A commercial whole blood polymerase chain reaction assay failed to diagnose histoplasmosis in cats confirmed by cytology or urine antigen detection.

Objective: To determine the sensitivity and specificity of a commercial whole blood real-time PCR assay (RT-PCR) for the diagnosis of histoplasmosis when compared to direct organism identification and/or urine antigen quantification by enzyme immunoassay (UA-EIA). A secondary objective was to compare the sensitivity and specificity of RT-PCR to anti-Histoplasma immunoglobulin G antibody detection by enzyme immunoassay (IgG-EIA) and IgG-EIA to UA-EIA.

Animals: Cats presented to the Kansas State University Veterinary Health Center from February through September of 2023 in which histoplasmosis was diagnosed or suspected.

Methods: From February through September of 2023, cats were tested by RT-PCR, IgG-EIA, and UA-EIA if histoplasmosis was diagnosed cytologically or was a differential diagnosis for the presenting clinical signs. Cats were excluded if all 3 tests were not submitted or if the diagnosis of histoplasmosis could not be excluded despite a negative UA-EIA result. Cats with cytologically or histologically confirmed histoplasmosis were designated as proven histoplasmosis cases, and cats with a positive UA-EIA result without cytological or histological confirmation were designated as probable histoplasmosis cases.

Results: 10 cats were diagnosed with either proven (n = 6) or probable (4) histoplasmosis, and 10 cats were considered true negatives. Whole blood RT-PCR results were negative in all 20 cats (sensitivity, 0%; 95% CI, 0% to 30.85%). The IgG-EIA was 90% sensitive (95% CI, 55.50% to 99.75%) and 70% specific (95% CI, 34.75% to 93.33%). The UA-EIA results were positive in all cats with proven histoplasmosis.

Clinical Relevance: This commercial RT-PCR is insensitive when used on whole blood collected in EDTA and should not be used to diagnose feline histoplasmosis. Further studies are required to determine whether alternate RT-PCR protocols for EDTA-collected whole blood could be useful for diagnosing histoplasmosis in cats.