In-office diagnostic screening tests for FeLV and FIV have facilitated the control of these infections. These tests can be used on a variety of samples, including serum, plasma, and whole blood. Higher rates of false positives may be seen with whole blood samples, particularly if samples are hemolyzed. Test systems developed for use with tears or saliva samples may be less reliable than testing blood. Considerable variability in test accuracy exists between different manufacturers. User error also contributes to false-positive results.

Although testing is relatively straight forward, no test is accurate 100% of the time. It is possible to have discordant test results, especially if the tests target different disease stages or require different specimens. In populations with a low prevalence of infection, >50% of cats with positive test results may be uninfected. Since the consequences of both false-positive and false-negative test results can be disastrous for individual cats or for multiple cat populations, confirming positive test results is crucial, especially in asymptomatic cats. Generally, screening tests are believed to be more sensitive for FeLV and FIV infection (fewer false-negatives), whereas the advanced confirma-tory tests are felt to be more specific (fewer false-positives).

Detection of the FeLV core protein p27, produced abundantly in most infected cats, is the mainstay of clinical FeLV testing. Immunochromatic lateral flow devices and microtiter ELISA detect cell-free viral antigens and can be performed in clinics. The immunofluorescent antibody test (IFA) detects p27 antigen within the cytoplasm of infected blood cells. Because it requires special processing and fluorescent microscopy, the IFA must be performed by a qualified reference laboratory. Blood or bone marrow smears are air-dried and mailed, unfixed, to the laboratory. The antigen is present at highest concentrations in neutrophils and platelets, and false-negatives may result when these 2 cell lines are deficient. False-positive results may occur when smears are too thick, when background fluorescence is high, and when the test is run by inexperienced personnel. IFA-positive tests indicate that the bone marrow is infected with FeLV. In this case, most cats remain persistently infected for life.

By contrast, serum antibody tests are not useful because of the high rate of exposure to FeLV in the environment. FeLV vaccination also induces FeLV antibodies, but titers are not necessarily related to level of protection.

The polymerase chain reaction (PCR) test has been used for the diagnosis of FeLV infection. This test detects viral nucleic acid sequences instead of protein antigens. PCR may be useful in helping to determine the true status of cats with discordant results from other testing techniques. PCR is capable of detecting FeLV infection in blood, solid tissues, tissue cultures, and fixed specimens.

Virus culture for FeLV is the confirmatory test of choice in Europe. Not all cats with FeLV infection are positive on virus culture. Because FeLV generally replicates in lymphoid tissue and other sites before the bone marrow, immunochromatic tests may detect infection a few weeks earlier than the IFA.

Immunochromatic tests are preferred for screening, with the IFA or a second different immunochromatic test recommended for confirmation of positive results. The combination of routine screening and confirmatory tests will accurately determine the FeLV infection status of most cats. Some animals, however, will have repeatedly discordant test results or occult infections that remain undetected. Rarely, some cats that test negative for viremia have been shown to secrete infectious virus in body fluids such as milk and urine. These cats are infectious to other cats, but test negative on routine screening tests.