Eduovisual
Blood & Lymphoreticular
Heparin-induced thrombocytopenia: 4T score and management
— Antibody binding to FcγRIIa on platelets triggers platelet activation, microparticle release, thrombin generation, and paradoxical thrombosis despite a falling platelet count.
— Incidence: ~0.1–5% with unfractionated heparin (UFH); ~0.1–1% with low-molecular-weight heparin (LMWH); essentially zero with fondaparinux.
— Highest risk: post-cardiac/orthopedic surgery patients on UFH for ≥5 days; lower risk in medical and obstetric patients.
— Earlier drops (within 24 hours) can occur as "rapid-onset HIT" if the patient was exposed to heparin within the prior 30–100 days (preformed antibodies).
— "Delayed-onset HIT" may appear up to 3 weeks after heparin is stopped — suspect in any patient with otherwise unexplained thrombocytopenia and recent hospitalization.
— Even heparin flushes, heparin-coated catheters, and a single intraoperative dose can trigger HIT.
— DVT/PE most common; also limb ischemia, stroke, MI, adrenal hemorrhage from bilateral adrenal vein thrombosis, and skin necrosis at heparin injection sites.
— Bleeding is uncommon despite thrombocytopenia (median nadir ~60 ×10⁹/L; rarely <20).
Board pearl: A platelet count that falls by >50% on hospital day 5–10 in a patient receiving any form of heparin is HIT until proven otherwise — stop heparin and start a non-heparin anticoagulant before lab confirmation. Waiting for the SRA risks limb or life.
Key distinction: HIT is a clinicopathologic diagnosis — neither clinical suspicion alone nor a positive antibody alone is sufficient; you need compatible clinical picture plus lab confirmation.
— Typical-onset HIT (most common): platelet fall begins 5–10 days after first heparin exposure. Counts often still "normal" but down >50% from peak.
— Rapid-onset HIT: fall within 24 hours of re-exposure in a patient with heparin exposure in the prior 30–100 days (residual circulating PF4 antibodies).
— Delayed-onset HIT: thrombocytopenia and thrombosis develop days to 3 weeks after heparin discontinuation; often presents post-discharge with new DVT/PE and unexplained low platelets.
— Any UFH or LMWH in the past 3 months (prophylactic or therapeutic)?
— Hemodialysis (heparin in circuit), cardiac/vascular surgery (intraoperative UFH bolus), heparin-bonded central lines/PA catheters, heparin flushes of arterial lines.
— Recent orthopedic surgery (hip/knee replacement) is the highest-risk surgical setting.
— Unilateral leg swelling, pleuritic chest pain, dyspnea, hypoxia.
— Acute limb ischemia (cold, pulseless extremity) — especially after cardiac surgery.
— Focal neuro deficits (stroke), chest pain (MI), flank/abdominal pain with hemodynamic collapse (adrenal hemorrhage → adrenal insufficiency).
— Painful, erythematous, or necrotic lesions at subcutaneous heparin injection sites — pathognomonic.
Step 3 management: On the CCS case, the moment you suspect HIT, your order set is: stop all heparin (including flushes and LMWH), CBC with platelet count, HIT antibody (PF4-ELISA), duplex ultrasound of bilateral lower extremities (to screen for asymptomatic DVT), and start a non-heparin anticoagulant. Do not wait for serology.
Board pearl: Skin necrosis at heparin injection sites without thrombocytopenia still counts as HIT — antibody testing is warranted.
— Petechiae, purpura, mucosal bleeding should make you reconsider the diagnosis (think ITP, DIC, TTP instead).
— Inspect, palpate, and compare both legs: asymmetric swelling, calf tenderness, Homans sign (low sensitivity, do not rely on it).
— Check all four extremity pulses — acute arterial occlusion can present as the 6 P's (pain, pallor, pulselessness, paresthesia, poikilothermia, paralysis).
— Examine subcutaneous heparin injection sites (abdomen, thighs): erythematous plaques, indurated nodules, or frank skin necrosis — highly specific for HIT.
— Tachycardia, tachypnea, hypoxia, pleural friction rub, or new right heart strain (loud P2, JVD) → suspect PE.
— Auscultate for new murmurs and listen for embolic stroke deficits on neuro exam.
— In cardiac surgery patients, examine the sternotomy site and check graft patency clinically.
— Hypotension + abdominal/flank pain + hyponatremia/hyperkalemia → bilateral adrenal vein thrombosis with hemorrhagic adrenal infarction → adrenal crisis. Give stress-dose hydrocortisone 100 mg IV empirically while obtaining cortisol/ACTH and CT abdomen.
— Hypotension + hypoxia + RV strain → massive PE; consider catheter-directed therapy (systemic thrombolytics are relatively contraindicated due to bleeding risk with concurrent argatroban).
— Mottled, cold limb with absent pulses → vascular surgery consult for embolectomy; do not delay for imaging if limb-threatening.
— Look for venous limb gangrene — a classic HIT complication when warfarin is started before adequate direct thrombin inhibition (protein C depletion + ongoing thrombin generation).
CCS pearl: On a CCS case, ordering "vital signs q4h," "neuro checks q4h," "extremity exam q shift," and "monitor injection sites" demonstrates the longitudinal vigilance the test rewards.
Key distinction: Bleeding is the exam signature of ITP/DIC/drug-induced thrombocytopenia; thrombosis is the exam signature of HIT.
— CBC with platelet count and trend (compare to admission and peak values).
— PT/INR, aPTT, fibrinogen, D-dimer (D-dimer often markedly elevated).
— Peripheral smear — exclude schistocytes (TTP/HUS/DIC).
— BMP, LFTs (baseline for argatroban dosing in hepatic dysfunction; bivalirudin for renal).
— Duplex ultrasound of both lower extremities at baseline regardless of symptoms (screening identifies subclinical DVT in up to 50%).
— Thrombocytopenia: 2 = >50% fall AND nadir ≥20; 1 = 30–50% fall OR nadir 10–19; 0 = <30% fall OR nadir <10.
— Timing of platelet fall: 2 = clear onset days 5–10, or ≤1 day with heparin exposure in prior 30 days; 1 = consistent with days 5–10 but unclear, or fall after day 10, or ≤1 day with exposure 30–100 days ago; 0 = fall <4 days without recent exposure.
— Thrombosis or other sequelae: 2 = new confirmed thrombosis, skin necrosis, or anaphylactoid reaction post-IV bolus; 1 = progressive/recurrent thrombosis, erythematous skin lesions, suspected but unproven thrombosis; 0 = none.
— Other causes of thrombocytopenia: 2 = none apparent; 1 = possible other cause; 0 = definite other cause.
— 0–3 = low probability (~<5% HIT) → HIT essentially excluded; investigate alternatives, generally do not need to send antibody testing.
— 4–5 = intermediate (~10–30%) → send PF4-ELISA, stop heparin, start non-heparin anticoagulant.
— 6–8 = high (~20–80%) → stop heparin, start non-heparin anticoagulant, send PF4-ELISA and confirmatory functional assay.
Board pearl: A 4T score of ≤3 has a negative predictive value >97% — the most useful feature of the score. Use it to rule out HIT and avoid unnecessary expensive alternative anticoagulants.
Step 3 management: Intermediate or high 4T scores mandate immediate discontinuation of all heparin and initiation of a non-heparin anticoagulant — do not wait for serology results, which may take 24–72 hours.
— Detects IgG (and often IgA/IgM, depending on assay) antibodies against PF4-heparin complexes.
— High sensitivity (~99%) but low specificity (~50–80%) — many patients exposed to heparin develop antibodies without clinical HIT ("seroconversion without disease").
— Result reported as optical density (OD). Higher OD = higher probability of true HIT:
— OD <0.4 → essentially excludes HIT.
— OD 0.4–1.0 → low positive; correlate with 4T score.
— OD 1.0–2.0 → moderate positive.
— OD >2.0 → strongly suggests true HIT (positive predictive value ~90%).
— IgG-specific ELISA improves specificity over polyspecific assays.
— Serotonin release assay (SRA) — gold standard; measures ¹⁴C-serotonin release from donor platelets activated by patient serum in the presence of low-dose heparin (positive) but not high-dose heparin (excludes nonspecific activation). Sensitivity and specificity >95%.
— Heparin-induced platelet activation (HIPA) assay — European equivalent.
— Send-out tests, typically 3–7 day turnaround; do not delay treatment while waiting.
— Intermediate/high 4T → ELISA first.
— Positive ELISA with low OD → reflex to SRA for confirmation.
— Strongly positive ELISA (OD >2.0) + compatible clinical picture → diagnosis essentially confirmed; SRA still recommended for documentation.
— Negative ELISA in low/intermediate 4T → HIT excluded, restart heparin if needed.
— Bilateral lower extremity duplex ultrasound in all suspected HIT patients, even asymptomatic — affects duration of anticoagulation.
— Upper extremity duplex if central venous catheter present.
— CT angiography for suspected PE; MRI/CT for suspected adrenal hemorrhage or CNS thrombosis.
Key distinction: ELISA = sensitive screening; SRA = specific confirmation. A positive ELISA alone in a patient with a low 4T score is not HIT — it is asymptomatic seroconversion. Do not start alternative anticoagulation based on serology alone.
Board pearl: Never document "HIT" in the chart based on a low-titer ELISA without clinical correlation — it permanently labels the patient and forecloses future heparin use, including for cardiopulmonary bypass.
— Stop all heparin — including LMWH, heparin flushes, heparin-coated catheters, heparin-bonded grafts where feasible.
— Start a non-heparin anticoagulant at therapeutic doses — prophylactic dosing is inadequate because of high thrombosis risk.
— Do not transfuse platelets prophylactically — adds fuel to a prothrombotic fire. Reserve for active bleeding or invasive procedure with platelets <20–30.
— Do not start warfarin acutely — risk of venous limb gangrene and skin necrosis from rapid protein C depletion while thrombin generation continues.
— Screen for asymptomatic DVT with bilateral lower extremity duplex.
— Argatroban — direct thrombin inhibitor, hepatically cleared. Preferred in renal failure. IV infusion; monitor aPTT (goal 1.5–3× baseline). Falsely elevates INR — complicates warfarin transition.
— Bivalirudin — direct thrombin inhibitor, ~80% enzymatic clearance, ~20% renal. Preferred in hepatic dysfunction and in patients needing PCI or cardiac surgery.
— Fondaparinux — factor Xa inhibitor, does not cross-react with HIT antibodies. Off-label but evidence-supported; SC dosing makes it convenient for stable patients.
— DOACs (rivaroxaban, apixaban, dabigatran) — increasingly used in clinically stable, non-critically ill HIT patients without active major thrombosis; oral convenience and no monitoring.
— Isolated HIT (no thrombosis): anticoagulate for at least 4 weeks (or until platelet recovery and at least 4 weeks if antibodies persist).
— HIT with thrombosis (HITT): anticoagulate for 3–6 months minimum.
Step 3 management: The single most common Step 3 trap is starting warfarin early or transfusing platelets. Both are wrong answers. Correct sequence: stop heparin → start argatroban/bivalirudin/fondaparinux → wait for platelet recovery to ≥150 → then overlap with warfarin for ≥5 days and INR therapeutic for ≥2 days.
Board pearl: Even "heparin flushes only" causes HIT — write "no heparin, including flushes" explicitly in orders.
— Dose: 2 mcg/kg/min IV continuous infusion (start at 0.5–1.2 mcg/kg/min in heart failure, critical illness, post-cardiac surgery, or hepatic dysfunction).
— Monitoring: aPTT every 2 hours until stable, then q6–12h; goal 1.5–3× baseline (not exceeding 100 sec).
— Onset: minutes; half-life ~45 min; cleared hepatically (avoid or reduce in cirrhosis/MELD elevation).
— Falsely elevates INR — when transitioning to warfarin, overlap ≥5 days and stop argatroban only when INR remains therapeutic 4–6 hours after holding argatroban.
— Off-label for HIT but commonly used, especially in PCI, cardiac surgery, ECMO.
— Dose: 0.15–0.20 mg/kg/hr IV (no bolus for HIT); adjust for renal function (reduce 50% if CrCl <30; further reduction on dialysis).
— Monitor aPTT q4–6h, goal 1.5–2.5× baseline.
— Half-life 25 min; ~80% proteolytic clearance, ~20% renal.
— Does not bind PF4, no cross-reactivity with HIT antibodies.
— Dose (HIT, off-label): weight-based SC daily — <50 kg: 5 mg; 50–100 kg: 7.5 mg; >100 kg: 10 mg.
— Renally cleared; avoid if CrCl <30.
— No routine monitoring; advantage for clinically stable patients transitioning out of ICU.
— Rivaroxaban 15 mg BID × 21 days, then 20 mg daily; or apixaban 10 mg BID × 7 days, then 5 mg BID.
— Reserved for clinically stable patients without limb-/life-threatening thrombosis, no need for procedures, no GI absorption issues.
— Avoid in CrCl <30 (apixaban somewhat more permissive), severe hepatic disease.
— Isolated HIT: ≥4 weeks (until antibody clearance and platelet recovery).
— HITT: 3 months minimum, often extended based on thrombosis location/provoking factors.
CCS pearl: Order "argatroban IV continuous infusion, titrate to aPTT 1.5–3× baseline" plus "aPTT q2h until stable" — this displays both drug knowledge and the monitoring discipline graders look for.
— Do not start warfarin until platelet count has recovered to ≥150 × 10⁹/L (or returned to baseline).
— Overlap parenteral agent with warfarin for minimum 5 days AND until INR therapeutic (2.0–3.0) for ≥2 consecutive days.
— Start at low doses (≤5 mg) — avoid loading doses (rapid protein C depletion → venous limb gangrene/skin necrosis).
— Argatroban falsely elevates INR — use a goal INR of >4 while on argatroban, then recheck INR 4–6 hours after stopping argatroban to confirm true therapeutic level.
— Simpler: stop parenteral agent and start DOAC at therapeutic dose once platelets recover and patient clinically stable.
— Preferred in many cases now for simplicity and outpatient management.
— Cardiac surgery / CPB: if surgery is unavoidable and antibodies still present → bivalirudin for CPB. If antibodies have cleared (typically >100 days from acute HIT, confirmed by negative ELISA and SRA) → UFH may be reused for the single intraoperative dose only, with bivalirudin for pre- and post-op anticoagulation.
— PCI: bivalirudin is preferred intraprocedural anticoagulant.
— Hemodialysis: argatroban or citrate regional anticoagulation; avoid heparin in circuit.
— Catheter flushes: use normal saline; remove heparin-coated central lines.
Board pearl: A HIT patient cleared for future cardiac surgery >100 days out, with negative antibodies, can receive a single intraoperative UFH dose for bypass — but pre- and post-op anticoagulation must be non-heparin.
Step 3 management: When asked "next step after platelets recover?" — the answer is overlap warfarin with the parenteral agent for ≥5 days AND until INR therapeutic for 2 days, not "stop argatroban and start warfarin."
— Higher baseline bleeding risk (HAS-BLED elements: age >65, prior bleed, polypharmacy, falls).
— Reduced renal and hepatic reserve → dose-adjust all non-heparin anticoagulants.
— Polypharmacy: argatroban interacts minimally; DOACs have CYP3A4/P-gp interactions (amiodarone, diltiazem, azoles).
— Frequent fall risk — DOACs preferred over warfarin where appropriate due to lower ICH risk.
— Argatroban = drug of choice in renal failure (including dialysis) — hepatic clearance, no dose adjustment for CrCl.
— Bivalirudin requires significant dose reduction: CrCl 30–60 → 80% of dose; CrCl <30 → 50%; dialysis → 25%; monitor aPTT closely.
— Fondaparinux — accumulates; avoid if CrCl <30, caution at 30–50.
— Rivaroxaban/apixaban — apixaban tolerated better in CKD; both contraindicated in CrCl <15; dabigatran is renally cleared (avoid in CrCl <30).
— Warfarin — generally safe across renal function but unstable INR in advanced CKD.
— Bivalirudin = drug of choice in hepatic dysfunction (proteolytic clearance dominant).
— Argatroban requires dose reduction in hepatic dysfunction: start at 0.5 mcg/kg/min if MELD elevated, bilirubin >1.5, or severe heart failure (low hepatic perfusion).
— DOACs (apixaban, rivaroxaban) contraindicated in Child-Pugh C; caution in B.
— Warfarin difficult in advanced cirrhosis — baseline INR elevation, variable response.
— Hepatic perfusion may be poor → reduce argatroban starting dose.
— Monitor more frequently (aPTT q1–2h initially).
— Drug-drug interactions, hemodilution, and circuit consumption complicate dosing — close pharmacist involvement.
Key distinction: Renal failure → argatroban. Hepatic failure → bivalirudin. Both failing → bivalirudin with cautious dose titration.
Board pearl: In the post-cardiac surgery patient (low hepatic perfusion, multi-organ dysfunction), argatroban must be started at 0.5 mcg/kg/min, not the standard 2 mcg/kg/min — bleeding from overdose is a tested complication.
— HIT in pregnancy is rare because LMWH (lower risk than UFH) is the standard for VTE prophylaxis and treatment in pregnancy.
— When HIT occurs, fondaparinux is the preferred alternative (does not cross placenta meaningfully; case series support safety).
— Danaparoid is preferred in regions where available (no placental transfer).
— Avoid warfarin in pregnancy (teratogenic in first trimester, fetal hemorrhage later); avoid DOACs (limited safety data, placental transfer).
— Argatroban and bivalirudin: limited pregnancy data; reserve for situations where fondaparinux unavailable or contraindicated.
— Postpartum: warfarin and LMWH are compatible with breastfeeding; fondaparinux likely safe.
— HIT is uncommon; highest risk in post-cardiac surgery children on prolonged UFH and adolescents.
— Diagnosis follows adult 4T-style criteria but pediatric platelet baselines differ.
— Argatroban and bivalirudin are used off-label in pediatrics with weight-based dosing; specialty hematology consult mandatory.
— Neonates: hepatic immaturity affects argatroban clearance; bivalirudin often preferred.
— High HIT incidence (1–3%) due to large UFH doses.
— Acute HIT requiring urgent CPB → bivalirudin-based CPB protocols (specialized centers).
— Elective surgery with history of HIT:
— Confirm antibody clearance (negative ELISA and SRA, generally >100 days from acute HIT).
— Use UFH only intraoperatively for CPB, with bivalirudin or argatroban for pre- and post-op anticoagulation.
— Avoid heparin-bonded circuits and heparin flushes.
— HIT management requires switch to bivalirudin or argatroban infusion through the circuit, with intensified monitoring and platelet count trending.
Step 3 management: For a pregnant patient on LMWH for prior VTE who develops HIT, the answer is stop LMWH, start fondaparinux, NOT warfarin or rivaroxaban.
Board pearl: History of HIT is not a permanent contraindication to heparin for CPB — once antibodies are undetectable (typically >100 days), a single intraoperative UFH exposure is acceptable.
— Venous thromboembolism (DVT/PE) — most common; PE may be fatal.
— Arterial thrombosis — limb ischemia (may require amputation), stroke, MI, mesenteric ischemia.
— Cerebral venous sinus thrombosis — present with headache, focal deficits, seizures.
— Bilateral adrenal hemorrhage from adrenal vein thrombosis → acute adrenal insufficiency (hypotension, hyponatremia, hyperkalemia, abdominal pain). Mortality high if missed; treat empirically with hydrocortisone while imaging.
— Heparin-induced skin necrosis at injection sites — erythematous plaques progressing to necrotic ulcers; pathognomonic.
— Venous limb gangrene — limb swelling, ischemia, and necrosis in a setting of supratherapeutic warfarin (high INR) early after HIT diagnosis; mechanism is protein C depletion with ongoing thrombin generation. Often requires amputation.
— Warfarin-induced skin necrosis — overlapping mechanism (protein C deficiency) on trunk, breast, thigh.
— Within 5–30 minutes of IV heparin bolus in a sensitized patient: fever, chills, dyspnea, hypertension, tachycardia, cardiac arrest. Mimics PE.
— Uncommon from HIT itself; more often iatrogenic from over-anticoagulation with the non-heparin agent. Monitor aPTT/anti-Xa carefully.
— Post-thrombotic syndrome from DVT.
— Chronic thromboembolic pulmonary hypertension from unresolved PE.
— Functional disability from amputation, stroke, or MI.
— Lifetime heparin avoidance — limits options for future hospitalizations, dialysis, surgery.
— Historical untreated HIT mortality ~20–30%; with prompt non-heparin anticoagulation, ~6–10%.
— Limb loss in 2–10% even with treatment.
Key distinction: Venous limb gangrene = HIT + early warfarin + supratherapeutic INR. This is iatrogenic and preventable — never start warfarin until platelets recover.
CCS pearl: Order MedicAlert bracelet and document HIT prominently in the problem list and allergy list before discharge — failure to do so risks fatal re-exposure on a future admission.
— Guides serology interpretation, anticoagulant selection, duration, and rechallenge decisions.
— Critical for atypical presentations, pregnancy, pediatrics, and complex comorbidities.
— Hemodynamic instability (massive PE, adrenal crisis, septic shock from limb ischemia).
— Acute limb ischemia requiring urgent vascular intervention.
— Need for continuous bivalirudin or argatroban with frequent aPTT monitoring.
— Post-cardiac surgery, ECMO, or VAD patients developing HIT.
— Active major bleeding on non-heparin anticoagulant.
— Acute limb ischemia (6 P's) → emergent thromboembolectomy or bypass.
— Mesenteric ischemia → ex-lap or endovascular intervention.
— Hemodynamically significant PE not responsive to anticoagulation alone — consider surgical embolectomy (thrombolytics risky with concurrent argatroban).
— Cardiac surgery patient who develops HIT with bypass graft thrombosis.
— Catheter-directed thrombectomy for limb-threatening DVT/iliofemoral DVT.
— Avoid IVC filter unless absolutely necessary (high thrombosis rate).
— Bilateral adrenal hemorrhage → long-term steroid replacement, adrenal crisis education.
— Inpatient → step-down: continue parenteral non-heparin agent until platelets recover and oral agent therapeutic.
— Discharge: ensure outpatient anticoagulation clinic follow-up within 1 week; HIT clearly documented in EHR allergy/problem list; patient given written discharge instructions including "no heparin, no LMWH" lifetime advisory.
— Sentinel event review if heparin re-exposed despite documented HIT.
— Pharmacy hard-stop alerts in EHR for heparin orders in HIT-flagged patients.
Step 3 management: Suspected HIT is never an outpatient diagnosis — admit, stop heparin, start non-heparin anticoagulant, screen for thrombosis with duplex, consult hematology.
Board pearl: Always confirm adrenal function in a HIT patient with refractory hypotension and abdominal pain — bilateral adrenal hemorrhage is rare but lethal if missed.
— Common culprits: vancomycin, linezolid, sulfonamides, quinine, quinidine, rifampin, GP IIb/IIIa inhibitors (abciximab, eptifibatide), carbamazepine, valproate, ibuprofen.
— Mechanism: drug-dependent antibodies bind platelet GP. Severe thrombocytopenia (often <20), bleeding (not thrombosis) dominates.
— Onset typically 1–2 weeks after starting drug; recovery within days of discontinuation.
— Key distinction from HIT: bleeding phenotype, no thrombosis, lower platelet nadir.
— Autoimmune anti-platelet antibodies; isolated thrombocytopenia, no thrombosis.
— Diagnosis of exclusion; consider in young adults (especially women) with petechiae and mucosal bleeding.
— Treat with corticosteroids, IVIG, anti-D, TPO agonists.
— Severe thrombocytopenia 5–10 days after transfusion in HPA-1a–negative patient.
— Treat with IVIG.
— Pentad: MAHA, thrombocytopenia, neurologic, renal, fever. ADAMTS13 <10%.
— Schistocytes on smear — KEY differentiator from HIT.
— Emergency plasma exchange + steroids + caplacizumab.
— MAHA + thrombocytopenia + AKI; Shiga toxin (typical) or complement-mediated (atypical).
— Supportive care; eculizumab for atypical HUS.
— Consumptive coagulopathy; prolonged PT/aPTT, low fibrinogen, elevated D-dimer, schistocytes.
— Underlying trigger (sepsis, malignancy, trauma, OB emergency). Treat underlying cause.
— HIT-like syndrome 5–30 days after adenoviral vector COVID vaccines (Ad26.COV2.S, ChAdOx1).
— Anti-PF4 antibodies without heparin exposure; cerebral venous sinus thrombosis and splanchnic thrombosis prominent.
— Treat like HIT: non-heparin anticoagulation + high-dose IVIG; avoid platelet transfusion.
Key distinction: HIT and VITT share PF4 antibody mechanism but differ in trigger — heparin vs adenoviral vector vaccine. Both use the same management approach.
Board pearl: Schistocytes on smear → think TTP/HUS/DIC, not HIT.
— Multifactorial: marrow suppression, splenic sequestration, DIC, hemophagocytosis.
— Trend with clinical course; treat underlying infection. No immune mechanism in pure sepsis.
— Hemodilution + CPB-induced platelet activation → platelet nadir typically day 1–4 post-op, recovering by day 5–7.
— Distinct from HIT, which falls again around day 5–10 after an initial recovery — "biphasic" pattern is the classic HIT signature in cardiac surgery patients.
— After large-volume crystalloid or RBC resuscitation; correlates temporally with transfusion volume.
— Cirrhosis causes splenic sequestration and decreased thrombopoietin; platelets typically 50–100.
— Splenomegaly on exam/imaging; chronic, stable thrombocytopenia.
— Aplastic anemia, MDS, leukemia, marrow metastases. Usually pancytopenia.
— Smear abnormalities, marrow biopsy diagnostic.
— EDTA-induced platelet clumping → falsely low automated count.
— Repeat CBC in citrate (blue top) — corrects to normal. Always exclude before workup.
— Gestational thrombocytopenia (mild, third trimester).
— HELLP syndrome (hemolysis, elevated LFTs, low platelets, hypertension).
— Preeclampsia, acute fatty liver of pregnancy.
— Thrombosis + thrombocytopenia + recurrent miscarriage.
— Lupus anticoagulant, anti-cardiolipin, anti-β2-glycoprotein I.
— Can mimic HIT clinically; check antibody panel.
Step 3 management: Before diagnosing HIT, exclude pseudothrombocytopenia (citrate-tube repeat), DIC (fibrinogen, PT/aPTT, D-dimer, schistocytes), and drug-induced thrombocytopenia from non-heparin agents (review medication list).
Key distinction: Post-cardiac-surgery platelet nadir on day 2 with steady recovery = normal post-op pattern; nadir on day 8 after initial recovery = HIT.
— Isolated HIT (no thrombosis): anticoagulate for at least 4 weeks — period of highest thrombosis risk persists even after heparin discontinuation due to ongoing antibody-mediated platelet activation.
— HIT with thrombosis (HITT): 3 months minimum; extend for unprovoked or recurrent thrombosis per standard VTE guidelines (often 6–12 months or indefinite).
— DOAC (rivaroxaban or apixaban) — increasingly first-line for stable patients without major drug interactions; no INR monitoring, simpler transition.
— Warfarin — acceptable; bridge with parenteral agent until INR therapeutic for ≥2 days AND ≥5 days overlap, only after platelets ≥150.
— Avoid restarting LMWH or UFH for the duration of treatment and indefinitely.
— Document HIT in allergy list, problem list, and discharge summary prominently.
— Provide MedicAlert bracelet and wallet card.
— Counsel patient to inform every future provider (hospital, dialysis, surgery, OB).
— Re-exposure within ~3 months can precipitate rapid-onset HIT; beyond 100 days, controlled single-dose intraoperative UFH for CPB may be acceptable if antibodies negative.
— Recognize symptoms of recurrent thrombosis: leg swelling, chest pain, dyspnea, neuro deficits, abdominal pain.
— Bleeding precautions on any anticoagulant.
— Avoid IM injections, contact sports; soft toothbrush, electric razor.
— Drug interactions (NSAIDs, antibiotics with warfarin; CYP3A4/P-gp inhibitors with DOACs).
— Aggressively manage VTE risk factors (mobility, hydration, treat malignancy).
— Address provoking factors and consider thrombophilia evaluation only if clinically indicated.
Step 3 management: Discharge checklist for HIT — (1) oral anticoagulant prescription with clear duration, (2) anticoagulation clinic follow-up within 1 week, (3) HIT documented in allergy list, (4) MedicAlert bracelet ordered, (5) patient counseled on lifetime heparin avoidance.
Board pearl: "Isolated HIT" still requires 4 weeks of therapeutic anticoagulation — undertreatment is a frequently tested error.
— Anticoagulation clinic visit within 1 week of discharge — verify platelet recovery, INR if on warfarin, no bleeding or new thrombosis.
— Hematology follow-up at 2–4 weeks to confirm platelet normalization and review duration of anticoagulation.
— Repeat lower extremity duplex if symptoms suggest recurrence.
— Re-image PE/DVT at end of treatment course only if extending anticoagulation decision pending.
— CBC weekly until platelets stable at baseline, then monthly during anticoagulation.
— INR per protocol if on warfarin (weekly initially, then every 2–4 weeks once stable).
— DOACs: no routine coagulation monitoring; check CBC, renal, and hepatic function every 3–6 months.
— Consider repeat PF4-ELISA at 3 months to document antibody clearance — useful if future heparin exposure may be needed (cardiac surgery, dialysis).
— Lifetime heparin avoidance — including LMWH, fondaparinux is OK, but all heparins (UFH, dalteparin, enoxaparin, tinzaparin) prohibited.
— Recognize signs of recurrent thrombosis and bleeding.
— Medication adherence and dose timing for DOAC/warfarin.
— Dietary considerations for warfarin (vitamin K consistency).
— Pregnancy planning: warfarin teratogenic; women of childbearing age may prefer DOAC bridge with planning for fondaparinux/LMWH switch (but LMWH contraindicated post-HIT) — preconception counseling essential.
— Post-DVT: gradual return to activity, consider compression stockings (no longer routinely recommended for PTS prevention but useful symptomatically).
— Post-stroke or post-MI: cardiac rehab, neurorehab as indicated.
— Post-amputation: PT, prosthetic fitting, psychosocial support.
— Lifetime steroid replacement (hydrocortisone, fludrocortisone).
— Stress-dose protocol education, MedicAlert.
CCS pearl: "Schedule follow-up with anticoagulation clinic in 1 week" and "Counsel patient on lifetime heparin avoidance" are the highest-yield CCS orders before final discharge.
Board pearl: A history of HIT becomes the patient's lifelong medical fingerprint — every future hospitalization decision (DVT prophylaxis, dialysis, CPB) flows from that single label.
— Most serious sentinel event in HIT care is inadvertent heparin re-administration after diagnosis.
— Required EHR safeguards: hard-stop alerts on heparin orders for HIT-flagged patients, with override requiring two-physician sign-off and pharmacy verification.
— Document HIT prominently in allergy list (not just problem list), discharge summary, and medication reconciliation.
— MedicAlert bracelet and wallet card before discharge.
— Hand-off between ED, ICU, floor, and outpatient providers is the highest-risk window.
— Use structured hand-off (e.g., I-PASS) explicitly noting HIT status, current anticoagulant, target levels, and platelet trajectory.
— Skilled nursing facilities, dialysis units, and surgical centers must receive explicit written notification.
— When using off-label agents (fondaparinux, DOACs in HIT), document discussion of evidence base, alternatives, and risks.
— For pregnant patients, consent for fondaparinux must include discussion of limited but supportive data.
— Refusal of anticoagulation: document capacity assessment, risks of refusal (thrombosis, death), and shared decision-making.
— Any iatrogenic heparin re-exposure in a known HIT patient triggers sentinel event review under Joint Commission standards.
— Adverse drug events should be reported to FDA MedWatch.
— Failure to recognize HIT and continued heparin administration despite a falling platelet count is a common malpractice claim.
— Documentation of 4T score calculation, antibody testing rationale, and switch to non-heparin agent provides defensible clinical reasoning.
— If HIT developed as a complication of heparin therapy, transparent disclosure to the patient (per AMA ethics) is required — even when therapy was appropriately indicated.
— Non-heparin anticoagulants are expensive; appropriate use (intermediate/high 4T) avoids waste from over-treating low-probability cases.
— Pharmacy stewardship and protocol-driven order sets improve cost-effectiveness.
Step 3 management: Before discharging any HIT patient, verify and document: (1) allergy list updated, (2) MedicAlert ordered, (3) written patient education provided, (4) outpatient providers notified, (5) anticoagulation clinic appointment within 7 days.
Board pearl: Heparin re-exposure in a HIT-labeled patient is a never event — system safeguards, not just memory, prevent it.
Board pearl: Thrombocytopenia + thrombosis + heparin exposure 5–10 days ago = HIT until proven otherwise. Stop heparin, start argatroban, do not transfuse platelets, do not start warfarin.
— Post-op day 7 orthopedic patient on enoxaparin prophylaxis; platelets 230 → 95. New unilateral leg swelling.
— Best next step? Stop enoxaparin, start argatroban, send PF4-ELISA, duplex bilateral lower extremities.
— Patient with prior cardiac surgery 6 weeks ago, now admitted for new MI; receives IV UFH; platelets drop within 12 hours.
— Diagnosis? Rapid-onset HIT from preformed antibodies. Treatment: bivalirudin.
— Patient discharged 10 days ago post-CABG returns with new DVT and platelets 70. No current heparin.
— Mechanism? Delayed-onset HIT. Send PF4-ELISA, start non-heparin anticoagulant.
— HIT patient on argatroban with platelets recovering to 80; resident starts warfarin 10 mg.
— Risk? Venous limb gangrene/skin necrosis from rapid protein C depletion. Correct approach: wait until platelets ≥150, low-dose warfarin, ≥5-day overlap.
— HIT patient with platelets 40, no bleeding, central line needed.
— Action? Do NOT transfuse prophylactically. Use ultrasound-guided line, hold pressure; transfuse only if active bleeding.
— Patient with 4T = 2 (mild fall day 12, no thrombosis, alternative cause likely).
— Action? HIT essentially excluded; continue heparin if needed; investigate alternative causes.
— HIT patient with abdominal pain, hypotension, Na 128, K 5.8.
— Diagnosis? Bilateral adrenal hemorrhage → adrenal insufficiency. Empiric hydrocortisone, CT abdomen.
— Pregnant patient on enoxaparin develops HIT.
— Anticoagulant? Fondaparinux (avoid warfarin/DOAC in pregnancy; LMWH contraindicated post-HIT).
— Dialysis patient with HIT → argatroban. Cirrhotic patient with HIT → bivalirudin.
— Patient with HIT 4 months ago, negative antibodies, needs urgent CABG.
— Approach? Single intraoperative UFH dose for CPB acceptable; bivalirudin pre- and post-op.
Key distinction: The two most common Step 3 wrong answers in HIT stems are (1) starting warfarin too early and (2) transfusing platelets prophylactically. Recognize and reject both.
Board pearl: When the stem includes "platelet count fell from 250 to 110 on hospital day 7 while on heparin DVT prophylaxis," the answer choices will tempt you with "ITP," "DIC," or "sepsis" — choose HIT unless schistocytes, bleeding, or low fibrinogen are explicitly stated.
HIT is an IgG-mediated, prothrombotic reaction to heparin presenting as a >50% platelet fall on day 5–10 (or earlier with re-exposure) — managed by immediately stopping all heparin, calculating the 4T score, sending PF4-ELISA, and starting a therapeutic non-heparin anticoagulant (argatroban for renal failure, bivalirudin for hepatic failure, fondaparinux or DOAC for stable patients) — never transfusing platelets prophylactically, never starting warfarin until platelets recover to ≥150, and committing the patient to lifetime heparin avoidance.
High-yield recap bullets:
Board pearl: Thrombocytopenia + thrombosis + heparin = HIT until proven otherwise — act first, confirm second.