Along with metamyelocytes and myelocytes, promyelocytes are the precursors of neutrophils, the largest class of white blood cells. These immature neutrophils are normally found only in bone marrow. In the blood, it is metamyelocytes that 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 as the number (#) of promyelocytes and, more usefully, as the ratio of promyelocytes (fraction of white cells made up of promyelocytes).
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) most likely indicates a myeloproliferative disorder (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.