Mature red blood cells (erythrocytes) do not contain a nucleus. Nucleated red blood cells represent a very immature form of erythrocytes that are released by the bone marrow when the body significantly lacks red blood cells, such as in severe anemia, thalassemia (lack of hemoglobin synthesis) and hypoxemia (chronically low oxygen levels).
Nucleated red blood cells are also found in miliary tuberculosis (severe form of tuberculosis with the bacteria spread throughout the body), as well as in bone marrow cancers and fibroses. The presence of nucleated red blood cells may be normal in infants a few days after birth. Results represent the number of nucleated red blood cells per 100 leukocytes.