Normoblasts (synonym erythroblasts) are the nucleated precursors of erythrocytes. The cytoplasm changes with the increasing maturity from basophilic, to polychromatophilic to orthochromatic. The nucleus is coarse to pycnotic, compact and partially eccentric.
In peripheral blood, the normoblast count is calculated relative to 100 leukocytes. If more than 5 normoblasts per 100 leukocytes are found, the white blood count must be corrected (correction for normoblasts), since the nucleated normoblasts are counted for the total blood cell count by automated cell counters. Most modern automated cell counters do this automatically.
|Formula:||Corrected leukocyte count||=||Automatically-counted leukocytes
100 + microscopically-counted normoblasts
|Example:||Corrected leukocyte count at 15 normoblasts/100 leukocytes||=||47.7 x 109/L
100 + 15
|x 100 = 41.5 x 109/L|
The occurrence of normoblasts in the peripheral blood is always pathological. They can be observed in accelerated erythropoiesis (e.g. in severe hemolysis or thalassemia major), in bone marrow metastatic disease with leukoerythroblastic blood picture, or in extramedullary blood formation.