What is the primary lab order to check for malignant cells on a blood smear?

Blood Smear Analysis for Malignant Cells

The primary lab order to check for malignant cells on a blood smear is a peripheral blood smear examination, which should be specifically requested as part of a complete blood count (CBC) with differential and manual review. 1, 2

Components of the Order

• Complete Blood Count (CBC) with differential and peripheral blood smear examination 1, 2
  • This should explicitly request "manual review" or "pathologist review" to ensure the smear is examined for malignant cells 2
  • The order should specify "evaluation for malignant cells" to direct attention to this specific concern 2, 3

When to Order

A peripheral blood smear examination for malignant cells is indicated in the following situations:

• Abnormal automated CBC results with flags for blast cells or atypical lymphocytes 2, 4
• Unexplained cytopenias (anemia, thrombocytopenia, leukopenia) 5
• Unexplained leukocytosis or other CBC abnormalities 5
• Suspected hematologic malignancy based on clinical presentation 3, 5
• Follow-up of known hematologic malignancy 1

Examination Process

The examination process for malignant cells on a blood smear typically includes:

• Blood smear scan: Initial verification of automated hematology results and determination if manual differential is needed 2
• Manual differential leukocyte count: Systematic examination of white blood cells with morphologic assessment 1, 2
• Blood smear review: Comprehensive examination by a laboratory professional or pathologist 2, 3

Additional Testing Considerations

If malignant cells are suspected or identified on peripheral blood smear:

• Bone marrow examination is typically the next step for definitive diagnosis 1
• Flow cytometry immunophenotyping may be performed on peripheral blood if sufficient abnormal cells are present 1
• Cytogenetic studies including FISH and/or conventional karyotyping may be indicated 1
• Molecular studies may be needed for specific genetic abnormalities 1

Important Caveats

• A peripheral blood smear alone may not be sufficient for definitive diagnosis of malignancy 1
• False negatives can occur if malignant cells are not circulating or present in low numbers 2, 5
• Proper specimen collection and smear preparation are critical for accurate assessment 3, 6
• For suspected leukemia, bone marrow examination remains the gold standard diagnostic test 1
• In cases with dry tap or inability to obtain bone marrow, peripheral blood with sufficient blast cells may be used as an alternative 1

Remember that while the peripheral blood smear is an important screening tool, definitive diagnosis of hematologic malignancies typically requires additional specialized testing including bone marrow examination, flow cytometry, cytogenetics, and molecular studies. 1