How is Glanzmann thrombasthenia diagnosed?

Diagnosis of Glanzmann Thrombasthenia

Glanzmann thrombasthenia is diagnosed through a stepwise approach starting with light transmission aggregometry (LTA) showing absent/severely reduced platelet aggregation to multiple agonists (ADP, collagen, arachidonic acid, epinephrine) with preserved ristocetin response, followed by flow cytometry demonstrating reduced or absent GPIIb/IIIa (CD41/CD61) expression, and confirmed by genetic testing of ITGA2B and ITGB3 genes 1.

Algorithmic Diagnostic Approach

Step 1: Clinical Presentation Recognition

Look for these specific features:

• Mucocutaneous bleeding starting in early childhood (epistaxis, easy bruising, petechiae, menorrhagia, gastrointestinal bleeding) 2, 3
• Normal platelet count (typically 319 ± 93 × 10⁹/L) 4
• Normal platelet morphology on blood smear (distinguishes from Bernard-Soulier syndrome which has giant platelets)

Step 2: First-Line Laboratory Testing

Light Transmission Aggregometry (LTA) - The gold standard 1:

• Test with standard agonist panel: ADP (2.5 mg/mL), collagen (1 mg/mL), arachidonic acid (0.5 mg/mL), epinephrine
• Diagnostic pattern: Absent or severely reduced aggregation to ALL physiological agonists
• Preserved response to ristocetin (1.5 mg/mL) - this distinguishes GT from von Willebrand disease and Bernard-Soulier syndrome 4
• Requires 21-28 mL of blood 1

PFA-100 testing is NOT recommended due to insufficient specificity/sensitivity 1.

Step 3: Flow Cytometry for Subtype Classification

Essential antibodies on resting platelets 1, 5:

• CD41 (GPIIb) and CD61 (GPIIIa) - quantify αIIbβ3 integrin expression
• CD42a and CD42b (GPIb/IX) - should be normal (excludes Bernard-Soulier syndrome)

Subtype classification based on CD41/CD61 expression 5, 6:

• Type I GT: <5% of normal GPIIb/IIIa expression (most common, 47% of cases)
• Type II GT: 5-20% of normal expression (11.8% of cases)
• Type III/Variant GT: Near normal levels (>20%) but qualitatively defective (41.2% of cases)

Important caveat: Mean CD41 expression is typically lower than CD61 expression in GT patients 5. Type III patients have significantly fewer severe bleeders compared to Type I/II 5.

Step 4: Confirmatory Genetic Testing

Sequence both genes 3:

• ITGA2B (encodes GPIIb/αIIb subunit)
• ITGB3 (encodes GPIIIa/β3 subunit)
• Analyze all exons including exon/intron boundaries
• If intronic mutations suspected, perform RT-PCR on platelet-derived RNA for splicing analysis

Genetic heterogeneity: 27 different mutations identified across both genes, with extensive variability 3. Approximately 16% of clinically suspected GT cases may have no identifiable mutations in these genes, suggesting regulatory element defects 3.

Alternative Testing Methods

Whole blood impedance aggregometry (MEA/Multiplate) can be used as an alternative to LTA 4:

• Uses whole blood (simpler than LTA which requires platelet-rich plasma)
• Shows absent aggregation to all agonists except ristocetin
• May detect minimal collagen response (14.3 ± 7 μg) in some Type II/III patients, potentially differentiating subtypes 4
• Must test within 4 hours of collection

Second-Line Tests (If Diagnosis Unclear)

Expand testing with 1:

• Additional LTA agonists: α-thrombin, TRAP-6, U46619, CRP, convulxin, PAR-4 peptide
• Clot retraction: Absent in Type I, reduced in Type II, may be preserved (>60%) in Type III 6
• Expanded flow cytometry: CD31, CD49b (GPIa/IIa), CD36 (GPIV), GPVI to exclude other platelet disorders

Critical Pitfalls to Avoid

1. Do not rely on bleeding time or PFA-100 - insufficient diagnostic accuracy 1
2. Always test ristocetin response - preserved response distinguishes GT from von Willebrand disease and Bernard-Soulier syndrome
3. Check CD42a/CD42b - must be normal to exclude Bernard-Soulier syndrome 4
4. Type III GT can be missed - near-normal glycoprotein expression but qualitative defect requires careful interpretation 5
5. Carrier detection possible: Family members with <35% normal GPIIb/IIIa levels may be carriers 6