Nucleated red blood cells (NRBCs) are immature red cell precursors that normally remain in the bone marrow. In healthy adults, the reference range is zero; they should not be present in the peripheral blood.

When NRBCs appear, it usually signals significant physiologic stress on the marrow or the body as a whole—such as severe anemia, hypoxia, hemolysis, major infection, marrow infiltration, or critical illness. For that reason, even low-level NRBCs are generally not treated as a normal variation in healthy adults.