Is the ristocetin-induced platelet aggregation (RIPA) test 100% sensitive in detecting type 2 von Willebrand disease with a normal platelet count?

Ristocetin-Induced Platelet Aggregation Test is Not 100% Sensitive for Type 2B von Willebrand Disease

The ristocetin-induced platelet aggregation (RIPA) test is not 100% sensitive in detecting Type 2B von Willebrand disease, even with a normal platelet count of 155, and additional specific testing is required for definitive diagnosis. 1

Limitations of RIPA Testing

• Traditional VWF:RCo assays (which include RIPA) have significant technical limitations:

  • Poor sensitivity with detection limits >10-20 IU/dL
  • High coefficients of variation (up to 20-30%)
  • Potential for false negatives, especially in milder cases 2

• A normal platelet count (such as 155) does not exclude Type 2B VWD, as thrombocytopenia may be intermittent or absent in some cases 1

• Single measurements showing normal VWF:RCo/VWF:Ag ratios can miss Type 2 VWD, necessitating repeat testing 3

Factors Affecting Test Accuracy

• Pre-analytical variables that can affect results:

  • Improper sample handling (samples should be processed at room temperature and tested within 2 hours)
  • VWF level fluctuations due to stress, exercise, or inflammation 1

• Genetic variants (e.g., p.P1467S, p.D1472H) can cause falsely decreased VWF:RCo levels without clinical significance 2

• The traditional RIPA test measures two parameters simultaneously:

  1. Binding of ristocetin to VWF
  2. Binding of ristocetin-activated VWF to test platelets

  This dual measurement introduces additional variables that can affect sensitivity 2

Recommended Diagnostic Approach

For accurate diagnosis of Type 2B VWD, a comprehensive testing panel is required:

1. Initial screening:

   • VWF antigen (VWF:Ag)
   • VWF activity (VWF:RCo or newer assays)
   • Factor VIII activity
   • Complete blood count 1

2. Specific diagnostic tests:

   • VWF:RCo/VWF:Ag ratio (ratio <0.5-0.7 suggests Type 2 VWD)
   • VWF multimer analysis (selective loss of high-molecular-weight multimers)
   • Genetic testing for mutations in the A1 domain of the VWF gene 1

3. Additional testing considerations:

   • RIPA mixing studies at lower doses of ristocetin (0.5 mg/mL) to differentiate Type 2B VWD from platelet-type VWD 4
   • Repeat functional testing of platelet aggregation, VWF:RCo, and VWF:CB if clinical suspicion remains high despite initial normal results 3

Newer Diagnostic Approaches

More advanced testing methods with improved sensitivity and specificity include:

• VWF:GPIbM (gain-of-function mutant GPIb fragment) assays
• VWF:GPIbR (recombinant GPIb fragment) assays
• VWF:Ab (monoclonal antibody to VWF A1 domain) assays 2, 1

These newer assays may better differentiate between VWD Type 2A and 2B than traditional RIPA testing 1.

Clinical Implications

The limited sensitivity of RIPA testing has important clinical implications:

• False negatives can lead to missed diagnoses and inadequate treatment of bleeding disorders
• Patients with typical bleeding histories should undergo comprehensive and repeated testing if clinical suspicion remains high
• The PFA-100 platelet function analyzer may be useful as an initial screening tool when VWD is suspected 3

Remember that a single normal RIPA test result does not conclusively rule out Type 2B VWD, particularly when clinical symptoms suggest the diagnosis.