What ancillary study can be done on a bone marrow aspirate in almost any laboratory to determine the lineage of blasts in a patient suspected of having acute leukemia with 40% blasts in the bone marrow?

Cytochemical Stains Are the Most Accessible Method to Determine Blast Lineage in Acute Leukemia

Cytochemical stains are the most widely available ancillary study that can be performed in almost any laboratory to determine the lineage of blasts in a patient with suspected acute leukemia. 1

Rationale for Cytochemical Stains

Cytochemical stains represent a traditional and accessible approach to lineage determination that remains valuable even in the era of advanced diagnostic techniques. These stains can be performed in virtually any laboratory setting, unlike more specialized techniques that require advanced equipment or expertise.

The key cytochemical stains used for lineage determination include:

• Myeloperoxidase (MPO) or Sudan Black B (SBB): Identifies myeloid differentiation when present in ≥3% of blasts 1
• Nonspecific Esterase (NSE): Shows diffuse cytoplasmic activity in monoblasts (usually 80% positive) and monocytes (usually 20% positive), indicating monocytic differentiation 1
• Periodic Acid-Schiff (PAS): May show large globules of PAS positivity in acute erythroid leukemia 1

Advantages of Cytochemical Stains Over Other Options

1. Accessibility: Unlike flow cytometry (option A), which requires specialized equipment and expertise typically found in larger centers, cytochemical stains can be performed in almost any laboratory 1, 2

2. Simplicity: Cytochemical stains require minimal specialized equipment compared to:

   • Complete immunophenotyping (option A): Requires flow cytometers and trained personnel
   • Cytogenetics (option B): Requires cell culture facilities and specialized cytogenetic expertise
   • Molecular diagnostics (option C): Requires PCR equipment and molecular biology expertise

3. Rapid Results: Results can be available within hours, whereas cytogenetics may take days to weeks 1

4. Established Reliability: Detection of MPO (if present in ≥3% of blasts) reliably indicates myeloid differentiation 1

Diagnostic Algorithm Using Cytochemical Stains

1. First-line stains:

   • MPO or SBB: Positive in myeloid lineage (≥3% of blasts)
   • NSE: Positive in monocytic lineage
   • PAS: May show block positivity in lymphoid lineage or large globules in erythroid leukemia

2. Interpretation:

   • MPO/SBB positive: Myeloid lineage
   • NSE positive with MPO/SBB negative: Monocytic lineage
   • All cytochemical stains negative: Consider lymphoid lineage or minimally differentiated AML (M0)

Limitations and Pitfalls

While cytochemical stains are accessible, they do have limitations:

• Early myeloblasts and monoblasts may lack MPO expression 1
• SBB staining parallels MPO but is less specific 1
• Some cases of acute leukemia may have negative or inconclusive cytochemical staining patterns, particularly in minimally differentiated AML (M0) 3
• PAS positivity alone has limited specificity (81%) for lymphoblastic leukemia, but when combined with negative MPO, SBB, and NSE stains, the specificity increases to 100% 4

Modern Context

While more advanced techniques like flow cytometry are now considered the gold standard for lineage determination 2, cytochemical stains remain valuable, especially in resource-limited settings or when rapid preliminary results are needed. The National Comprehensive Cancer Network and European LeukemiaNet recognize the continued utility of cytochemical stains in the diagnostic workup of acute leukemia, particularly in settings where advanced technologies may not be readily available 1, 2.

In conclusion, cytochemical stains (option D) represent the most universally accessible method for determining blast lineage in acute leukemia that can be performed in almost any laboratory setting.