Laboratory Changes Supporting a Diagnosis of Immune-Mediated Anemia in Cats With Initial Suspicion of Progressive Feline Leukemia Virus.

BACKGROUND: Progressive feline leukemia virus (FeLV) infection and anemia of suspected immune-mediated origin (ASIMO) are associated with similar hematologic abnormalities, including anemia, neutropenia, and lymphocytosis. Cats with ASIMO may test positive for FeLV using point-of-care (POC) ELISA tests. OBJECTIVE: To describe laboratory changes that may result in the misclassification of cats with ASIMO as having progressive FeLV. METHODS: Cats with ASIMO and laboratory results mimicking FeLV infection were retrospectively identified via searches of laboratory and clinical records. Inclusion criteria were: 1) a positive POC FeLV result with negative confirmatory testing, 2) hematocrit < 20%, with or without macrocytosis, neutropenia, lymphocytosis, or suspicion of circulating atypical cells, and 3) a clinical diagnosis of ASIMO. Clinical and laboratory data, treatment, and outcome were recorded. RESULTS: Nine cats met the inclusion criteria. All were vaccinated against FeLV. Seven (78%) had outdoor access and two (22%) were indoor only. Hematocrit at presentation ranged from 7-16%, and macrocytosis was recorded in seven cases (78%), of which six were normochromic by analyzer data. Seven cats (78%) had one or more laboratory indicators of hemolysis, including an abnormal RBC histogram in all five cases in which this was available for review. Other hematologic abnormalities included neutropenia (n = 6, 67%), lymphocytosis (n = 1, 11%), and circulating atypical cells (n = 1, 11%). CONCLUSIONS: Cats with ASIMO may present with macrocytic, normochromic anemia and other laboratory features mimicking progressive FeLV. Evaluation of RBC histograms alongside detailed blood film evaluation is helpful in identifying immune-mediated anemia as a likely diagnosis.