What are the immediate steps for a patient suspected of having heparin-induced thrombocytopenia (HIT)?

Diagnosis of Heparin-Induced Thrombocytopenia

When HIT is suspected, immediately calculate the 4T score to determine pre-test probability, stop all heparin exposure (including flushes and heparin-coated catheters), order anti-PF4 antibody testing, and start therapeutic-dose alternative anticoagulation without waiting for laboratory results if the 4T score is intermediate (4-5) or high (≥6). 1, 2

Immediate Clinical Assessment: The 4T Score

The 4T score is your first diagnostic tool and determines your immediate management pathway 1, 3:

• Thrombocytopenia: Platelet fall >50% and nadir ≥20,000/μL (2 points); fall 30-50% or nadir 10,000-19,000/μL (1 point); fall <30% or nadir <10,000/μL (0 points) 1
• Timing: Platelet fall day 5-10, or ≤1 day if recent heparin exposure (2 points); consistent with days 5-10 but unclear, or >10 days (1 point); platelet fall <4 days without recent exposure (0 points) 1
• Thrombosis or other sequelae: New thrombosis, skin necrosis, or acute systemic reaction (2 points); progressive/recurrent thrombosis or erythematous skin lesions (1 point); none (0 points) 1
• Other causes of thrombocytopenia: None apparent (2 points); possible (1 point); definite (0 points) 1

Management Based on 4T Score

Low Probability (Score ≤3)

• HIT is excluded - continue heparin with close platelet monitoring every 2-3 days 1, 2
• Pursue alternative causes of thrombocytopenia 3
• No need for anti-PF4 antibody testing unless clinical picture changes 1

Intermediate Probability (Score 4-5)

• Stop all heparin immediately (including IV flushes, subcutaneous prophylaxis, and heparin-coated catheters) 1, 2, 3
• Start therapeutic-dose alternative anticoagulation without waiting for test results 1, 2
• Order anti-PF4 antibody testing (ELISA or chemiluminescent assay) 1, 3
• If anti-PF4 antibodies are positive, proceed to functional testing (serotonin release assay or HIPA test) for confirmation 2, 3
• If anti-PF4 antibodies are negative, HIT is excluded and heparin can be resumed with close monitoring 2

High Probability (Score ≥6)

• Stop all heparin immediately 1, 2, 3
• Start therapeutic-dose alternative anticoagulation immediately - do not wait for laboratory confirmation 1, 2
• Order anti-PF4 antibody testing, but treatment should not be delayed 1, 3
• Proceed to functional testing if anti-PF4 antibodies are positive 2, 3

Laboratory Testing Strategy

Initial Test: Anti-PF4 Antibodies

• Order immunoassay (ELISA or chemiluminescent test) as the first laboratory test 3
• These tests have high sensitivity and negative predictive value but lower specificity 2, 4
• A negative result effectively excludes HIT in patients with intermediate probability 2

Confirmatory Test: Functional Assays

• If anti-PF4 antibodies are positive with intermediate or high 4T score, perform functional testing 2, 3
• Options include serotonin release assay (SRA) or heparin-induced platelet activation (HIPA) test 2, 3
• These tests detect platelet-activating antibodies and have higher specificity 4, 5

Special Diagnostic Considerations

Post-Cardiac Surgery Patients

• The 4T score is less reliable in this population 2
• Look for a "biphasic" platelet count pattern - initial postoperative drop followed by recovery, then a second drop - which strongly suggests HIT 2

Patients with Recent Heparin Exposure (Within 100 Days)

• Obtain baseline platelet count before starting heparin 6
• Repeat platelet count 24 hours later if feasible, as rapid-onset HIT can occur within 24 hours in patients with circulating HIT antibodies 6

Acute Systemic Reactions

• If a patient develops fever, chills, hypertension, tachycardia, dyspnea, chest pain, or cardiopulmonary symptoms within 30 minutes of an IV heparin bolus, immediately check platelet count 6
• This presentation is strongly suggestive of acute HIT 6

Critical Pitfalls to Avoid

• Never delay stopping heparin while waiting for antibody test results if clinical probability is intermediate or high - the thrombotic risk is immediate with 30-50% of untreated patients developing thrombosis 1, 2, 7
• Do not rely solely on laboratory tests - HIT is primarily a clinical diagnosis that must be confirmed by testing, but treatment decisions are based on clinical probability 4, 5
• Remember that antibody formation alone is not diagnostic - anti-PF4 antibodies occur in many clinical settings without causing thrombocytopenia or thrombosis 7, 8
• Do not use low molecular weight heparin (LMWH) as an alternative - it cross-reacts with HIT antibodies in 80-90% of cases 1, 9
• Biological confirmation should never delay treatment - if clinical suspicion is high, act immediately 2

Monitoring During Diagnostic Workup

• Check platelet counts every 2-3 days until recovery (>150,000/μL or return to baseline) 3
• Obtain baseline coagulation studies (aPTT, INR) before starting alternative anticoagulation 3
• Monitor for new thrombotic events, as thrombosis may be the initial presentation even without significant thrombocytopenia 10, 5