Several companies make and market ELISA or similar assays to test whole blood, serum, plasma or saliva for FeLV antigen. Infection should be confirmed by virus isolation or by immunofluorescence as some cats may be positive ELISA but negative by virus isolation as antigen can occasionally be present in the circulation although virus infection has been eliminated from the blood. In addition, ELISA kits may sometimes give false positive results, for example, if the cat serum sample has haemolysed. Cats should be retested after 12 weeks to determine whether the viraemia is transient or persistent.

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