Classification: Type 1 vs. Type 2
| Type 1 | Type 2 | |
|---|---|---|
| Frequency | 10-20% | 1-3% |
| Timing | 1-4 days | 5-10 days (and even longer) |
| Nadir of platelet count | 100,000 | 30-55,000 |
| Antibody mediated | No | Yes |
| Thromboembolic events | None | 30-80% |
| Hemorrhagic events | None | Rarely |
| Management | Observe | Discontinue heparin and start alternate anticoagulant. |
Evaluate the pre-test probability of HIT: unfortunately, the laboratory tests to diagnose HIT take at least a few days to come back. Therefore, use the following clinical scoring system (4-T score) to determine the pre-test probability of HIT and the appropriate management plan. Remember to periodically re-assess, as new information can change pre-test probability (e.g. positive blood cultures).
| Clinical Factor | 2 points | 1 point | 0 points |
|---|---|---|---|
| Thrombocytopenia | Nadir 20-100, or >50% platelet fall | Nadir 10-19 or 30-50% platelet fall | Nadir <10, or <30% platelet fall |
| Timing of onset of platelet fall | Day 5-10, or ≤ day 1 with recent heparin (past 30 days) | > day 10 or timing unclear (but fits with HIT) | ≤ day 1 (no recent heparin) |
| Thrombosis or other sequelae | Proven thrombosis, skin necrosis, or acute systemic reaction following bolus of heparin IV | Progressive, recurrent, or silent thrombosis; erythematous skin lesions | None |
| Other cause of platelet fall | None evident | Possible | Definite |
Initial Management
- Based on the total pre-test probability score, come up with a management plan (total score 0 to 8):
- Score 0-3: continue heparin; no need for further testing.
- Score 4-5: physician judgment; consider laboratory testing.
- Score 6-8:
- Perform laboratory testing for confirmation (see below).
- Stop all heparin, including heparin flushes and heparin-coated catheters. Give alternative, non-heparin anticoagulant (direct thrombin inhibitor or non-heparin factor Xa inhibitor).
- Avoid warfarin until platelet count >100,000/μL. Warfarin starting dose should be low and therapy should be overlapped with a direct thrombin inhibitor for a minimum of 5 days.
Laboratory Diagnosis
- Based on your pre-test probability score, seek further laboratory testing starting with a platelet factor 4 antibody assay (“HIT” antibodies). Proceed as follows:
| Pretest Score | PF4Ab NEGATIVE | PF4Ab POSITIVE |
|---|---|---|
| 6-8 | Send Serotonin Release Assay | STOP. HIT confirmed |
4-5 or recent CABG | STOP. No HIT | Send Serotonin Release Assay |
| 0-3 | No testing indicated | No testing indicated |
Treatment
- Includes stopping all heparin products (including heparin flushes and heparin-coated catheters) and instituting alternate anticoagulation, to prevent thrombotic complications. Choices include bivalirudin, lepirudin, or argatroban (direct thrombin inhibitors).
- With liver disease best choices are lepirudin, danaparoid, or fondaparinux and with renal disease can use argatroban or lower doses of lepirudin. See section Hematology/Oncology: IV Direct Thrombin Inhibitors.
Key Points
- Type I is a nonimmune phenomenon and limited while Type II is an antibody-mediated disorder including HIT and requires treatment.
- If HIT is suspected, assess the pretest probability to guide management.
- Complications include arterial or venous thromboses; often occur at sites of pre-existing pathology.
Alving BM. How I treat heparin-induced thrombocytopenia and thrombosis. Blood 2003;101:31-37.
Rice L, Attisha WK, Drexler A, et al. Delayed-onset heparin-induced thrombocytopenia. Ann Intern Med 2002;136:210-215.
UCSF Comprehensive Hemostasis and Antithrombotic Service (CHAS) guidelines, 2004