How to investigate giant platelets further?

Investigation of Giant Platelets

The diagnostic approach to giant platelets should begin with first-step tests including blood smear examination, light transmission aggregometry (LTA), assessment of platelet granule release, and flow cytometry analysis of platelet surface glycoproteins to identify specific inherited platelet function disorders (IPFD). 1

Initial Evaluation

When giant platelets are identified on a peripheral blood smear, a systematic approach is needed to determine the underlying cause:

1. Blood smear examination: Careful examination of platelet size and morphology, looking for:

   • Platelet size variation
   • Presence of other cell abnormalities (e.g., neutrophil inclusion bodies in MYH9-related disorders)
   • Stomatocytosis or other red cell abnormalities

2. Clinical history assessment:

   • Bleeding history and severity
   • Family history of thrombocytopenia or bleeding disorders
   • Presence of syndromic features (hearing loss, cataracts, renal dysfunction)
   • Drug intake that might affect platelet function

First-Step Laboratory Tests

The following tests should be performed as part of the initial investigation 1:

1. Light Transmission Aggregometry (LTA):

   • Test with epinephrine, ADP, collagen, arachidonic acid, and ristocetin
   • Specific patterns can suggest different disorders:
     • Absent ristocetin-induced aggregation suggests Bernard-Soulier Syndrome (BSS)
     • Normal aggregation with ADP/collagen but absent with ristocetin is characteristic of BSS 2

2. Platelet Granule Release Assessment:

   • Measure ATP/ADP release (dense granules)
   • Assess α-granule release markers
   • Defective granule release may indicate Gray Platelet Syndrome, storage pool deficiencies, or other IPFDs

3. Flow Cytometry Analysis:

   • Evaluate major platelet surface glycoproteins:
     • GPIIb/IIIa (CD41)
     • GPIIIa (CD61)
     • GPIb (CD42b)
     • GPIb/IX (CD42a)
   • Reduced GPIb/IX expression is diagnostic for Bernard-Soulier Syndrome 2, 3

Second-Step Tests

If first-step tests are inconclusive, proceed with 1:

1. Expanded LTA panel:

   • Additional agonists: α-thrombin, TRAP-6, U46619, CRP, convulxin, PAR-4 activating peptide, PMA, and A23287

2. Extended flow cytometry:

   • Additional glycoproteins: GPIa/IIa (CD31, CD49b), GPIV (CD36), and GPVI
   • Assessment of platelet procoagulant activity (annexin V binding)

3. Transmission Electron Microscopy (TEM):

   • Evaluate platelet ultrastructure
   • Assess granule content and distribution

4. Genetic testing:

   • Targeted gene sequencing based on suspected disorder:
     • Bernard-Soulier Syndrome: GP1BA, GP1BB, GP9 genes
     • MYH9-related disorders: MYH9 gene
     • Gray Platelet Syndrome: NBEAL2 gene

Common Giant Platelet Disorders

Bernard-Soulier Syndrome (BSS)

• Characterized by giant platelets, thrombocytopenia, and bleeding tendency
• Diagnostic findings:
  • Absent ristocetin-induced platelet aggregation
  • Reduced GPIb/IX expression on flow cytometry
  • Normal aggregation with ADP or collagen 2, 4
• Often misdiagnosed as immune thrombocytopenia (ITP) 3

MYH9-Related Disorders

• Include May-Hegglin anomaly, Sebastian syndrome, Fechtner syndrome
• Associated with Döhle-like inclusion bodies in leukocytes
• May have extra-hematological manifestations (hearing loss, cataracts, nephritis)

Gray Platelet Syndrome

• Characterized by defective α-granule formation
• Platelets appear gray on Wright-Giemsa stain
• Defective α-granule release on testing

Clinical Pitfalls

1. Misdiagnosis as ITP: Giant platelets are frequently misdiagnosed as ITP, leading to inappropriate treatment with steroids 3. Always consider inherited disorders when ITP is treatment-resistant.

2. Specimen collection challenges: Giant platelets may be difficult to separate from other blood cells. Whole blood assays may be more reliable than methods requiring platelet-rich plasma 2.

3. Acquired conditions: Not all giant platelet disorders are inherited. Consider acquired causes such as myelodysplastic syndromes 5.

4. Automated counters: Automated blood counters may underestimate platelet counts in patients with giant platelets, as large platelets may be counted as small lymphocytes.

By following this systematic approach, the specific cause of giant platelets can be identified, allowing for appropriate management and genetic counseling when indicated.