What labs are recommended to diagnose heparin-induced thrombocytopenia (HIT) in an adult patient with thrombocytopenia and/or thrombosis?

Laboratory Diagnosis of Heparin-Induced Thrombocytopenia

When HIT is suspected, immediately order basic coagulation studies (PT, aPTT, fibrinogen, D-dimers) and calculate the 4Ts score to determine whether anti-PF4 antibody testing is needed; if the 4Ts score is intermediate (4-5) or high (6-8), order an immunoassay (ELISA or chemiluminescent assay) for anti-PF4 antibodies as first-line testing, and if positive with significant titer, confirm with a functional assay such as the serotonin release assay (SRA). 1, 2, 3

Initial Laboratory Workup

When HIT is suspected, begin with basic hemostasis tests before specific HIT testing:

• Order PT, aPTT, fibrinogen, and D-dimers to evaluate for disseminated intravascular coagulation (DIC), which can occur in severe HIT and does not exclude the diagnosis 1
• Examine the blood sample tube for clots and review the blood smear to exclude platelet aggregates, as false thrombocytopenia on EDTA samples is common; a new citrate sample often rules out pseudothrombocytopenia 1
• Perform ultrasound screening of lower limbs (or upper limbs if catheter present) to assess for thrombosis 1

Clinical Probability Assessment: The 4Ts Score

The 4Ts score guides the testing strategy and has excellent negative predictive value:

• Calculate the 4Ts score based on: (1) degree of thrombocytopenia, (2) timing of platelet fall, (3) thrombosis or other sequelae, and (4) other causes of thrombocytopenia 1, 3
• Low probability (0-3 points): HIT probability is 0-3%; no further testing needed and heparin can be continued if clinically indicated 3, 4
• Intermediate (4-5 points) or high probability (6-8 points): Proceed with laboratory testing 1, 3

Important caveat: The 4Ts score is difficult to assess and less reliable after cardiac surgery due to comorbidities and multiple treatments; in this setting, analyze platelet count evolution after surgery instead 1. Additionally, 24-61% of patients with high 4Ts scores do not actually have HIT, so laboratory confirmation is essential 3.

First-Line Testing: Immunoassays for Anti-PF4 Antibodies

For intermediate or high 4Ts scores, order an immunoassay as soon as possible:

• Immunoassays (ELISA or chemiluminescent tests) detect IgG, IgM, and IgA antibodies against PF4-heparin complexes 1, 2
• These tests have excellent sensitivity and negative predictive value, allowing rapid exclusion of HIT if negative 1, 2
• A negative immunoassay effectively rules out HIT, and heparin can be continued if clinically appropriate 2, 5

Interpreting Immunoassay Results

The specificity of immunoassays is limited, requiring careful interpretation:

• Specificity improves when using IgG-specific assays with quantitative results (absorbance values) rather than detecting all antibody isotypes 1
• Anti-PF4 antibodies can appear without causing HIT, particularly after cardiac surgery where they are present in nearly 50% of patients 1
• Higher antibody titers and affinity correlate with higher probability of true HIT 5
• Positive immunoassays detect antibody seroconversion but cannot distinguish pathogenic from non-pathogenic antibodies, necessitating confirmatory testing 3, 5

Confirmatory Testing: Functional Assays

If the immunoassay is positive with significant titer in a patient with intermediate or high clinical probability, perform a functional test to confirm HIT:

• The serotonin release assay (SRA) is the gold standard functional test, with 97.2% sensitivity and near 100% specificity 1, 2, 3
• SRA detects only platelet-activating IgG antibodies capable of causing clinical HIT, performed at both low and high heparin concentrations 1, 5
• A positive SRA in a patient with clinical features of HIT strongly supports the diagnosis and justifies continued alternative anticoagulation 2, 3

Alternative Functional Assays

Since SRA requires radioactive isotopes and specialized expertise, alternatives exist:

• Heparin-induced platelet activation (HIPA) test is another washed platelet assay with near 100% specificity, though rarely used 1
• Platelet aggregation tests (PAT) can be performed on platelet-rich plasma or whole blood, with whole blood aggregation providing a reasonable alternative to SRA 6
• Flow cytometry assays are emerging as more accessible functional tests available to laboratories without radioactive capabilities 5
• Heparin-induced multi-electrode aggregometry (HIMEA) is being developed as a more accessible functional assay 2

Critical Timing Considerations

Do not delay clinical management while awaiting laboratory results:

• Discontinue all heparin immediately and initiate alternative anticoagulation when clinical suspicion is intermediate or high, without waiting for test results 1, 3
• Thrombosis risk is approximately 5% per day without treatment in untreated HIT 3
• Collect blood samples at least 4 hours after cessation of unfractionated heparin and ≥12 hours after low-molecular-weight heparin dosing 2
• Immunological tests should be performed as first-line tests, with functional testing only if anti-PF4 antibodies are positive 1, 5

Diagnostic Algorithm Summary

The evidence-based approach follows this sequence:

1. Basic labs + 4Ts score → If low probability (0-3), stop; no HIT testing needed 3, 4
2. If intermediate/high probability → Order immunoassay (ELISA/chemiluminescent) for anti-PF4 antibodies 1, 2, 3
3. If immunoassay negative → HIT excluded; consider continuing heparin 2, 5
4. If immunoassay positive with significant titer → Order functional assay (SRA preferred) for confirmation 1, 3
5. If functional assay positive → HIT diagnosis confirmed; continue alternative anticoagulation 1, 3

Essential point: No single approach (clinical or laboratory) provides optimal sensitivity and specificity, so the combination of clinical evaluation using the 4Ts score with sequential laboratory testing (immunoassay followed by functional assay if positive) ensures diagnostic accuracy 6, 7.