Acute myeloid leukaemia
The morphological forms of blast cells in blood do not allow AML to be classified without additional samples and tests.
However, when describing blasts on blood films it is helpful to try and ascribe the cells to a morphological type, since these criteria still form the basis of division when other class-defining features (genetic or cytogenetic) are not found (AML not otherwise classified).
AML with minimal differentiation (FAB M0, 5% of AML)
Major features: Typically blasts cells have medium size with the typical diffuse chromatin pattern similar to AML without maturation, but appearances may vary. Nuclei may be round, angular or indented (usually with one or more visible nucleoli); the cytoplasm is agranular; the degree of cytoplasmic basophilia may vary between cases. Cases may sometimes resemble lymphoblasts and rarely monoblasts.
AML without maturation (FAB M1, 10-20% of AML)
Major features: Most often this form of AML has blasts of medium or large size. Nuclei may vary but are most often round, and a nucleolus (or several) will usually be found, cytoplasm is basophilic and usually agranular or granules may be indistinct (in marrow fewer then 10% of cells in marrow will have granules); Auer rods may be present.
AML with maturation (FAB M2, 30-40% of AML)
Major features: The blast cells usually are of medium size with primitive chromatin often with a visible nucleolus, and is most often offset by the Golgi body (visible as a pale area adjacent to the nucleus). Cytoplasm is basophilic with granulation that may be indistinct or obvious (in marrow this must involve >10% of cells). The granules may coalesce and form one or more masses, cells with Auer rods are often found. Normal or abnormal maturing forms (often including neutrophils) typically accompany the blast cell population.
Acute promyelocytic leukaemia (FAB M3, 10-15% of AML)
There are two forms of APL:
Hypergranular APL: The number of abnormal cells in blood is usually low. The abnormal promyelocytes tend to be large, and to vary markedly in size. The most characteristic features of the cells are the densely packed granules in cytoplasm which are coarse and red or purple and may obscure the nuclear appearance. In some cells, rod-like structures may occur in bundles (basket cells), but this is more frequently observed in marrow samples than in blood.
Microgranular (hypogranular) APL: The blasts are generally more numerous than in the hypergranular form and have a distinctive appearance - most commonly there is a bi-lobed nucleus with very dispursed chromatin that makes its appearance almost transparent. There is little or no granulation, although occasional hypergranular forms or Auer rods may be detected (particularly in marrow).
Acute myelomonocytic leukaemia (FAB M4, 10-15%)
Major features: The appearance is mixed, with cells that have irregular nuclear shape and scattered granulation or Auer rods suggesting granulocytic maturation and other cells that have the appearance of more typical monoblasts with a round or folded nucleus that has lacy nuclear chromatin and basophilic cytoplasm (with or without fine azurophilic granules and vacuolation).
Acute monoblastic and monocytic leukaemia (FAB M5, 10-20% of AML)
Monoblastic (FAB M5a): These have a roughly circular nucleus, delicate lacy chromatin, and abundant, often basophilic cytoplasm. These cells may also have cytoplasmic budding or extension
Monocytic (FAB M5b): The monoblasts have more obvious mature features, with convoluted nucleus and the cytoplasm that may contain metachromatic granules and vacuoles (promonocytes are regarded as blast equivalents)
Acute erythroid leukaemia (FAB M6, <5% of AML)
Major features The blast cells have nuclei that are usually round, but may be irregular with lobulation, and cytoplasm may show the intense basophilia associated with early erythroid forms although this is by no means always the case and appearances can be non-specific.
Acute megakaryoblastic leukaemia (FAB M7, <5% of AML)
Major features: Medium to large-sized blasts. Typically the nucleus is round (or occasionally indented) with one or more nucleoli, and may have a high nucleocytoplasmic ratio (similar to lymphoblasts). The cytoplasm of the typical blasts is agranular and generally basophilic, pseudopod formation may be seen.
Acute basophilic leukaemia (rare)
Major features: Medium to large sized blasts that may show very few or absent granules and have basophilic characteristics discovered only by immunophenotype, through to cells with very obvious basophilic granulation and differentiation.