Thrombocytopenia in the ICU & HIT

📘 1 · Washington Manual of Critical Care

Thrombocytopenia — Common, and a Marker of Illness

Thrombocytopenia (platelet count <150 ×10⁹/L) develops in up to a third or more of ICU patients and is an independent marker of severity and mortality — usually reflecting how sick the patient is rather than a primary platelet disease. The key questions are not "how low?" alone but "why?", "is the patient bleeding or clotting?", and "is this one of the few diagnoses that will kill or maim if missed — HIT, TTP, DIC or drug-induced immune thrombocytopenia?" The count itself rarely causes spontaneous bleeding until it is very low (<10–20 ×10⁹/L).

Summarised from the Washington Manual of Critical Care — Thrombocytopenia.

First, define the pattern

Timing — thrombocytopenia present on admission vs developing after day 4–5 (the classic window for HIT).
Severity — mild (100–150), moderate (50–100), severe (<50), or critical (<20 ×10⁹/L, spontaneous-bleeding risk).
Bleeding vs thrombosis — most consumptive states (DIC) bleed; HIT and TTP paradoxically clot despite a low count.
Rule out artefact first — EDTA-induced pseudothrombocytopenia (platelet clumping in the purple tube): repeat in a citrate tube and look at the film before acting on a surprise low count.

📗 2 · Marino's The ICU Book, 5th Edition (2025)

The Four Mechanisms — a Framework That Fits Every Case

Every thrombocytopenia in the ICU can be assigned to one (often more than one) of four mechanisms: reduced production, increased destruction or consumption, sequestration in an enlarged spleen, and dilution from massive fluid or blood resuscitation. Sorting the case into these bins — rather than reaching straight for a platelet transfusion — is what points to the specific, treatable diagnosis.

Summarised from Marino PL. The ICU Book, 5th Ed — Platelet Disorders in Critical Illness.

🔬 Why is the platelet count low?

Low plateletsConfirmed on film (not clumping / pseudothrombocytopenia)

↓ ProductionSepsis, marrow suppression, drugs, alcohol, B12/folate, malignancy

↑ DestructionDIC, sepsis, HIT, TTP/HUS, ITP, drug-immune, mechanical (ECMO/IABP/valve)

SequestrationSplenomegaly (portal hypertension, liver disease)

DilutionMassive transfusion / large-volume resuscitation

Act on the causeSepsis & DIC are commonest; but always screen for the "can't-miss" trio — HIT · TTP · drug-induced

📗 Marino — the diagnoses that clot, not bleed

HIT — an IgG antibody to the platelet-factor-4/heparin complex activates platelets → arterial & venous thrombosis with a falling count. Bleeding is rare; the danger is limb- and life-threatening clot.
TTP — deficiency of ADAMTS13 → ultralarge vWF multimers → microvascular platelet thrombi. A haematological emergency; plasma exchange is life-saving.
Never transfuse platelets reflexively in HIT or TTP — in TTP it can precipitate thrombosis; reserve for serious bleeding only.

📗 Marino — timing is the single best clue to HIT

HIT classically causes a platelet fall of >50% beginning day 5–10 after starting heparin (or within a day if there was heparin exposure in the last 100 days). A count that was low from admission, or that falls in the first 4 days without prior exposure, is not HIT. Use the 4Ts score to gate testing — do not send a HIT assay on every low count.

📋 3 · HIT, TTP & When to Transfuse

Evidence-Based Management

HIT — the 4Ts score gates testing ASH 2018

Score 0–2 points each for Thrombocytopenia (fall >50% & nadir ≥20 = 2), Timing (fall day 5–10 = 2), Thrombosis (new confirmed clot = 2), and no oTher cause (2). Low (0–3): HIT effectively excluded — do not test, continue heparin. Intermediate (4–5) / High (6–8): stop ALL heparin (including flushes & heparin-bonded lines), start a non-heparin anticoagulant, and send an anti-PF4 ELISA (confirm a positive with a functional serotonin-release assay).

HIT — anticoagulate; never just "stop heparin" STRONG

HIT is a prothrombotic state — stopping heparin alone leaves a ~50% thrombosis risk. Start a non-heparin anticoagulant (argatroban, danaparoid, fondaparinux, or bivalirudin) immediately when HIT is likely. Do not start warfarin until platelets recover >150 (early warfarin can cause venous limb gangrene); avoid prophylactic platelet transfusion.

TTP — recognise the pentad, act on the triad EMERGENCY

Suspect TTP with microangiopathic haemolytic anaemia (MAHA — schistocytes, ↑LDH, ↓haptoglobin) + thrombocytopenia, with or without neurological signs, AKI and fever (the classic pentad is rarely complete). The combination of MAHA + thrombocytopenia with a normal coagulation screen (normal PT/APTT/fibrinogen) is TTP until proven otherwise → urgent plasma exchange (+ steroids, caplacizumab, rituximab); send ADAMTS13 but do not wait for it. This distinguishes TTP from DIC, where the coagulation screen is deranged.

Platelet transfusion — restrictive, threshold-based

Transfuse for <10 ×10⁹/L (prophylaxis against spontaneous bleed), <20 with sepsis/fever or planned minor procedure, <50 for active bleeding, major surgery or invasive procedure, and <100 for CNS/ocular bleeding or neurosurgery. Withhold platelets in HIT and TTP unless there is life-threatening bleeding. Treat the cause — a transfusion is a bridge, not a treatment.

🇮🇳 Indian Context

ICU thrombocytopenia in India is dominated by sepsis and tropical infections — dengue, malaria, scrub typhus, leptospirosis and enteric fever — where the count can fall dramatically but usually recovers with treatment of the infection, and prophylactic platelet transfusion in stable dengue is discouraged (transfuse for bleeding, not for a number). Single-donor apheresis platelets (SDP) are costly and not always available; pooled random-donor platelets (RDP) are the workhorse. For HIT, argatroban and fondaparinux are the practical non-heparin options; the anti-PF4 ELISA and functional assays may need to be sent to a reference lab, so act on a high 4Ts score without waiting. For TTP, arrange plasma exchange early — refer to a centre with apheresis if not available locally.

💊 4 · Drug Doses

Non-Heparin Anticoagulants & Adjuncts

Drug	Indication	Dose	Notes
Argatroban	HIT (esp. hepatic OK, renal impairment)	0.5–2 µg/kg/min IV infusion, titrate to APTT 1.5–3×	Hepatically cleared — reduce in liver failure; prolongs INR (complicates warfarin bridging)
Fondaparinux	HIT (off-label but widely used)	Weight-based SC once daily (e.g. 7.5 mg for 50–100 kg)	Renally cleared — avoid if CrCl <30; convenient in stable patients
Bivalirudin	HIT with cardiac surgery / PCI / ECMO	IV infusion titrated to APTT	Short half-life; part renal, part enzymatic clearance
Danaparoid	HIT (where available)	IV bolus then infusion, anti-Xa monitored	Limited availability
Platelets (RDP pool / SDP)	Threshold-based (see §3)	1 adult dose ≈ ↑ 20–40 ×10⁹/L	Withhold in HIT/TTP unless life-threatening bleeding
Plasma exchange	TTP (definitive)	1–1.5 plasma volumes daily until remission	Life-saving; start on clinical suspicion — do not wait for ADAMTS13
Methylprednisolone	TTP / immune thrombocytopenia adjunct	1 mg/kg/day (higher in TTP)	With PEX in TTP; IVIg/steroids for ITP

🗺 5 · Clinical Flowchart

Approach to the Low Platelet Count in the ICU

Confirm it is real & check for bleeding

Exclude clumping/pseudothrombocytopenia — repeat in citrate, review the blood film
Assess bleeding vs new thrombosis; is the patient haemodynamically stable?

Assign the mechanism

↓ production · ↑ destruction/consumption · sequestration · dilution
Send coagulation screen, fibrinogen, D-dimer, film, LDH, haptoglobin, bilirubin, and review drug/heparin chart & timing

Screen for the "can't-miss" diagnoses

DIC? — deranged PT/APTT, ↓fibrinogen, ↑D-dimer, schistocytes → treat the trigger
HIT? — score the 4Ts; if ≥4, stop all heparin & start a non-heparin anticoagulant
TTP? — MAHA + low platelets with a normal coagulation screen → urgent plasma exchange

Treat the cause; transfuse by threshold

Source-control sepsis, stop the offending drug, replace deficiencies
Platelets only by threshold (<10 prophylaxis; <50 bleeding/procedure; <100 CNS) — not in HIT/TTP unless life-threatening

Reassess the trend

Recovery follows treatment of the cause; failure to recover → re-examine the mechanism
In confirmed HIT, continue non-heparin anticoagulation and delay warfarin until platelets >150

⚠️ 6 · Common Mistakes

Common Mistakes in ICU Thrombocytopenia

❌ Mistake 1 — Transfusing platelets reflexively for a number

Most ICU thrombocytopenia does not bleed and needs no transfusion. Worse, platelets can be harmful in HIT and TTP. Transfuse for bleeding or by threshold, and always ask "why is it low?" first.

❌ Mistake 2 — Just stopping heparin in suspected HIT

HIT is a clotting disease. Stopping heparin without starting a non-heparin anticoagulant leaves a ~50% thrombosis risk. Stop ALL heparin (including flushes and bonded lines) and anticoagulate.

❌ Mistake 3 — Sending a HIT assay on every low count

The anti-PF4 ELISA is sensitive but not specific; testing low-probability patients generates false positives and needless harm. Gate with the 4Ts score.

❌ Mistake 4 — Missing TTP by lumping it with DIC

TTP has a normal coagulation screen; DIC does not. MAHA + thrombocytopenia + normal PT/APTT/fibrinogen = TTP → plasma exchange, an emergency that must not wait for ADAMTS13.

❌ Mistake 5 — Starting warfarin early in HIT

Early warfarin, while still prothrombotic and thrombocytopenic, can cause venous limb gangrene and skin necrosis. Bridge with a parenteral non-heparin agent and delay warfarin until platelets recover >150.

❌ Mistake 6 — Forgetting pseudothrombocytopenia

EDTA-induced platelet clumping produces a spuriously low count and triggers unnecessary work-up and transfusion. Look at the film and repeat in a citrate tube.

🎓 7 · Exam Pearls — DrNB / PDCC / IFCCM / EDIC

Exam Pearls

Q: What are the four mechanisms of thrombocytopenia?
Decreased production, increased destruction/consumption, sequestration (splenomegaly) and dilution — assign every case to one or more before transfusing.

Q: What are the 4Ts of HIT?
Thrombocytopenia (magnitude), Timing (fall day 5–10), Thrombosis, and no oTher cause — each 0–2 points. Score ≥4 → stop heparin, anticoagulate, test.

Q: How is HIT managed?
Stop ALL heparin, start a non-heparin anticoagulant (argatroban/fondaparinux/bivalirudin/danaparoid), avoid platelet transfusion, and delay warfarin until platelets >150.

Q: How do you distinguish TTP from DIC at the bedside?
TTP has a normal coagulation screen (normal PT/APTT/fibrinogen) with MAHA + thrombocytopenia; DIC has a deranged screen with low fibrinogen and high D-dimer.

Q: What is the definitive treatment of TTP?
Urgent plasma exchange (plus steroids, caplacizumab, rituximab) — started on clinical suspicion, before ADAMTS13 results return.

Q: What platelet count triggers prophylactic transfusion?
<10 ×10⁹/L (or <20 with sepsis/fever); <50 for bleeding/procedures; <100 for CNS bleeding or neurosurgery.

Q: Which infections cause marked thrombocytopenia in India?
Dengue, malaria, scrub typhus, leptospirosis, enteric fever and sepsis — treat the infection; avoid prophylactic platelets in stable dengue.

📚 8 · References

References

Cuker A, Arepally GM, Chong BH, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia. Blood Adv. 2018;2:3360–3392.
Greinacher A. Heparin-Induced Thrombocytopenia. N Engl J Med. 2015;373:252–261.
Zheng XL, Vesely SK, Cataland SR, et al. (ISTH). ISTH guidelines for the diagnosis and treatment of thrombotic thrombocytopenic purpura. J Thromb Haemost. 2020;18:2486–2521.
Thachil J, Warkentin TE. How do we approach thrombocytopenia in critically ill patients? Br J Haematol. 2017;177:27–38.
Kaufman RM, Djulbegovic B, Gernsheimer T, et al. (AABB). Platelet transfusion: a clinical practice guideline. Ann Intern Med. 2015;162:205–213.
Marino PL. The ICU Book, 5th Edition. Platelet Disorders in Critical Illness. Wolters Kluwer; 2025.
Washington Manual of Critical Care, 4th Edition. Kollef MH, Witt CA (eds). Thrombocytopenia. Wolters Kluwer; 2023.