Making The Diagnosis

On a routine complete blood count (CBC), most patients with AML are anemic and thrombocytopenic.78 The white blood count is variable, with 20% of patients having white blood counts less than 5000/mm3 and 20% of patients with white blood counts greater than 100,000/mm3.78 High white blood counts and hyperleukocytosis are more common in the monocytic leukemias.36 79 Although blasts are usually present in the peripheral blood, a subset of patients present with "aleukemic leukemia."

A review of the peripheral blood film and a bone marrow aspirate/biopsy is part of the initial diagnostic work-up and is essential for distinguishing AML from other hematologic disorders such as ALL, MDS, or AML arising in the setting of MDS (Table 3.1).3 Occasionally, immature blasts can be confused with metastatic carcinoma, plasma cell neoplasms, or lymphoma.77 The peripheral blood film and bone marrow slides should be air-dried and stained with a polychrome dye such as Wright-Giesma.12

Based on the new WHO criteria, the diagnosis of AML is made when at least 20% of nucleated cells in the bone marrow or peripheral blood are myeloid blasts.11 The previous FAB classification system required 30% blasts to make a diagnosis of AML. The major reason for lowering the blast threshold is that patients with 20-30% blasts (previously considered as RAEB-t, refractory anemia with excess blasts in transformation) have an identical prognosis to those with 30% blasts.2 The percentage of blasts should be determined on a 500 cell count on well-stained bone marrow aspirate slides or a 200 cell differential on peripheral blood smears.1011 In certain

Table 3.1 Specimen submission requirements and diagnostic utility

Diagnostic study

Specimen requirements

Tests performed

Diagnostic utility

Comments

CBC and differential

2.5 mL whole blood in 4 mL (EDTA) lavender top tube. Fill tube to at least half of fill volume

Automated CBC with differential; manual review of smear

Determine absolute leukocyte count and blast count; assess for anemia and thrombocytopenia; evaluate blast morphology; rule out quantitative or qualitative abnormalities in other cell types, including dysplastic features, presence of nucleated red blood cells, and/or microangiopathic changes

Blood remaining after CBC may be used for cytochemistry, flow cytometry, and molecular analysis

Bone marrow aspirate

Bone marrow aspirate smears with extra unstained smears for cytochemistry, iron stain, or other studies as necessary

Wright-Giemsa stain for routine morphology and cell differential count

Determine blast percentage and evaluate blast morphology; assess quantitative and qualitative abnormalities in myeloid, erythroid, and megakaryocyte lineages

High quality smears are essential for accurate diagnosis; iron stain useful to assess iron stores and presence of ringed sideroblasts; make touch preps of biopsy if dry tap

Bone marrow biopsy

Bone marrow core biopsy ideally >1 cm in length. Place in appropriate fixative (B5, acid zinc formalin, or buffered formalin)

Hematoxylin-eosin stain for routine histologic examination with immunostains (CD34, TdT, CD79a, Hgb, CD20, CD3, MPO, CD61, CD10, and CD31) as necessary

Determine overall cellularity and percentage and lineage of blasts; assess residual normal hematopoietic elements; evaluate for associated fibrosis and/or dysplastic features; rule out associated disorders that mimic leukemia

Immunostains may aid in diagnosis, especially in the absence of flow cytometry (dry tap)

Cytochemical stains

Unfixed, fresh air-dried bone marrow aspirate smears

May include MPO, nonspecific esterase, chloroacetate esterase, PAS, and Sudan black B

Cytochemical stains

Unfixed, fresh air-dried bone marrow aspirate smears

May include MPO, nonspecific esterase, chloroacetate esterase, PAS, and Sudan black B

Assess lineage and differentiation of blasts. MPO and Sudan black B are used as sensitive markers of myeloid differentiation (positive defined as staining >3% blasts). Nonspecific esterase is a marker of monocytic differentiation. Abnormal erythroid precursors show "block-like" cytoplasmic staining with PAS

Negative MPO and/or SBB stains do not exclude a diagnosis of AML; enzyme activity degrades over time, so fresh unfixed material is necessary table continues

0 0

Post a comment