Is the Ristocetin Induced Platelet Aggregation (RIPA) test 100% sensitive at 0.5 mcg/mL of Ristocetin (restocin) with a platelet count of 155,000/μL (thrombocytopenia)?

Ristocetin-Induced Platelet Aggregation (RIPA) Test Sensitivity in Thrombocytopenia

The RIPA test at 0.5 mg/mL ristocetin is not 100% sensitive with a platelet count of 155,000/μL, and additional testing should be performed for definitive diagnosis of platelet function disorders.

Limitations of RIPA Test in Mild Thrombocytopenia

The RIPA test has important limitations when used as a standalone diagnostic tool, especially in patients with borderline platelet counts:

• A platelet count of 155,000/μL (mild thrombocytopenia) can affect the reliability of RIPA test results, potentially leading to false negatives or false positives 1
• The RIPA test at low ristocetin concentrations (0.5 mg/mL) is primarily designed to detect enhanced ristocetin response seen in Type 2B von Willebrand Disease (VWD) and platelet-type VWD, not general platelet function disorders 2
• Test sensitivity varies based on laboratory technique, reagent quality, and platelet donor characteristics 1

Recommended Additional Testing

For a comprehensive evaluation of platelet function with a borderline platelet count of 155,000/μL, consider:

1. Expanded Light Transmission Aggregometry (LTA) with multiple agonists:

   • ADP, collagen, arachidonic acid, and additional agonists like α-thrombin, TRAP-6, U46619, CRP, convulxin 1
   • This provides a more complete picture of platelet function across multiple activation pathways

2. VWF Panel to rule out von Willebrand Disease:

   • VWF:Ag, VWF:RCo, FVIII activity
   • VWF:RCo/VWF:Ag ratio (ratio <0.5-0.7 suggests Type 2 VWD) 2
   • VWF multimer analysis if VWF:RCo/VWF:Ag ratio is abnormal 2

3. Flow Cytometry for platelet surface glycoproteins:

   • Evaluates GPIIb/IIIa, GPIb/IX, and other surface markers
   • Can detect qualitative platelet defects even with normal aggregation tests 1

4. Granule Release Assays:

   • Measures ATP/ADP release and α-granule secretion
   • Can identify storage pool deficiencies that may be missed by aggregation tests alone 1

Interpretation of RIPA Results

When interpreting RIPA results at 0.5 mg/mL ristocetin with a platelet count of 155,000/μL:

• Enhanced aggregation (threshold <0.7 mg/mL) suggests Type 2B VWD or platelet-type VWD 3
• Normal aggregation does not exclude other platelet function disorders or different VWD subtypes 2
• Reduced aggregation may indicate Type 1 or Type 3 VWD, Bernard-Soulier Syndrome, or other platelet disorders 4

Important Considerations

• The International Society on Thrombosis and Haemostasis (ISTH) recommends a comprehensive approach to diagnosing platelet function disorders rather than relying on a single test 1
• Mild thrombocytopenia (platelet count 150,000-450,000/μL range) can affect test results without necessarily indicating a primary platelet disorder 1
• Medications (including NSAIDs, antiplatelet agents) can affect RIPA results and should be discontinued at least 7-10 days before testing 1
• Test conditions matter significantly - hirudin anticoagulation provides better sensitivity than citrate for platelet function testing 1

When to Consider Repeat Testing

Consider repeat testing if:

• Initial results are borderline or inconclusive
• Strong clinical suspicion persists despite normal test results
• Patient was taking medications that could interfere with test results
• Sample collection or processing issues occurred

For accurate diagnosis, testing should be performed when the patient is not actively bleeding and has not taken interfering medications for at least 7 days 1.