Free erythrocyte protoporphyrin (FEP) II. The FEP test is clinically useful in classifying microcytic RBC disorders in adults.

Microcytic red blood cells (RBC) are commonly encountered in clinical medicine and are caused by disorders of heme synthesis [usually iron deficiency anemia (IDA) or anemia of chronic disease (ACD)] or disorders of globin synthesis (usually thalassemia syndromes or HbE). Using the clinical history and standard laboratory tests (hematocrit, per cent saturation of transferrin (% sat), serum ferritin, Hb electrophoresis, HBA2, and HbF) we classified 198 adults with microcytic RBC as follows: 48 IDA, 11 probable IDA, 11 iron-deficient erythropoiesis without anemia, 13 ACD, 42 alpha-thalassemia trait, 35 probable alpha-thalassemia trait, 20 beta-thalassemia trait, and 15 unclassified. In addition, we demonstrated that the FEP test reliably (83-90% of the time, depending on FEP methodology) classifies microcytic RBC states into disorders of heme synthesis vs. disorders of globin synthesis. Because of reliability and ease of measurement, we recommend the hematofluorometer FEP as the first step in the clinical laboratory evaluation of microcytic RBC disorders in both adults and children.