Coagulation Studies: PT/INR, aPTT, D-dimer, and Fibrinogen Demystified
Coagulation Studies: PT/INR, aPTT, D-dimer, and Fibrinogen Demystified When a prolonged aPTT is lupus anticoagulant vs. factor deficiency vs. heparin contamination—and why D-dimer is the most overordered test in the ED. Coagulation studies get ordered constantly but are interpreted correctly far less often. The most common mistakes: ordering a D-dimer on a patient who clearly needs imaging, panicking about a mildly prolonged aPTT in a preop patient, and missing the lupus anticoagulant connection that ties directly into this series. Let's decode the panel. The Tests and What They Measure Test Pathway What It Assesses Prolonged By PT (Prothrombin Time) / INR Extrinsic + common pathway (Factors VII, X, V, II, fibrinogen) Warfarin monitoring; liver synthetic function Warfarin, liver disease, vitamin K deficiency, DIC, factor VII deficiency aPTT (Activated Partial Thromboplastin Time) Intrinsic + common pathway (Factors XII, XI, IX, VIII, X, V, II, fibrinogen) Heparin monitoring; intr...