Thrombocytopenia in the ICU

Contents

Overview

Thrombocytopenia (platelet count <150 × 10⁹/L) occurs in 40–60% of ICU patients and is independently associated with increased mortality. It may reflect sepsis, medication effects, consumptive processes, immune mechanisms, or primary haematological disease. The most clinically urgent distinction is between conditions that require specific treatment — heparin-induced thrombocytopenia (HIT), thrombotic thrombocytopenic purpura (TTP), disseminated intravascular coagulation (DIC) — and those that can be managed expectantly.

Classification and Causes

Thrombocytopenia results from three mechanisms:

Decreased production: Bone marrow suppression (sepsis, drugs — particularly chemotherapy, linezolid, heparin at myelosuppressive doses, alcohol), infiltration (haematological malignancy, myelofibrosis), vitamin B12/folate deficiency.

Increased destruction: Immune-mediated (HIT, ITP, drug-induced immune thrombocytopenia, post-transfusion purpura), thrombotic microangiopathy (TTP, HUS, DIC), mechanical destruction (IABP, ECMO, prosthetic heart valves).

Sequestration: Hypersplenism (platelets trapped in enlarged spleen in portal hypertension, haematological conditions).

Common ICU causes:

  1. Sepsis (direct bone marrow suppression, DIC, consumption)
  2. Drug-induced: heparin (HIT), linezolid, glycoprotein IIb/IIIa inhibitors, antibiotics
  3. Dilutional: massive blood product transfusion
  4. DIC
  5. HIT
  6. TTP/HUS
  7. Post-cardiac surgery (CPB-related)

Heparin-Induced Thrombocytopenia

HIT is an immune-mediated, prothrombotic drug reaction caused by IgG antibodies against platelet factor 4 (PF4) complexed with heparin. It paradoxically causes thrombosis despite thrombocytopenia, and is associated with significant morbidity and mortality if not diagnosed and managed promptly.

Pathophysiology

Heparin binds PF4 (released from activated platelets), forming a multimolecular complex. In susceptible patients, IgG antibodies form against this complex. These antibodies bind to FcγRIIA receptors on platelets, causing massive platelet activation and aggregation — generating thrombin and causing both thrombocytopenia (platelet consumption) and thrombosis. Simultaneously, antibody binding to endothelium and monocytes activates tissue factor pathways, further promoting a prothrombotic state.

Clinical Features

  • Platelet count typically falls by >50% from the pre-heparin baseline (not necessarily below 150 × 10⁹/L)
  • Onset: 5–10 days after heparin exposure (or within 1 day if prior heparin exposure in the last 3 months — rapid-onset HIT)
  • Thrombocytopenia resolves within 1–2 weeks of stopping heparin
  • Thrombosis occurs in 30–75% of patients with HIT: venous (DVT, PE, limb vein thrombosis) more common than arterial (stroke, MI, limb ischaemia)

The 4Ts Score

The 4Ts score provides pre-test probability assessment:

Category 2 points 1 point 0 points
Thrombocytopenia >50% fall, nadir ≥20 30–50% fall, nadir 10–19 <30% fall, nadir <10
Timing Days 5–10, or ≤1 day (prior heparin within 30 days) >10 days, or day 1 (prior heparin 30–100 days) <4 days without recent heparin
Thrombosis New thrombosis, skin necrosis Progressive/recurrent thrombosis, erythematous skin lesions None
Other causes of thrombocytopenia None Possible Definite

Score: 0–3 (low, <5% probability), 4–5 (intermediate, ~14%), 6–8 (high, ~64%).

Diagnosis

ELISA for anti-PF4/heparin antibodies: high sensitivity (>95%), low specificity. A positive ELISA in a low-probability 4Ts patient may be a false positive.

Platelet activation assay (SRA — serotonin release assay): high specificity (>95%), less widely available. Gold standard for confirming HIT.

Management

Stop all heparin immediately — including LMWH, heparin flushes, heparin-bonded catheters, and heparin-coated lines.

Anticoagulation with a non-heparin agent is mandatory (because of the high thrombosis risk):

  • Argatroban: IV direct thrombin inhibitor; preferred in renal impairment (hepatically eliminated). Monitor APTT (target 1.5–3× baseline).
  • Fondaparinux: subcutaneous anti-Xa; not licensed for HIT in UK but widely used. Renally cleared — avoid in severe renal impairment.
  • Bivalirudin: IV direct thrombin inhibitor; used in cardiac procedures in HIT patients.
  • Danaparoid: anti-Xa; licensed for HIT in the UK but limited availability.

Do not give warfarin acutely — it can cause venous limb gangrene in HIT by depleting protein C before thrombin inhibition is established.

Platelet transfusions are contraindicated in HIT unless there is life-threatening haemorrhage — they may fuel further thrombosis.

Thrombotic Thrombocytopenic Purpura

TTP is a thrombotic microangiopathy (TMA) caused by severely deficient ADAMTS13 activity (the protease that cleaves large von Willebrand factor multimers). Uncleaved ultra-large VWF multimers cause platelet aggregation in small vessels, leading to thrombocytopenia, microangiopathic haemolytic anaemia (MAHA), and end-organ ischaemia.

Classic Pentad

Thrombocytopenia, MAHA, neurological symptoms (confusion, seizure, stroke), renal failure, fever. The pentad is rarely complete — MAHA + thrombocytopenia should trigger investigation.

Causes

Immune TTP (iTTP): Autoantibodies against ADAMTS13 — most common form. Idiopathic or triggered by drugs (quinine, clopidogrel, ticlopidine), pregnancy, HIV.

Congenital TTP (Upshaw-Schulman syndrome): ADAMTS13 gene mutation.

Investigations

ADAMTS13 activity <10% confirms TTP diagnosis. Peripheral blood film: schistocytes (fragmented red cells) — hallmark of MAHA. Elevated LDH, unconjugated bilirubin, reticulocyte count; negative direct antiglobulin test (Coombs). Normal coagulation (distinguishes from DIC).

Management

Plasma exchange (PEX): Daily sessions with FFP replacement — removes antibodies and provides ADAMTS13. Emergency PEX reduces mortality from >90% to approximately 10–20%. Start urgently; do not wait for ADAMTS13 result.

Corticosteroids: Prednisolone 1 mg/kg/day to suppress the immune response.

Caplacizumab: Anti-VWF nanobody preventing platelet-VWF interaction. Added to PEX + corticosteroids in confirmed iTTP — significantly reduces time to platelet normalisation and exacerbation rate (HERCULES trial).

Rituximab: Anti-CD20 monoclonal antibody; reduces relapse rate and ADAMTS13 autoantibody production. Given during acute episode and for relapse prevention.

Platelet transfusion: Contraindicated unless life-threatening haemorrhage — may fuel TMA.

Disseminated Intravascular Coagulation

DIC is widespread activation of coagulation causing microvascular thrombosis (causing end-organ ischaemia) and consumption of clotting factors and platelets (causing bleeding). It is always secondary to an underlying trigger.

Triggers

  • Sepsis (most common ICU cause)
  • Major trauma
  • Obstetric emergencies (amniotic fluid embolism, placental abruption, HELLP)
  • Malignancy (especially AML-M3)
  • Transfusion reactions
  • Burns
  • Liver failure

Diagnosis

ISTH DIC score: elevated D-dimer or FDPs (2–3 points), prolonged PT (0–2 points), low fibrinogen (0–1 point), low platelet count (0–2 points). Score ≥5 = overt DIC.

Management

Treat the underlying cause — the most important step.

Blood products for active bleeding or before invasive procedures:

  • Platelets: keep >50 × 10⁹/L if bleeding, >20 × 10⁹/L if not bleeding
  • FFP: 15 mL/kg to replace clotting factors (corrects PT, APTT)
  • Cryoprecipitate: two pools for fibrinogen <1 g/L (target >1.5–2 g/L)
  • Tranexamic acid: avoid in most DIC — there is already reduced fibrinolytic activity; TXA may worsen thrombosis in microcirculation. Exception: fibrinolysis-predominant DIC (APL/AML-M3, prostate cancer).

Other Causes

Immune thrombocytopenic purpura (ITP): Autoantibodies against platelet surface antigens (GPIb/IX, GPIIb/IIIa). Causes isolated thrombocytopenia without anaemia or coagulopathy. Treat with IV immunoglobulin, corticosteroids; anti-D in Rh-positive patients; rituximab; thrombopoietin receptor agonists (romiplostim, eltrombopag) for chronic ITP.

Drug-induced thrombocytopenia: Many drugs can cause immune-mediated thrombocytopenia. Common culprits: quinine, glycoprotein IIb/IIIa inhibitors (tirofiban, eptifibatide, abciximab), vancomycin, sulphonamides, cephalosporins, rifampicin. Resolves within days of stopping the drug.

Post-cardiac bypass: CPB causes platelet activation, haemodilution, and consumption; counts typically nadir on day 1–2 post-operatively and recover within a week.

Investigations

For new thrombocytopenia in ICU:

Mandatory:

  • Full blood count with differential and peripheral film (schistocytes → TMA; blast cells → haematological malignancy)
  • Coagulation screen: PT, APTT, fibrinogen, D-dimer (normal in HIT and TTP; abnormal in DIC)
  • Blood film review by haematologist
  • Drug history — identify potential culprits; calculate timing relative to heparin exposure (4Ts score)
  • Blood cultures if sepsis suspected

If TMA suspected (thrombocytopenia + schistocytes on film):

  • ADAMTS13 activity (sample before plasma exchange)
  • LDH, unconjugated bilirubin, reticulocyte count
  • Direct antiglobulin test (Coombs — negative in TTP)
  • Stool culture/Shiga toxin if HUS suspected

If HIT suspected (4Ts ≥4): anti-PF4/heparin ELISA; SRA if ELISA positive

Management Principles

  • Platelet transfusion threshold: In the ICU, transfuse to maintain >50 × 10⁹/L for active bleeding or invasive procedures, >20–25 × 10⁹/L if no bleeding risk.
  • Never transfuse platelets in HIT or TTP — contraindicated.
  • Stop the cause: Remove offending drugs immediately in drug-induced thrombocytopenia and HIT.
  • Treat the underlying condition: sepsis, DIC, etc.
  • Seek haematology input for complex cases (HIT, TTP, DIC, haematological malignancy).

Viva Questions

How do you diagnose and manage heparin-induced thrombocytopenia?

HIT should be suspected whenever a patient develops a fall in platelet count of more than 50% from baseline between days 5 and 10 of heparin exposure, particularly if associated with new thrombosis. The 4Ts score provides a structured pre-test probability: a score of 4–5 indicates intermediate and 6–8 indicates high probability. If the 4Ts score is 4 or above, all heparin must be stopped immediately — including LMWH, heparin flushes, and heparin-coated lines — without waiting for laboratory confirmation. Anti-PF4/heparin ELISA is highly sensitive but not specific; the serotonin release assay (SRA) is the gold standard but takes longer. A non-heparin anticoagulant must be started promptly, because HIT carries a very high thrombosis risk: argatroban is preferred in renal impairment as it is hepatically eliminated. Platelet transfusion is contraindicated as it may precipitate further thrombosis. Warfarin should not be started acutely as it depletes protein C rapidly and can cause limb venous gangrene. Once platelet count recovers above 150 × 10⁹/L, transition to warfarin or a DOAC can occur for ongoing anticoagulation. Patients must be counselled that they must never receive heparin again.

What is TTP and how does it differ from DIC?

Thrombotic thrombocytopenic purpura (TTP) and DIC both cause thrombocytopenia, but they have fundamentally different mechanisms, clinical features, and treatments — confusing them is dangerous. TTP is caused by deficiency of ADAMTS13, the metalloprotease that cleaves ultra-large von Willebrand factor multimers. Without ADAMTS13, these multimers accumulate and cause platelet aggregation in small vessels, causing microangiopathic haemolytic anaemia (MAHA), thrombocytopenia, and end-organ ischaemia. Coagulation tests are characteristically normal in TTP. DIC results from widespread activation of coagulation by a systemic trigger (sepsis, trauma, obstetric emergency), causing simultaneous microvascular thrombosis and consumption of clotting factors — hence coagulation is abnormal: elevated PT, APTT, D-dimer; low fibrinogen. On the blood film, both may show schistocytes in TTP, but DIC does not produce the microangiopathic picture unless there is concomitant TMA. The treatment differs fundamentally: TTP requires urgent plasma exchange with FFP replacement to provide ADAMTS13 and remove antibodies. Platelet transfusion is contraindicated. DIC is managed by treating the underlying cause and replacing blood products for bleeding. Platelet transfusion is appropriate in DIC with bleeding. ADAMTS13 activity below 10% confirms TTP.

What are the indications for platelet transfusion in the ICU and when is it contraindicated?

Platelet transfusion is indicated to prevent or treat bleeding in thrombocytopenia. The threshold depends on the clinical context: in patients with active haemorrhage, platelets should be maintained above 50 × 10⁹/L (or 100 × 10⁹/L for intracranial haemorrhage or neurosurgery). Before invasive procedures — central line insertion, chest drain, bronchoscopy — a threshold of 50 × 10⁹/L is appropriate, though many procedures can be safely performed at lower counts with adequate coagulation. For stable ICU patients without bleeding, transfusion is recommended if platelets fall below 10–20 × 10⁹/L as a prophylactic measure. Platelet transfusion is contraindicated in HIT — where it may fuel thrombosis by providing more substrate for platelet activation — and in TTP, where transfused platelets are rapidly consumed in the TMA process and may worsen microvascular thrombosis. In these conditions, platelet transfusion should only be given if there is life-threatening haemorrhage where the risk of withholding outweighs the risk of worsening TMA or HIT-associated thrombosis.