Is a serotonin release assay (SRA) necessary for confirming heparin-induced thrombocytopenia (HIT)?

Serotonin Release Assay (SRA) for HIT Confirmation

The serotonin release assay (SRA) is not routinely necessary for all suspected HIT cases but is recommended as a confirmatory test when immunoassay results are positive and clinical suspicion remains high, as it detects platelet-activating antibodies that are pathogenic in HIT.

Diagnostic Approach to HIT

Initial Testing Strategy

• The diagnosis of HIT requires careful review of clinical history and laboratory assays 1
• Pre-test clinical scores (such as the 4Ts score) should guide initial testing strategies 1
• Immunoassays should be used as the first-line screening test for HIT 1
• A negative immunoassay result can effectively rule out HIT due to high sensitivity 1

When to Consider Functional Testing

• Positive immunoassay results have modest specificity and include many patients with non-pathogenic antibodies (false-positive HIT) 1
• Over-diagnosis of HIT is common with immunoassays alone, leading to unnecessary treatments, increased costs, and bleeding risk 1
• Functional assays that detect platelet-activating heparin-dependent antibodies (like SRA) can confirm the diagnosis in cases with positive immunoassay results 1

Role of Serotonin Release Assay

Strengths of SRA

• SRA is considered the gold standard functional assay for HIT with high sensitivity (95%) and specificity (95%) 2
• SRA is especially suited for detecting highly pathogenic HIT sera containing both heparin-dependent and heparin-independent platelet-activating antibodies 2
• SRA can identify unusual forms of HIT including "autoimmune HIT" disorders (delayed-onset, persisting, spontaneous, heparin "flush," fondaparinux-associated) 2

Limitations of SRA

• SRA is technically difficult, requires human platelets from known reactive donors, and involves working with radiation 1
• SRA is only available at reference laboratories, limiting its real-time utility 1
• Recent evidence shows that SRA-negative HIT can occur in approximately 2.8% of cases, where patients have subthreshold levels of platelet-activating antibodies 3

Practical Recommendations

Sample Collection

• Blood samples should be collected from all patients with clinical suspicion of HIT 1
• Samples should be collected at least 4 hours after cessation of unfractionated heparin infusion and ≥12 hours after a dose of LMW heparin to avoid interference 1
• Collection should include both serum and citrate (0.109 M) tubes to allow for comprehensive testing 1

Testing Algorithm

• Begin with an immunoassay (such as ELISA) for anti-PF4/heparin antibodies 1
• If immunoassay is negative, HIT can be ruled out 1
• If immunoassay is positive and clinical suspicion remains high (high 4Ts score), order a functional assay like SRA for confirmation 1
• For patients with positive immunoassay but negative SRA who still have strong clinical suspicion of HIT, consider PF4-enhanced platelet activation assays 3

Interpretation of Results

• Positive laboratory confirmation of HIT/HITTS by SRA is valuable for definitive diagnosis 4
• In clinical trials of argatroban, only 57% of clinically diagnosed HIT patients had positive laboratory confirmation by SRA or other functional assays 4
• A positive SRA result in a patient with clinical features of HIT strongly supports the diagnosis and justifies continued alternative anticoagulation 1

Emerging Alternatives

• Heparin-induced multi-electrode aggregometry (HIMEA) is being developed as a more accessible functional assay 1
• Non-radioactive enzyme immunoassay techniques to quantify serotonin release from platelets provide reliable alternatives to traditional radioactive SRA 5
• Automated rapid immunoassays such as chemiluminescence immunoassay (CLIA) and latex immunoturbidimetric assay (LIA) are facilitating real-time laboratory diagnosis 2

Remember that the diagnosis of HIT remains a clinical-pathological diagnosis requiring both laboratory evidence and appropriate clinical presentation 2. While SRA is valuable for confirmation, treatment decisions often need to be made before results are available.