A Review of IRF and RMI in Evaluating Regenerative and Non-regenerative Anemia in Dogs and Cats

Anemia is one of the most common hematological topics in dogs and cats, and it has various causes and outcomes. The immature reticulocyte fraction (IRF) and reticulocyte maturity index (RMI) serve as emerging indicators that identify highly immature reticulocytes containing large amounts of ribonucleic acid (RNA). These parameters are critical for the immediate evaluation of bone marrow response, the precise differential diagnosis of various causes of anemia, the examination of treatment efficacy, and the control of bone marrow regeneration in different healthcare settings. The maturity of reticulocytes and the RNA fluorescence intensity are identified by systems such as Sysmex XN-1000V and ADVIA 2120i. These hematology analyzers precisely calculate these indices. The IRF and RMI in dog and cat species with hemolytic anemias are significantly higher than those in hemorrhagic anemia. Additionally, these reticulocyte parameters in dogs and cats with bone marrow failure (BMF) are considerably lower than in other non-regenerative anemia, and both groups compared to pre-regenerative anemia. This review article addresses the concepts, clinical applications, and comparison of the IRF and RMI determined by the Sysmex XN-1000V and ADVIA 2120i hematology analyzers in various types of regenerative, non-regenerative, and pre-regenerative anemia in dogs and cats.