10-Year Study Comparing Enzyme-Linked-Immunosorbent-Assay With The Immunofluorescent Antibody-Test For Detection Of Feline Leukemia-Virus Infection In Cats

Immunodetection tests for feline retroviruses are powerful tools used in modern veterinary practice. Veterinarians must fully understand the characteristics-strengths and weaknesses-of the FeLV tests so that the information gained from them can be used properly. Any FeLV ELISA or immunofluorescent antibody (IFA) test is a method for detection of FeLV infection (the virus) and is not a diagnostic test for leukemia or other feline disease.From previous studies, it was determined that the most accurate test for detection of persistent FeLV infection is the IFA test, which detects FeLV antigens in cytoplasm of leukocytes in the blood of infected cats. In the study reported here, 1,142,600 FeLV IFA tests were performed between June 1972 and December 1990. During this period 19.8% of the IFA test results were positive and 78% were negative. Evaluation was not possible for the remaining 2.2% of the tests because of lack of enough leukocytes in the smears to evaluate, or nonspecific staining reactions.In 1979, 7 years after introduction of the IFA test, in-hospital FeLV ELISA were introduced, which enabled veterinarians to test for FeLV in their hospitals. Ever since that time, continual discrepancies have been reported between results of FeLV ELISA and IFA tests, particularly between positive ELISA results and their IFA test confirmation. A 10-year comparison was made between practitioner-performed in-hospital FeLV ELISA (n = 20, 240 tests) results and FeLV IFA test performed by a commercial laboratory. All samples tested by ELISA were submitted (for confirmation of results) by veterinarians from the United States, Canada, Europe, Japan, and Australia. There was 86.9% agreement between negative ELISA results and the IFA test result, but only 46.3% agreement between positive ELISA results and the IFA test result. Overall agreement was 49.0% for all confirmatory IFA tests. However, there was bias toward confirmation of positive ELISA results because 93% of confirmatory tests requested were of ELISA-positive samples.It is apparent from this study that between 26 and 69% of in-hospital positive ELISA results and 13% of in-hospital negative ELISA results are incorrect. On the basis of these observations, we recommend that veterinarians immediately confirm all FeLV positive ELISA results by IFA testing. In addition, negative ELISA results in cats that the veterinarian suspects have FeLV infection or FeLV-induced disease should also be confirmed by IFA testing. We also recommend that all cats be vaccinated for FeLV and that they be tested for FeLV and the feline immunodeficiency virus at time of first vaccine dose.