What labs should be run for a patient with leukopenia (low White Blood Cell (WBC) count) and thrombocytopenia (low platelet count)?

Laboratory Evaluation for Very Low WBC and Platelets

When a patient presents with leukopenia (low white blood cell count) and thrombocytopenia (low platelet count), a comprehensive laboratory workup should include a complete blood count with differential, coagulation studies, bone marrow examination, and specific tests for underlying causes.

Initial Laboratory Evaluation

• Complete Blood Count (CBC) with differential and peripheral blood smear examination to confirm true thrombocytopenia and leukopenia, identify potential alternative diagnoses, and assess for abnormal cells 1
• Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT) to evaluate coagulation function 1
• D-dimer measurement and fibrinogen assay to assess for disseminated intravascular coagulation 1
• Liver function tests to rule out liver disease as a cause 1
• Chemistry profile to evaluate renal function and nutritional status 1

Bone Marrow Evaluation

• Bone marrow aspiration and biopsy should be considered, especially if:
  • Abnormal cells are seen in peripheral blood 1
  • No obvious cause is identified from initial testing 1
  • There is suspicion for hematologic malignancy 1

Specific Disease Testing

For Hematologic Malignancies

• Flow cytometry of peripheral blood and/or bone marrow to identify leukemia or lymphoma 1
• Cytogenetic analysis and molecular studies (including FISH, RT-PCR, and next-generation sequencing) to detect genetic abnormalities associated with leukemia 1
• Testing for BCR-ABL1 and other gene fusions if acute lymphoblastic leukemia is suspected 1

For Immune-Mediated Causes

• Anti-platelet antibody testing if immune thrombocytopenia is suspected 1
• Antinuclear antibody (ANA) test to screen for autoimmune disorders 1
• Direct antiglobulin test (Coombs) to evaluate for immune-mediated hemolysis 1

For Infectious Causes

• HIV testing, as HIV infection can cause both cytopenias 1
• Testing for rickettsial diseases if clinically suspected (particularly important in febrile patients with rash) 1
• Blood cultures if infection is suspected 1

For Drug-Induced Causes

• Review of medication history for drugs associated with bone marrow suppression 1
• HLA B*5701 testing if abacavir therapy is being considered (particularly in HIV patients) 1

For Vaccine-Related Concerns

• Anti-PF4 antibody testing if vaccine-induced immune thrombocytopenia and thrombosis (VITT) is suspected 1

Special Considerations

• If von Willebrand disease is suspected as a cause of bleeding with thrombocytopenia, include VWF antigen, VWF ristocetin cofactor activity, and factor VIII coagulant activity 1
• For suspected inherited platelet function disorders, consider light transmission aggregometry with various agonists (epinephrine, ADP, collagen, arachidonic acid, and ristocetin) 1
• For patients with suspected leukemia, additional tests for minimal residual disease may be needed for risk stratification 1

Pitfalls to Avoid

• Do not assume that leukopenia and thrombocytopenia are always due to the same cause; multiple pathologies may coexist 1
• Avoid delaying bone marrow examination in patients with unexplained cytopenias, especially when clinical suspicion for malignancy is high 1
• Be aware that normal platelet count at presentation does not rule out developing thrombocytopenia; repeat testing may be necessary if clinical suspicion remains high 1
• Consider that some inherited platelet disorders may present with mild thrombocytopenia and should not be excluded from further testing 1