Diagnostic utility of CD4%:CD8low% T-lymphocyte ratio to differentiate feline immunodeficiency virus (FIV)-infected from FIV-vaccinated cats

Antibody testing based on individual risk assessments is recommended to determine feline immunodeficiency virus (FIV) status, but ELISA and Western blot tests cannot distinguish between anti-FIV antibodies produced in response to natural infection and those produced in response to FIV vaccination. The aim of this cross-sectional study was to test the hypothesis that FIV-infected cats could be differentiated from FIV-vaccinated uninfected cats using lymphocyte subset results, specifically the CD4%:CD8low% T-lymphocyte ratio. Comparisons of the CD4%:CD8low% T-lymphocyte ratio were made among the following four groups: Group 1 - FIV-infected cats (n = 61; FIV-antibody positive by ELISA and FIV PCR positive); Group 2 - FIV-uninfected cats (n = 96; FIV-antibody negative by ELISA); Group 3 - FIV-vaccinated uninfected cats (n = 31; FIV-antibody negative by ELISA before being vaccinated against FIV, after which they tested FIV ELISA positive); and Group 4 - FIV-uninfected but under chronic/active antigenic stimulation (n = 16; FIV-antibody negative by ELISA; all had active clinical signs of either upper respiratory tract disease or gingival disease for ≥ 21 days). The median CD4%:CD8low% T-lymphocyte ratio was lower in Group 1 (1.39) than in each of the other three groups (Group 2 - 9.77, Group 3 - 9.72, Group 4 - 5.64; P < 0.05). The CD4%:CD8low% T-lymphocyte ratio was also the most effective discriminator between FIV-infected cats and the other three groups, and areas under ROC curves ranged from 0.91 (compared with Group 4) to 0.96 (compared with Group 3). CD4%:CD8low% shows promise as an effective test to differentiate between FIV-infected cats and FIV-vaccinated uninfected cats.