drug-induced thrombocytopenia

**Mechanisms** 1. **Immune-mediated (drug-dependent antibodies)**: The drug binds a platelet glycoprotein (GP IIb/IIIa or GP Ib/IX). The antibody then binds the drug–GP complex, and the spleen and macrophages clear the opsonized platelets. Onset is typically 5-10 days after first exposure, or within hours on re-exposure. Platelet counts are often very low (<20,000/µL), with mucocutaneous bleeding such as petechiae, purpura, epistaxis, and wet purpura. Common culprits are quinine (including tonic water), vancomycin, TMP-SMX, beta-lactams (piperacillin), carbamazepine, rifampin, and NSAIDs. Abciximab and other GP IIb/IIIa inhibitors (eptifibatide, tirofiban) can cause acute thrombocytopenia within hours, sometimes from pre-existing antibodies. 2. **Heparin-induced thrombocytopenia (HIT)**: - *Type I*: mild, non-immune, occurs in the first 1-2 days, resolves spontaneously, and heparin can be continued. - *Type II*: IgG antibodies against the **platelet factor 4 (PF4)–heparin complex** activate platelets through FcγRIIa. This causes platelet activation, aggregation, and thrombin generation. Onset is 5-10 days after exposure (earlier if heparin was used in the last ~100 days). The platelet count falls >50% (usually to 20,000-100,000/µL, rarely <20,000/µL), and the result is **thrombosis** (venous more than arterial: DVT, PE, limb ischemia, skin necrosis at injection sites, cerebral venous thrombosis, adrenal infarction), not bleeding. It is more common with unfractionated heparin than LMWH. - *Diagnosis*: 4T score (Thrombocytopenia, Timing, Thrombosis, oTher causes), then PF4-heparin ELISA, then confirmation with the functional serotonin release assay. - *Treatment*: **Stop all heparin, including flushes and heparin-coated catheters, and start a non-heparin anticoagulant**: argatroban (hepatic clearance; use in liver failure with dose adjustment), bivalirudin, or fondaparinux/DOACs. Avoid **warfarin** until platelets recover (>150,000/µL), because it can precipitate venous limb gangrene and skin necrosis through protein C depletion. Avoid platelet transfusion, which may fuel thrombosis. 3. **Non-immune marrow suppression**: Chemotherapy (alkylators, antimetabolites), linezolid (dose- and duration-dependent), valproate, thiazides, ethanol, and ganciclovir. These cause a decrease in platelet production, often with other cytopenias. Valproate also causes platelet dysfunction. 4. **Drug-induced thrombotic microangiopathy (TMA)**: Quinine, gemcitabine, mitomycin C, cyclosporine/tacrolimus, and clopidogrel (rare) cause thrombocytopenia with microangiopathic hemolytic anemia (schistocytes) and renal injury. Unlike TTP, ADAMTS13 activity is typically normal.

**Diagnosis** - Exclude pseudothrombocytopenia (EDTA-induced clumping; repeat in a citrate tube or review the peripheral smear). - Review the timeline of drug exposure. Platelet recovery after stopping the drug supports the diagnosis (positive dechallenge). Rechallenge is the gold standard but is rarely performed. - Peripheral smear: isolated thrombocytopenia with normal RBC and WBC morphology in immune DITP. The marrow shows normal or increased megakaryocytes (increased destruction), unlike marrow suppression. - Differential includes ITP, TTP/HUS, DIC, sepsis, and HELLP.

**Management** - Stop the suspected drug and avoid structurally related drugs. - For severe bleeding or a platelet count <10,000/µL, give platelet transfusion. Corticosteroids and IVIG are sometimes used but have limited evidence. - HIT is treated as above. - Document the drug allergy.

**High-yield boards points** - Quinine is the classic cause of immune DITP; it can also cause TMA. - HIT causes a *prothrombotic* state, not bleeding. - The HIT antigen is the PF4–heparin complex. - The first step in HIT is to stop heparin and start argatroban or bivalirudin. - Do not give warfarin or platelet transfusion in acute HIT. - Drug-induced immune thrombocytopenia typically begins 1-2 weeks after starting a new drug. It resolves in about 1 week after stopping the drug.