What labs are used to diagnose Heparin-Induced Thrombocytopenia (HIT)?

Diagnostic Laboratory Tests for Heparin-Induced Thrombocytopenia (HIT)

The diagnosis of HIT requires both clinical assessment using the 4T score and specific laboratory tests including anti-PF4/heparin antibody immunoassays and functional platelet activation assays. 1

Initial Clinical Assessment

Before ordering laboratory tests, clinical probability should be assessed using the 4T score:

• Thrombocytopenia: Severity and timing of platelet count fall
• Timing: Relationship between heparin exposure and platelet count fall
• Thrombosis: Presence of new thrombosis or other sequelae
• Other causes: Likelihood of other causes of thrombocytopenia

Score interpretation:

• Low probability (0-3 points): HIT unlikely
• Intermediate probability (4-5 points): HIT possible
• High probability (6-8 points): HIT likely

Laboratory Testing Algorithm

First-Line Testing

1. Complete Blood Count (CBC) with platelet count

   • Verify thrombocytopenia
   • Rule out pseudothrombocytopenia by examining blood smear
   • Consider recollecting in citrate tube if EDTA-induced platelet clumping suspected 1

2. Basic Coagulation Panel

   • PT, aPTT, fibrinogen, D-dimers
   • Helps rule out DIC (which can coexist with HIT) 1

3. Anti-PF4/Heparin Antibody Immunoassays

   • Should be ordered for all patients with intermediate or high 4T score 1
   • Types:
     • ELISA (enzyme-linked immunosorbent assay)
     • Chemiluminescent immunoassay (CLIA)
     • Particle gel immunoassay (PaGIA)
     • Latex immunoturbidimetric assay (LIA)

Second-Line Testing (Confirmatory)

1. Functional Platelet Activation Assays
   • Should be performed when immunoassay is positive 1, 2
   • Types:
     • Serotonin Release Assay (SRA) - gold standard
     • Heparin-Induced Platelet Activation (HIPA)
     • Heparin-Induced Multi-Electrode Aggregometry (HIMEA) 1

Sample Collection Requirements

• Timing: Collect blood at least 4 hours after stopping unfractionated heparin or ≥12 hours after LMWH dose 1, 2
• Tubes: Both serum and citrate (0.109 M) tubes recommended 1
• Processing: Do not heat-inactivate samples as this may decrease antibody titers 1, 2
• Storage: Store remaining serum and plasma at -80°C for potential additional testing 1

Interpretation of Results

Anti-PF4/Heparin Antibody Immunoassays

• Negative result: Virtually excludes HIT (excellent negative predictive value >99%) 1, 3, 4
• Positive result: Requires confirmation with functional assay due to limited specificity 1
• IgG-specific assays: Better specificity than assays detecting all isotypes 2
• Quantitative results: Higher optical density values correlate with higher likelihood of HIT 4

Functional Assays

• SRA: ~95% sensitivity and specificity for HIT 4
• Positive result: Confirms HIT when clinical probability is intermediate or high 1
• Negative result: Generally excludes HIT, but "SRA-negative HIT" can occur in rare cases 4

Clinical Decision Making Based on Test Results

1. Low 4T score (0-3):

   • HIT is unlikely (negative predictive value 100%)
   • Laboratory testing generally not necessary
   • Continue heparin if indicated 1, 3, 5

2. Intermediate 4T score (4-5):

   • Order anti-PF4/heparin antibody immunoassay
   • If negative: HIT excluded, continue heparin with monitoring
   • If positive: Perform functional assay and consider non-heparin anticoagulant while awaiting results 1

3. High 4T score (≥6):

   • Order both immunoassay and functional assay
   • Stop heparin immediately and start non-heparin anticoagulant at therapeutic dose without waiting for test results 1, 2
   • Adjust treatment based on test results when available

Common Pitfalls to Avoid

• Overdiagnosis: Relying solely on immunoassays leads to overdiagnosis due to detection of non-pathogenic antibodies 5, 4
• Delayed treatment: Waiting for laboratory results before stopping heparin in high-probability cases 1
• Inappropriate testing: Testing patients with low clinical probability leads to false positives and unnecessary treatment 5
• Inadequate sampling: Collecting samples too soon after heparin administration can interfere with test results 1, 2
• Misinterpretation: Failing to recognize that a positive immunoassay alone is insufficient for diagnosis 1, 4

Special Considerations

• Post-cardiac surgery: 4T score is less reliable; analysis of platelet count evolution pattern (biphasic pattern) is more useful 1
• ICU patients: 4T score may be difficult to apply due to multiple comorbidities and treatments 1
• Early testing: Anti-PF4 antibodies are detectable at the onset of platelet count fall 4
• Antibody transience: Antibodies may disappear over time (seroreversion) 4

Remember that HIT is a clinical-pathological disorder requiring both clinical assessment and laboratory confirmation for accurate diagnosis.