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Blasts on the film and cytochemistry

15 min

  • Recognize myeloblasts and Auer rods that call for an acute myeloid leukemia workup
  • Recognize lymphoblasts that call for an acute lymphoblastic leukemia workup
  • Match an MPO, Sudan black B, or esterase pattern to blast lineage

Read the full reference

Try first

Try first

A film shows occasional blasts with fine chromatin, a nucleolus, and scant blue cytoplasm. The WBC count is 4.2 × 10³/µL, within the reference interval. What do you do?

The next section explains it.

Right. The next section explains why.

The next section explains it.

The next section explains it.

Get the idea

Blasts and Auer rods

Acute myeloid leukemia (AML) can present with a normal or low WBC count, and anemia, thrombocytopenia, and neutropenia can accompany few circulating blasts.1 A myeloblast has fine chromatin, one or more nucleoli, and scant blue cytoplasm. An Auer rod is a red, needle-shaped cytoplasmic inclusion that adds weight to the finding when present.1 Dismissing blasts because the total count is not high delays the workup.

Two blast sizes, one shared limit

Lymphoblasts fall into two size classes. The more common small form measures 1.0 to 2.5 times a normal lymphocyte, with scant blue cytoplasm and indistinct nucleoli. The larger form measures 2 to 3 times lymphocyte size, with a prominent nucleolus and an irregular nuclear membrane, and it can resemble a myeloblast.2 Reactive lymphocytes vary in size and chromatin pattern and carry abundant cytoplasm, so calling a repeating blast population reactive delays the urgent workup. Morphology establishes that a blast process is present. Lineage and classification need immunophenotyping and genetics.2

Reading a cytochemical stain

Cytochemical stains support lineage when morphology is ambiguous, and a stain pattern cannot establish a disease entity by itself.2 Myeloperoxidase (MPO) stains granulocytes and Auer rods strongly, monocytes weakly, and lymphoid cells not at all.2 Naphthol AS-D chloroacetate esterase, the specific esterase, marks granulocytic cells including Auer rods. α-Naphthyl acetate or butyrate esterase, the nonspecific esterase, shows strong diffuse positivity in monocytic cells, confirmed by inhibition with added sodium fluoride. Megakaryoblasts and lymphoblasts can show focal or dot positivity on the nonspecific esterase, so a reaction called monocytic without the fluoride-inhibition step can belong to a nonmonocytic lineage.2 A stain pattern supports a lineage. Naming the leukemia type takes immunophenotyping and genetic testing.2

References
  1. Arber DA, Borowitz MJ, Cessna M, et al. Initial diagnostic workup of acute leukemia: guideline from the College of American Pathologists and the American Society of Hematology. Arch Pathol Lab Med. 2017;141(10):1342-1393. doi:10.5858/arpa.2016-0504-CP
  2. Keohane EM, Preston MM, Mirza KM, Walenga JM, eds. Rodak's Hematology: Clinical Principles and Applications. 7th ed. Elsevier; 2025. Accessed September 26, 2026. https://www.us.elsevierhealth.com/rodaks-hematology-9780323936507.html

Watch one

A marrow film shows a blast population with strong diffuse cytoplasmic positivity on the α-naphthyl acetate esterase stain. What do you do to read this pattern?

  1. Read the pattern: the positivity is strong and diffuse, the pattern monocytic cells show. Megakaryoblasts and lymphoblasts typically show only focal or dot positivity.

    Nonspecific esterase can show focal positivity in nonmonocytic blasts, so the pattern alone is not yet conclusive.

  2. Run the fluoride-inhibition step: the reaction is markedly reduced with added sodium fluoride.

    Sodium fluoride inhibits the nonspecific esterase reaction in monocytic cells specifically, so it separates a true monocytic reaction from one that only resembles it.

  3. Check MPO and the specific esterase on the same blasts: both are negative.

    MPO and the specific esterase mark the granulocytic component of a mixed population, so checking them shows whether granulocytic cells are also present.

  4. Record the pattern as supporting monocytic differentiation, and refer the specimen for immunophenotyping and genetic testing to classify the leukemia.

    A stain pattern supports a lineage, and the leukemia type itself needs more than morphology and cytochemistry.

The fluoride-inhibited, diffusely positive nonspecific esterase with negative MPO and specific esterase supports monocytic differentiation. Immunophenotyping and genetic testing classify the leukemia.

Your turn

Problem 1 of 3

A film shows abnormal promyelocytes packed with granules and cells with multiple Auer rods, and the same patient has thrombocytopenia with a low fibrinogen. What does the laboratory do?

Incorrect. Suspected acute promyelocytic leukemia is a medical emergency, so notification comes first, and rapid PML::RARA testing and coagulation studies proceed without waiting for the complete workup.

Incorrect. AML with PML::RARA has no fixed minimum blast percentage, and the urgency comes from the coagulopathy.

Correct. In suspected acute promyelocytic leukemia the coagulopathy can be severe, so abnormal promyelocytes or multiple Auer rods with thrombocytopenia, prolonged clotting times, low fibrinogen, or increased D-dimer call for immediate notification under your laboratory's urgent-result procedure.

Hint
  1. Look at the granules on the abnormal cells together with the coagulation findings.
  2. Acute promyelocytic leukemia has no fixed minimum blast percentage.
  3. The coagulopathy is what makes this urgent, whatever the blast count.

Review Acute myeloid leukemia

Problem 2 of 3

A film from a child with fatigue and bruising shows a repeating population of small cells with round nuclei, indistinct nucleoli, and scant blue cytoplasm, similar in size across the population. What do you report?

Reactive lymphocytes vary in size and chromatin pattern and carry abundant cytoplasm. A population that repeats with the same fine chromatin and scant cytoplasm is not that variable pattern.

Counted lymphoblasts as reactive lymphocytes

Lymphoblasts have fine chromatin, a high nucleus-to-cytoplasm ratio, and scant blue cytoplasm, with nucleoli in the larger form, and similar cells repeat across the film. Reactive lymphocytes vary in size and chromatin and have abundant cytoplasm. Calling a blast population reactive delays the urgent acute-leukemia workup.

Fine chromatin, a high nucleus-to-cytoplasm ratio, and scant blue cytoplasm that repeats across the film describe lymphoblasts. The finding calls for urgent referral.

Normal small lymphocytes have dark, condensed chromatin. This population's fine chromatin and indistinct nucleoli describe a lymphoblast.

Hint
  1. Compare this population against the variation expected in reactive lymphocytes.
  2. A repeating, similar-looking population across the film is a clue in itself.

Review Morphology and immunophenotype

Problem 3 of 3

A different blast population shows strong diffuse α-naphthyl acetate esterase positivity. No fluoride-inhibition step was run. What do you do before reporting a lineage?

A strong diffuse pattern still needs the fluoride-inhibition step. Without it, a nonmonocytic reaction can be read as monocytic.

Called esterase positivity monocytic without fluoride

Monocytic cells show strong diffuse nonspecific esterase that sodium fluoride inhibits. Megakaryoblasts and lymphoblasts can show focal or dot positivity. Without the inhibition step, a nonmonocytic reaction can be called monocytic. A stain pattern supports a lineage. Naming the leukemia type takes immunophenotyping and genetic testing.

Monocytic cells show strong diffuse nonspecific esterase that sodium fluoride inhibits. The inhibition step is what confirms the lineage the pattern suggests.

The stain is read once the inhibition step is added. Running fluoride settles what the uninhibited pattern only suggests.

Review Special stains

Use it

Try this at the MicroscopePendingWBC0 / 100PENDINGPeripheral blood differentialPerform manual diff

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