Metamyelocytes in Peripheral Blood: Diagnostic Significance

Pathophysiological Significance

Metamyelocytes are immature granulocytic cells that normally develop and mature within the bone marrow before being released into circulation as mature neutrophils, and their presence in peripheral blood represents an abnormal release of immature granulocytic precursors from the bone marrow, which may suggest myelodysplastic syndromes, myeloproliferative disorders, bone marrow infiltration, or a reactive process to infection or inflammation 1, 2
This finding may be seen in both benign reactive conditions and malignant hematologic disorders, indicating premature release of immature cells 2, 3

Peripheral blood dysplasia including the presence of immature myeloid cells like metamyelocytes can be a key diagnostic feature of MDS, according to the World Health Organization (WHO) classification 1, 3
The presence of dysplastic cells in peripheral blood is an important criterion for diagnosing various MDS subtypes, with metamyelocytes showing dysplastic features such as abnormal nuclear configuration or cytoplasmic granulation 1, 4

Atypical CML and chronic myelomonocytic leukemia (CMML) may also present with circulating metamyelocytes, which can comprise a significant percentage of the differential count 5, 6

Severe infections can trigger significant left shift with metamyelocytes in peripheral blood, and other reactive causes include inflammation, tissue necrosis, and severe hemorrhage 7, 8

A complete blood count and differential should be evaluated to assess for other cytopenias or abnormal cell counts, and to determine the percentage and absolute count of metamyelocytes and other immature cells 1, 2
Peripheral blood smear examination should be performed to look for dysplastic features in all cell lines, and to check for presence of blasts, which would suggest more advanced disease 3, 4, 7
Bone marrow examination is indicated when persistent unexplained cytopenias or suspicion of hematologic malignancy exists, and should include assessment of cellularity, dysplasia, blast percentage, and megakaryocyte morphology 1, 2, 3, 8

In MDS, the degree of dysplasia and presence of immature cells correlates with disease severity and prognosis, and persistent circulation of metamyelocytes without obvious cause warrants close follow-up, as it may represent early manifestation of a myeloid neoplasm 1, 2, 4, 6
Transient appearance of metamyelocytes can occur in severe infections and should resolve with treatment of the underlying condition, and if only unilineage dysplasia is present without increased blasts or specific cytogenetic abnormalities, observation for 6 months with repeat evaluation is recommended before diagnosing MDS 3, 8