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PCR tests are designed to detect short segments of a virus's genetic material. While antibody-based tests are ideal screening tests for infection. In certain situations PCR-based tests would be superior. New research suggests that most cats exposed to FeLV remain infected for life. Some of these cats may become nonviremic, a state called regressive infection. In these cats, neither antigen nor virus is detectable in the blood. However, proviral DNA can be found using PCR testing. These cats are not likely to shed virus but may be able to transmit disease if used as blood donors. While testing is not generally affected by vaccination, blood collected immediately after vaccination may give false positive results, so it is suggested that samples be collected before vaccination. When FeLV screening and confirmatory testing results do not agree, repeated testing at 60 days and then yearly is encouraged. Usually the cat's status will emerge with repeated testing. When performed by a qualified laboratory, PCR testing may be the most sensitive test for FeLV and could help identify regressive infections and resolve cases where other testing methods disagree. Most laboratories do not routinely perform FeLV or FIV PCR testing. PCR assay can detected very few copy numbers of viral genome, and methods for extraction of nucleic acid have been optimized to detect low copy numbers of pathogens, even in large volume of sample materials.

Emir Hodzic, D.V.M., Ph.D.
Center for Comparative Medicine
Real-Time PCR Research & Diagnostic Core Facility
School of Veterinary Medicine
University of California, Davis