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

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.

To describe laboratory changes that may result in the misclassification of cats with ASIMO as having progressive FeLV.

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.

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%).

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.