PT/INR
PT (prothrombin time): 11-13.5 seconds. Measures extrinsic pathway (Factors VII, then common). INR (international normalized ratio): standardizes PT, 0.8-1.2. Warfarin: therapeutic INR 2-3 (most), 2.5-3.5 (mechanical valve). Elevated: warfarin, liver disease, DIC, vitamin K deficiency, factor deficiency. Monitor: warfarin therapy, frequency varies by stability. Vitamin K: reverses warfarin (dietary, oral, IV — IV risk of anaphylaxis). Reversal: vitamin K, FFP, PCC (faster).
aPTT
aPTT (activated partial thromboplastin time): 25-35 seconds. Measures intrinsic pathway (Factors XII, XI, IX, VIII, then common). Elevated: heparin, hemophilia (VIII, IX), DIC, lupus anticoagulant. Heparin: therapeutic aPTT 1.5-2.5x control. Low molecular weight heparin: anti-Xa monitoring (routine aPTT not needed). Reversal: protamine sulfate (for unfractionated heparin). Direct thrombin inhibitors: argatroban, bivalirudin (monitor aPTT).
Other Studies
Platelets: 150-450 x10^3/mcL. Low: thrombocytopenia (bleeding risk <50, high risk <20). Bleeding time: platelet function (rarely used). D-dimer: fibrin degradation product. Elevated: DVT, PE, DIC, inflammation, pregnancy, malignancy. High sensitivity, low specificity. Fibrinogen: 200-400 mg/dL. Low: DIC, severe bleeding. Thrombin time (TT): measures fibrinogen to fibrin. Anti-Xa: monitors LMWH, direct Xa inhibitors (rivaroxaban, apixaban). Mixing studies: factor deficiency vs. inhibitor.
Important Notes
- Therapeutic INR for warfarin: 2-3 for most conditions, 2.5-3.5 for mechanical heart valves.
- Protamine sulfate reverses unfractionated heparin — 1 mg neutralizes 100 units of heparin (max 50 mg).
Clinical Tips
- D-dimer is sensitive but not specific — a normal D-dimer can rule out DVT/PE in low-risk patients, but elevated does not confirm.
- Vitamin K reverses warfarin — oral for slow reversal (24-48h), IV for rapid (6-12h, but risk of anaphylaxis).
Summary
Coagulation studies (PT/INR for extrinsic/warfarin, aPTT for intrinsic/heparin, platelets, D-dimer) guide anticoagulation monitoring and bleeding risk assessment.
