Discordant FeLV p27 immunoassay and PCR test results in cats with hematologic disorders

The authors of this case series described some cats with hematologic disease and a positive feline leukemia virus (FeLV) serum p27 with lateral flow immunoassay (LFI)/ELISA, but negative PCR test result.

A total of 1692 medical records from a primary care feline practice and a veterinary referral hospital were evaluated retrospectively to assess discordant feline leukemia virus (FeLV) test results. 

In total, 73 cats were positive for FeLV using serum in a (LFI) or laboratory-based ELISA. Of these cats, 21 subsequently tested negative for FeLV proviral DNA by non-quantitative PCR on EDTA whole blood (16/21, 76.2%), bone marrow (4/21, 19%) or both (1/21, 4.7%). The proportional morbidity (an estimate of prevalence in a sample of the total population) for FeLV by LFI/ELISA and PCR assays was 3.1%, consistent with that reported in previous studies for cats in North America. Cats with discordant LFI/ELISA and PCR results had either primary bone marrow disease (18 autoimmune, one neoplastic), a bone marrow insult (hemotrophic mycoplasmosis) or systemic inflammation (pyothorax with a marked neutrophilic leukocytosis). The percentage of cats with a positive LFA/ELISA result and negative PCR assay surviving to discharge was 85.7% (18/21). Of these, 88.9% (16/18) survived 4 months to 6 years. Seven cats (33.3%) were re-tested with LFI or ELISA once primary disease was controlled, and all tested negative.

In conclusion, these findings indicate that in cats with bone marrow disease that shares features of progressive FeLV infection, positive LFI and ELISA FeLV test results should be followed up with FeLV proviral DNA PCR testing, particularly in populations where disease prevalence is low.

“Discordant FeLV p27 immunoassay and PCR test results in 21 cats with hematologic disorders”. Matthew Kornya, et al. J Feline Med Surg. 2023 Jul;25(7):1098612X231183297.  doi: 10.1177/1098612X231183297.


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