surprisingly, 10% of cats negative
for FeLV p27 antigen by ELISA were positive by PCR (Hofmann-Lehmann, Huder et al. 2001). However, the proviral loads of these
cats were 300-fold lower than for ELISA-positive cats. One possible explanation for the PCR-positive, ELISA-negative cat is
that such cats are truly infected with FeLV but were able to overcome antigenemia (regressive infection). It is also possible
that these cats were in the early stages of FeLV-infection and had not yet become ELISA-positive. Research such as this suggests
it may be very difficult to determine the true status of cats that appear to be transiently infected and recovered.
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