Heparin-induced thrombocytopenia (HIT) is one of those “don’t miss” diagnoses: the platelet count drops, but the patient clots. If you can rapidly recognize the timing, the mechanism, and the correct next step, you’ll pick up easy points on Step—and avoid dangerous management mistakes on the wards.
Tip #1: Think “HIT = Hypercoagulable thrombocytopenia” (not a bleeding problem)
One-liner: HIT is an IgG-mediated reaction against PF4–heparin complexes that activates platelets, causing thrombosis despite thrombocytopenia.
The core mechanism (Step-friendly)
- Heparin binds platelet factor 4 (PF4) → forms a complex
- Patient makes IgG antibodies to the PF4–heparin complex
- IgG binds Fc receptors on platelets → platelet activation + consumption
- Activated platelets release procoagulant microparticles → arterial and venous thrombosis
High-yield clinical consequences
- Thrombosis is the main danger (DVT/PE, limb ischemia, stroke, MI)
- Platelets typically fall ≥ 50% from baseline
- Bleeding is uncommon (platelets usually not low enough to bleed)
Tip #2: Timing + % drop beats “absolute platelet number”
One-liner: HIT usually shows up 5–10 days after starting heparin (or within 24 hours if previously exposed).
The classic timing patterns
- Typical onset: day 5–10
- Rapid-onset HIT: within 24 hours if patient had heparin exposure in the last ~100 days (preformed antibodies)
- Delayed-onset HIT: thrombocytopenia and thrombosis can occur after stopping heparin (antibodies persist)
Quick “when to suspect” checklist
Suspect HIT if:
- Platelets drop ≥ 50%, even if nadir is still >150k
- New thrombosis occurs while on heparin
- Skin necrosis at injection sites or systemic reaction after heparin bolus
Mini-table: HIT vs DIC vs TTP/HUS (fast differentiation)
| Feature | HIT | DIC | TTP/HUS |
|---|---|---|---|
| Platelets | ↓ (often moderate; ≥50% drop) | ↓ | ↓ |
| PT/PTT | Usually normal | Prolonged | Normal |
| Fibrinogen | Normal | ↓ | Normal |
| D-dimer | May be ↑ | ↑↑ | Normal/variable |
| Smear | No schistocytes (typically) | ± schistocytes | Schistocytes |
| Key clue | Thrombosis on heparin | Sepsis/trauma/malignancy | Neuro/renal + MAHA |
Tip #3: Management is “Stop heparin + start a non-heparin anticoagulant” (and don’t give warfarin yet)
One-liner: If HIT is suspected, stop all heparin immediately and anticoagulate with a non-heparin agent—because the patient is at high risk of clotting right now.
What to do immediately (USMLE algorithm)
- Stop heparin (including heparin flushes and heparin-coated catheters)
- Start a non-heparin anticoagulant, such as:
- Argatroban (direct thrombin inhibitor; IV; hepatically cleared)
- Bivalirudin (often used in PCI/cardiac settings)
- Fondaparinux (factor Xa inhibitor; not heparin, low cross-reactivity)
- DOACs (increasingly used in practice; Step questions still love argatroban/bivalirudin)
- Order confirmatory testing:
- PF4 ELISA (sensitive)
- Serotonin release assay (SRA) (specific; classic “confirmatory”)
Two classic pitfalls to avoid
- Do NOT start warfarin during acute HIT
- Early warfarin drops protein C first → can worsen thrombosis and cause skin necrosis
- If already given, reverse with vitamin K
- Do NOT transfuse platelets unless there’s life-threatening bleeding
- You can “fuel the fire” by providing more platelets to activate
A quick mnemonic + visual to make HIT stick
Mnemonic: “HIT = Heparin → Immune → Thrombosis”
- Heparin exposure
- Immune IgG to PF4–heparin
- Thrombosis (the scary part)
Tiny visual (burn it into memory)
Heparin + PF4 → IgG → platelet activation → clots
…and the platelet count drops as a side effect of activation/consumption.
The “3-second Step takeaway”
If platelets fall 5–10 days after heparin and the patient clots, it’s HIT: stop heparin and start argatroban/bivalirudin/fondaparinux—not warfarin.