Myelocytes, along with metamyelocytes and promyelocytes, are the precursors of neutrophils, the largest class of white blood cell. These immature neutrophils are normally found only in the bone marrow. In the blood, metamyelocytes are the most often observed, accompanied by a few myelocytes. Promyelocytes are rarely observed and, if seen, are often a sign of blood cancer. Results are expressed in the number (#) of myelocytes and, more usefully, in the ratio of myelocytes (the fraction of white blood cells made up of myelocytes).
The transient presence of a fraction of neutrophil precursors (particularly metamyelocytes and some myelocytes) of less than 0.05 (5%) of white blood cells is generally benign and can be found in an infection, during pregnancy or during bone marrow stimulation to correct anemia or to recover from a phase of myelodepression (very low production of white blood cells, red blood cells and platelets by the bone marrow). The presence of a fraction of all precursors greater than 0.10 (10% of white blood cells) usually indicates a myeloproliferative syndrome (chronic myeloid leukemia, etc.). The presence of both neutrophil and red blood cell precursors (nucleated red blood cells, dacrocytes, etc.) may indicate, among other things, that the bone marrow is infiltrated by myelofibrosis or metastases.