Hematology & Internal Medicine & Emergency Medicine
Promyélocytes | Clinique Omicron Québec
Promyelocytes are precursor cells of the granulocytic (neutrophilic) lineage, occupying the second stage of myelopoiesis after myeloblasts - their name reflects their position in the maturation chain preceding myelocytes, then metamyelocytes, banded polynuclears and finally mature neutrophil polynuclears. In normal bone marrow, promyelocytes account for around 3 to 5 % of marrow cellularity, and are never present in the peripheral bloodstream of healthy adults outside a specific context. Morphologically, the promyelocyte is distinguished by its large size (15 to 20 µm), its voluminous nucleus with fine nucleolated chromatin (still reminiscent of myeloblast) slightly eccentric, and above all by the presence of abundant azurophilic primary granulation in the blue-violet cytoplasm - these primary granules containing myeloperoxidase, neutrophil elastase, lactoferrin and other proteolytic enzymes. These primary granules, characteristic of early granulocyte differentiation, are gradually replaced by secondary (specific) granules in later stages of maturation. Their discovery in peripheral blood - whether isolated or accompanied by other immature cells - always constitutes a significant abnormality requiring urgent hematological evaluation. While their presence in low numbers may simply reflect an intense marrow reaction to a severe infection or stimulation by G-CSF (granulocyte colony-stimulating factor), their presence in large numbers - particularly in the form of cells with hypergranular granulation and characteristic Auer rods - should immediately alert to the diagnosis of acute promyelocytic leukemia (APL or AML-M3), one of the most lethal forms of acute myeloblastic leukemia in the short term due to the fulminant coagulopathy it generates, but also one of the most curable thanks to ATRA (all-transretinoic acid) and arsenic trioxide (ATO).
Granulocyte morphology and maturation — promyelocyte positioning
- Myeloblast (Stage I): earliest precursor cell + large nucleus with very fine chromatin + 2–5 visible nucleoli + basophilic cytoplasm without granules + represents less than 5 % normal marrow
- Promyelocyte (Stage II): larger than myeloblast + slightly irregular nucleus + nucleolated + basophilic cytoplasm with numerous azurophilic primary granules (myeloperoxidase-positive) + FIRST STAGE where azurophilic granules appear + 3 to 5 % from normal marrow + absent from healthy peripheral blood
- Myelocyte (Stage III): More condensed nucleus + rounded + nucleoli not visible + appearance of specific secondary granules (gray or purple) + progressive disappearance of primary granules + represents 10-15 % of the marrow
- Metamyelocyte (Stage IV): kidney-shaped (reniform) or bean-shaped nucleus + clearly visible secondary granules + non-segmented nucleus + represents 15–20 % of the marrow
- Bands (Stage V) noyau en forme de bande ou de fer à cheval non segmenté + représente 15–20 % de la moelle + présent en faible proportion dans le sang normal (< 10 %)
- Mature polymorphonuclear neutrophil (stage VI): segmented nucleus in 2–5 lobes + abundant secondary granules + effector cell of innate immunity + represents 50–70 % of circulating leukocytes
Causes of promyelocytes in peripheral blood
| Cause | Mechanism and context | Proportion and other associated anomalies |
|---|---|---|
| Acute promyelocytic leukemia (APL — LAM-M3) — a medical emergency | Translocation t(15;17) → PML-RARA fusion → blockade of granulocyte maturation at the promyelocyte stage + accumulation of hypergranular malignant promyelocytes → release of procoagulant factors (tissue factor + ANCA) → fulminant DIC + major risk of fatal cerebral and visceral hemorrhage | Promyelocytes > 20 % + often very numerous + hypergranular + Auer rods (often in bundles - faggot cells) + DIC (prolonged PT + PTT + decreased fibrinogen + very high D-dimers + profound thrombocytopenia) + absolute therapeutic urgency must take precedence over complete diagnostic investigation |
| Leukemoid reaction (severe infections + G-CSF) | Intense bone marrow stimulation → early release of granulocyte precursors + severe bacterial infections (sepsis) + G-CSF (filgrastim + pegfilgrastim) administration + extensive burns + massive tissue necrosis | Promyelocytes in low numbers (1-3 %) + presence of myelocytes + metamyelocytes + bandemia (left shift) + without Auer rods + without associated DIC + obvious infectious or therapeutic context |
| Other acute myeloid leukemias (non-M3 AML) | Maturation arrest at various stages + AML with granulocytic differentiation (AML-M2 + AML-M4) can present with circulating promyelocytes. | Variable promyelocytes + high blast count (> 20 % blasts on marrow aspirate) + absence of t(15;17) translocation + without DIC as fulminant as in APL (except some AML-M5) |
| Myeloproliferative Neoplasms (CML + Myelofibrosis) | In chronic myeloid leukemia (CML) → hyperplasia of all granulocytic lineages → circulating promyelocytes + myelocytes + metamyelocytes (left shift) + Philadelphia chromosome (BCR-ABL1) | Promyelocytes + myelocytes + metamyelocytes + basophils + elevated eosinophils + very high neutrophils (often > 50 × 10⁹/L) + splenomegaly + positive BCR-ABL1 PCR |
Acute promyelocytic leukemia (APL) — a hematologic emergency
- Epidemiology and genetics: 10–15 % de toutes les LAM + âge médian 40–50 ans (plus jeune que les autres LAM) + translocation t(15;17)(q24;q21) dans > 95 % des cas → fusion PML-RARA (protéine de leucémie promyélocytaire + récepteur alpha de l'acide rétinoïque) + le gène de fusion PML-RARA est détectable par cytogénétique conventionnelle (caryotype) + FISH + RT-PCR (méthode la plus sensible — détecte 1 cellule parmi 10 000)
- Classic clinical presentation: massive hemorrhagic syndrome (ecchymoses + purpura + mucosal bleeding + cerebral hemorrhage) at diagnosis + biological DIC (fibrinogen < 1.5 g/L + very high D-dimers + collapsed PT + thrombocytopenia) + sometimes leukopenia + monocytopenia + general signs (asthenia + fever) + the diagnosis should be suspected in cases of unexplained hemorrhagic syndrome with pancytopenia or with hyperleukocytosis and DIC
- Clinical presentations: hyperleukocytic form (leukocyte count > 10 × 10⁹/L + poor prognosis + higher risk of differentiation syndrome) + hyperleukocytic form with hypogranular promyelocytes (M3v variant — sometimes difficult diagnosis: less typical morphology + but identical biology) + classic hyperleukocytic form (leukopenic with deep thrombocytopenia)
- Treatment Principle — ATRA + ATO: ATRA (all-trans retinoic acid — Vesanoid®) initiated emergently upon clinical suspicion BEFORE molecular confirmation → forces terminal differentiation of APL promyelocytes into mature granulocytes → resolution of DIC in 3 to 7 days + arsenic trioxide (ATO — Trisenox®) acts by degrading the PML-RARA fusion protein → apoptosis of APL cells + the combination of ATRA + ATO (APL0406 trial + GIMEMA 2013 trial — NEJM) is curative in > 95 % of standard-risk APL (leukocytes < 10 × 10⁹/L) without requiring conventional cytotoxic chemotherapy
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The golden rule of APL: initiate ATRA (all-transretinoic acid) upon clinical suspicion of APL—without waiting for cytogenetic or molecular confirmation. APL-related coagulopathy can kill the patient within hours to days before diagnosis is even confirmed. A peripheral blood smear showing hypergranular promyelocytes with bundled Auer rods + biological DIC is sufficient to start ATRA as an emergency measure. This early therapeutic decision has transformed the prognosis of APL from an almost universally fatal disease in the short term to a curable disease in over 90 % of cases.
Differentiation syndrome (formerly ATRA syndrome) — treatment complication
- Mechanism: rapid differentiation of malignant promyelocytes under ATRA and/or ATO → activation and release of cytokines (IL-1 + IL-6 + IL-8 + TNF-α) by differentiating cells → systemic inflammatory activation syndrome
- Manifestations: Fever + acute respiratory distress (pleural effusion + pericardial effusion + ARDS) + hypotension + renal failure + weight gain (fluid retention) + leukocytosis + generally occurs within 2 to 21 days after the start of ATRA
- Processing : dexamethasone 10 mg × 2/day IV → at the first signs → effective in almost all cases if initiated early + temporary interruption of ATRA if severe form + resumption upon clinical improvement + prophylaxis with dexamethasone if hyperleukocytosis > 10 × 10⁹/L at diagnosis
Absolute hematologic emergency — call 911
Call 911 or go to the nearest emergency room immediately if a complete blood count shows promyelocytes in the circulating blood + a hemorrhagic syndrome (multiple bruises + mucosal bleeding + hemoptysis + hematuria) + or if lab tests reveal pancytopenia with DIC (collapsed PT + low fibrinogen + very high D-dimers) — these findings suggest acute promyelocytic leukemia, a hematological emergency with an early mortality rate (before treatment) of 5 to 10 % from cerebral hemorrhage. ATRA should be initiated as soon as possible, ideally within one hour of diagnostic suspicion on blood smear.
All peripheral blood smears showing promyelocytes with significant granulation or Auer rods must be immediately reported by the laboratory to the requesting physician, who must refer the patient to an emergency hematology-oncology center.
Consult at Clinique Omicron
Clinique Omicron's physicians and specialized nurse practitioners (SNPs) may be called upon to see a patient whose blood smear reveals immature cells, including promyelocytes. In this context, Clinique Omicron's role is to immediately refer the patient to an emergency department or hematology center for specialized care, initiate coagulation tests (PT, aPTT, fibrinogen, D-dimers) if available urgently, and contact the on-call hematologist for advice. For all routine hematological assessments or follow-ups of abnormal CBCs, consultations are available at several service points in Quebec and via telemedicine. To make an appointment, visit cliniqueomicron.ca.
The content of this page is for informational purposes only and does not substitute for the advice of a physician or hematologist. The presence of promyelocytes in peripheral blood is always a significant abnormality requiring urgent hematologic evaluation — acute promyelocytic leukemia is a medical emergency whose prognosis depends on the rapid initiation of ATRA.
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