Immune-mediated platelet activation by anti-PF4/heparin antibodies. Paradoxically a PRO-thrombotic state despite low platelets. The platelets drop, but the patient clots. Stop ALL heparin immediately and switch to a non-heparin anticoagulant.
๐ Overview
4T Score (Pretest Probability) -likelihood of heparin-induced thrombocytopenia (HIT); guides whether to send anti-PF4 and stop heparin
Criterion
2 Points
1 Point
0 Points
Thrombocytopenia
Drop > 50% AND nadir โฅ 20K
Drop 30โ50% OR nadir 10โ19K
Drop < 30% OR nadir < 10K
Timing
Days 5โ10 OR โค 1 day if prior heparin within 30 days
Days 5โ10 (unclear) OR > day 10
< day 4 (no prior exposure)
Thrombosis
New thrombosis, skin necrosis, or anaphylaxis post-heparin bolus
Progressive or recurrent thrombosis
None
Other causes
No other cause for thrombocytopenia
Possible other cause
Definite other cause
0โ3: low probability (< 5%) โ HIT unlikely, no further testing. 4โ5: intermediate โ send PF4 antibody, start non-heparin anticoagulant. 6โ8: high probability โ treat as HIT while awaiting confirmatory tests.
Key Features
Timing: platelet drop typically 5โ10 days after heparin initiation (or within 24h if prior heparin exposure within 30 days -rapid onset HIT)
Platelet drop: usually > 50% from baseline (not absolute count -a drop from 300K to 130K is HIT)
Thrombosis in 50%: DVT/PE (most common), arterial (stroke, MI, limb ischemia), skin necrosis at heparin injection sites
UFH >> LMWH for HIT risk (but LMWH can also cause it -cross-reactivity ~90%)
๐จ Management
Immediate Actions
Stop ALL heparin immediately -IV, SC, line flushes, heparin-coated catheters. Even trace amounts perpetuate the immune response.
Drug
Dose
Notes
Argatroban 1ST LINE
0.5โ2 mcg/kg/min IV (no bolus). Monitor aPTT.
Hepatically cleared -preferred in renal failure. Falsely elevates INR (complicates warfarin bridging). Reduce dose in hepatic impairment, ICU, post-cardiac surgery.
Bivalirudin 1ST LINE
0.15โ0.2 mg/kg/hr IV. Monitor aPTT.
Short half-life (~25 min). Preferred for PCI and cardiac surgery settings. Partially renally cleared.
Fondaparinux ALTERNATIVE
Weight-based SC (same as VTE dosing)
Pentasaccharide -does NOT cross-react with HIT antibodies. Off-label for HIT but widely used. No monitoring needed. Renally cleared -avoid if CrCl < 30.
Do NOT transfuse platelets in HIT unless life-threatening bleeding. Platelets are prothrombotic in HIT, they become activated by HIT antibodies โ more thrombosis. Do NOT start warfarin until platelets recover to โฅ 150K, early warfarin in HIT can cause protein C depletion โ warfarin-induced skin necrosis and venous limb gangrene. When transitioning: overlap DTI (argatroban/bivalirudin) with warfarin for โฅ 5 days AND until INR is therapeutic on two consecutive days.
๐งช Workup
Workup
4T score -pre-test probability. โฅ 6 = high probability โ start non-heparin anticoagulation immediately while awaiting confirmatory testing. 4-5 = intermediate โ test. โค 3 = low โ HIT unlikely. Components: Thrombocytopenia (% fall + nadir), Timing (day 5-10), Thrombosis (new), oTher cause (none identified). [4T Score Validation, 2006]
PF4/heparin ELISA (PF4 antibody) -screening test. High sensitivity (~97%), moderate specificity (~75%). Negative ELISA essentially rules out HIT. Positive โ need confirmatory test. OD > 2.0 strongly predictive.
Serotonin release assay (SRA) -confirmatory gold standard. High specificity (~95%). Takes 3-7 days to result. Functional assay -measures actual platelet activation.
CBC trend -platelet nadir typically 5-10 days after heparin exposure. Classic: > 50% drop from baseline (e.g., 250K โ 80K). Nadir usually 20-80K. If < 20K โ consider other diagnoses (DIC, TTP).
Bilateral lower extremity duplex US -30-50% of HIT patients have occult DVT at diagnosis even without symptoms. Screen all confirmed/suspected HIT patients.
Review ALL heparin exposure -IV drips, SQ prophylaxis, line flushes, heparin-coated catheters (dialysis, PICC lines), heparin in OR tubing. Even brief exposure counts.
Timing clues: Typical onset day 5-10. Rapid-onset HIT (< 24h) = prior heparin exposure within 100 days (pre-formed antibodies). Delayed-onset HIT = develops after heparin stopped (rare, up to 3 weeks).
๐ Medications
Medications
Drug
Dose
Route
Notes
Argatroban
2 mcg/kg/min (0.5-1.2 in liver disease)
IV drip
Direct thrombin inhibitor.Argatroban HIT Trial, 2001 Hepatically metabolized -reduce dose in liver failure. Titrate to aPTT 1.5-3ร baseline. Falsely elevates INR โ complicates warfarin transition.
Bivalirudin
0.15-0.25 mg/kg/hr
IV drip
Alternative DTI. Shorter half-life (25 min vs 45 min) -preferred for PCI, cardiac surgery, or renal failure. Does not elevate INR.
Fondaparinux
5-10 mg SQ daily
SQ
Factor Xa inhibitor. Off-label for HIT but widely used. No IV monitoring needed. Minimal cross-reactivity with HIT antibodies (< 1%). Renally cleared -avoid if CrCl < 30.
DO NOT use LMWH
-
-
90% in-vitro cross-reactivity with HIT antibodies. Enoxaparin, dalteparin, tinzaparin are ALL contraindicated.
Warfarin transition
Start ONLY when platelets โฅ 150K
PO
Overlap argatroban for โฅ 5 days + INR โฅ 2 for 2 consecutive days. Starting warfarin too early depletes protein C Warfarin Limb Gangrene Study, 1997 โ paradoxical thrombosis (venous limb gangrene, skin necrosis). Warfarin Limb Gangrene Study, 1997
DOAC transition
Per agent dosing
PO
Rivaroxaban or apixaban increasingly used as alternatives to warfarin for long-term anticoagulation post-HIT. Start when platelets recovered. Duration: 3-6 months minimum (or longer if provoked thrombosis).
๐ On Rounds
Why can't you bridge to warfarin immediately in HIT?
Warfarin inhibits vitamin K-dependent factors (II, VII, IX, X) but also inhibits protein C and S (natural anticoagulants). Protein C has the shortest half-life (~8h) โ it drops first when warfarin is started. In HIT, the patient is already in a prothrombotic state. Starting warfarin creates a transient hypercoagulable window (protein C depleted before factors drop) โ venous limb gangrene and skin necrosis.
A patient on heparin has a platelet drop. How do you decide if it's HIT vs other causes?
Use the 4T score: (1) Thrombocytopenia -fall > 50% and nadir โฅ 20K scores highest, (2) Timing -days 5โ10 after heparin start (or < 1 day if prior heparin in last 30 days) scores highest, (3) Thrombosis -new confirmed thrombosis scores highest, (4) oTher causes -no other explanation scores highest. 4T score 0โ3 = low probability (HIT essentially ruled out, NPV > 99%). 4โ5 = intermediate โ send PF4 antibody.
Why can you NOT start warfarin until platelets recover above 150K in HIT?
In the acute thrombotic phase of HIT, protein C levels are already depleted (consumed by the ongoing thrombotic process). Warfarin further suppresses protein C (vitamin K-dependent, shorter half-life than other clotting factors) โ transient hypercoagulable state โ microvascular thrombosis โ venous limb gangrene or skin necrosis.
What non-heparin anticoagulants can you use in HIT?
Argatroban (first-line): direct thrombin inhibitor, IV drip, hepatically metabolized (reduce dose in liver failure). Titrate to aPTT 1.5-3ร baseline. Prolongs INR โ makes warfarin transition tricky (need to check INR after holding argatroban for 4h). Bivalirudin: direct thrombin inhibitor, shorter half-life, preferred in PCI setting or renal failure (cleared by plasma proteases, not kidneys).
โ What are the components of the 4T score?
T hrombocytopenia: > 50% fall + nadir โฅ 20K = 2pts. T iming: day 5-10 (or < 1 day with prior heparin in last 30 days) = 2pts. T hrombosis: new confirmed thrombosis = 2pts. oT her cause: none apparent = 2pts. Score: 0-3 = low (HIT unlikely), 4-5 = intermediate, 6-8 = high probability โ start non-heparin anticoag immediately. [4T Score Validation, 2006]
โ Why is warfarin dangerous if started too early in HIT?
Warfarin depletes protein C faster than procoagulant factors (protein C half-life = 6h vs Factor II = 60h). In the prothrombotic state of HIT, this early protein C drop โ paradoxical thrombosis โ venous limb gangrene, skin necrosis (especially at fat-rich sites). Start warfarin ONLY when platelets โฅ 150K and overlap with DTI ร โฅ 5 days. Warfarin Limb Gangrene Study, 1997
โ How do you transition from argatroban to warfarin given that argatroban elevates INR?
Argatroban (a direct thrombin inhibitor) falsely elevates INR because it affects the thrombin-dependent step in the PT assay. Strategies: (1) Start warfarin when platelets โฅ 150K while continuing argatroban. (2) Target combined INR > 4 on overlap. (3) Hold argatroban ร 4 hours, then check INR -if โฅ 2, the warfarin effect is therapeutic. (4) Some centers use chromogenic Factor X assay instead (not affected by argatroban).
โ Can you use DOACs in HIT?
DOACs (rivaroxaban, apixaban) are increasingly used as alternatives to warfarin for the ongoing anticoagulation phase after platelet recovery. Advantages: no INR monitoring, no interaction with argatroban, rapid onset. Limited prospective data but growing retrospective evidence supports safety and efficacy. Start after platelets recovered (โฅ 150K). Duration: 4 weeks (HIT without thrombosis) or 3-6 months (HIT with thrombosis).
โ What is rapid-onset HIT and how does it differ from typical HIT?
Rapid-onset HIT: platelet drop within < 24 hours of heparin re-exposure in a patient with prior heparin exposure within the last 100 days. Pre-formed PF4/heparin antibodies cause immediate platelet activation. Key clue: patient was recently hospitalized or had surgery and now presents with acute thrombocytopenia upon heparin re-exposure. Contrast with typical HIT (day 5-10 onset in heparin-naive patients).
Clinical Examples
๐ Case 1, Post-Surgical HIT with DVT
Patient: 68M, PMH HTN, DM2. POD 8 from R total knee arthroplasty on enoxaparin prophylaxis. Nursing reports new R calf swelling and pain.
Key findings: Platelets dropped from 245K to 88K (64% decline) over 3 days. Duplex US: acute R popliteal DVT. 4T score: 7 (high probability).
Management:
Stop ALL heparin products immediately (enoxaparin, flushes, line locks)
Start argatroban 2 mcg/kg/min IV, titrate to aPTT 1.5-3x baseline
Send PF4/heparin antibody (ELISA) and SRA (confirmatory)
Do NOT start warfarin until platelets โฅ 150K; overlap with argatroban โฅ 5 days
Anticoagulate for 3-6 months (HIT with thrombosis = HITT)
Teaching point: LMWH cross-reacts with HIT antibodies in ~90% of cases. A patient with HIT on UFH should NOT be switched to enoxaparin. Always use a non-heparin anticoagulant (argatroban, bivalirudin, fondaparinux).
๐ Case 2, Rapid-Onset HIT
Patient: 55F, PMH breast cancer s/p port placement 3 weeks ago (received heparin flushes). Admitted for chemo, started on UFH DVT prophylaxis. Platelets drop from 190K to 45K within 12 hours.
Key findings: Rapid platelet decline within 24h of heparin re-exposure. Prior heparin exposure < 30 days ago. 4T score: 6 (high).
Management:
Stop heparin immediately
Start argatroban; consider bivalirudin if hepatic dysfunction
Bilateral LE duplex (30-50% have occult DVT)
Send PF4 Ab + SRA
Teaching point: Rapid-onset HIT occurs within < 24h of re-exposure in patients with pre-formed antibodies from heparin exposure within the last 100 days. The 4T score timing criteria awards 2 points for platelet fall < 1 day with prior heparin exposure.
๐ Case 3, Intermediate 4T Score with Low Clinical Suspicion
Patient: 72M in MICU on UFH for AF. Day 6, platelets drop from 180K to 105K (42% decline). Patient is septic from pneumonia with new pressor requirement.
Key findings: 4T score: 4 (intermediate). Sepsis is a common cause of thrombocytopenia in the ICU. No new thrombosis.
Management:
Send PF4 antibody (ELISA) given intermediate probability
If PF4 ELISA negative (OD < 0.40) โ HIT essentially excluded (NPV > 99%)
If PF4 positive โ stop heparin, start argatroban, send SRA for confirmation
Consider alternative causes: sepsis-associated thrombocytopenia, drug-induced (vancomycin, linezolid), dilutional
Teaching point: The 4T score's greatest value is its NPV at low scores (0-3), which essentially rules out HIT. At intermediate scores (4-5), the PF4 ELISA helps decide next steps. Sepsis is the #1 cause of thrombocytopenia in the ICU, not HIT.
๐ฃ Sample Presentation
One-Liner
"Mrs. Liu is a 64-year-old post-op day 7 from hip replacement on heparin prophylaxis whose platelets dropped from 220K to 82K (62% decline). No bleeding. 4T score 6 (high probability). PF4 antibody sent."
Key Points to Cover on Rounds
High-probability HIT (4T score 6: >50% drop, day 5-10 timing, no thrombosis yet, no other cause). Immediate actions: (1) ALL heparin stopped (drips, flushes, line locks), (2) argatroban started at 2 mcg/kg/min, titrate to aPTT 1.5-3ร baseline, (3) bilateral LE duplex ordered (30-50% have occult DVT). PF4 Ab pending, SRA pending. No warfarin until plt >150K (protein C depletion risk โ skin necrosis, venous limb gangrene). Platelet transfusion NOT indicated. Plan: argatroban until plt recovery, then transition to warfarin with โฅ5 days overlap.
Monitoring
Platelet count daily -should begin recovering within 4-10 days of stopping heparin + starting alternative anticoagulation. If NOT recovering โ reconsider diagnosis or check for new thrombosis.
aPTT q6h while on argatroban -target 1.5-3ร baseline. Avoid supratherapeutic levels (bleeding risk).
INR for warfarin transition -argatroban falsely elevates INR. Check INR after holding argatroban ร 4h to get true INR. Some centers use chromogenic factor X assay instead.
Bilateral LE duplex US at diagnosis -even if no symptoms (30-50% occult DVT)
Clinical assessment for new thrombosis daily -arterial (stroke, limb ischemia, MI) and venous (DVT, PE). HIT is a prothrombotic state -thrombosis risk highest in first 30 days.
Skin exam -skin necrosis at injection sites (heparin-induced skin necrosis occurs before overt HIT)
Duration of anticoagulation: minimum 4 weeks if HIT without thrombosis. 3-6 months if HIT with thrombosis (HIT-T). Some experts recommend extended anticoagulation for arterial HIT events.
โก Summary
Summary
4T Score
Thrombocytopenia (timing, fall > 50%), Timing (days 5-10), Thrombosis (new), oTher causes excluded. Score โฅ 6 = high probability โ act immediately.
Immediate Actions
Stop ALL heparin (drips, flushes, line locks). Start argatroban. Bilateral LE duplex (30-50% have occult DVT). Send PF4 antibody + SRA.
Anticoagulation
Argatroban 2 mcg/kg/min IV (reduce in liver failure). Alternative: bivalirudin. NO LMWH (90% cross-reactivity).
Warfarin Timing
Do NOT start until plt โฅ 150K (risk of venous limb gangrene from protein C depletion). Overlap with argatroban โฅ 5 days.
Labs
PF4 antibody (screening, high NPV). SRA (serotonin release assay -confirmatory, high specificity). Don't wait for SRA to treat if high 4T score.
Duration
If no thrombosis: anticoag ร at least 4 weeks after plt recovery. If thrombosis (HITT): anticoag ร 3 months minimum.
๐ One Pager
Hematology ยท One Pager
HIT -Heparin-Induced Thrombocytopenia
4T score โฅ 6 โ stop ALL heparin + start argatroban. Bilateral LE duplex (30-50% occult DVT). No warfarin until plt โฅ 150K. No LMWH (90% cross-reactivity).
๐งช Diagnosis
4T score: Thrombocytopenia > 50% fall, Timing days 5-10, Thrombosis, oTher causes excluded. Score โฅ 6 = high probability โ treat immediately. Send PF4 Ab + SRA.
๐จ Treatment
STOP all heparin (drips, flushes, line locks). Start argatroban 2 mcg/kg/min IV (reduce in liver failure). Bilateral LE duplex US (30-50% have occult DVT).
โ ๏ธ Warfarin Rules
Do NOT start until plt โฅ 150K (protein C depletion โ venous limb gangrene). Overlap argatroban โฅ 5 days. DOACs emerging as option after acute phase but limited evidence.
๐ Key Drugs
Argatroban2 mcg/kg/min IV drip
BivalirudinAlternative DTI
FondaparinuxSQ (off-label for HIT)
NO LMWH90% cross-reactivity
โ ๏ธ Pitfalls
Not stopping ALL heparin (including flushes and line locks)
Warfarin before plt โฅ 150K (limb gangrene risk)
LMWH for HIT (cross-reacts in 90%)
Not checking bilateral LE duplex (30-50% occult DVT)