Hemostasis and Thrombosis
Hemostasis and thrombosis
Read coagulation tests as patterns. Prolonged aPTT with normal PT points to the intrinsic pathway (factors VIII, IX, XI, XII, and von Willebrand factor); prolonged PT with normal aPTT to the extrinsic pathway (factor VII); both prolonged to the common pathway (X, V, II, fibrinogen) or DIC. In mixing studies, immediate correction suggests factor deficiency, failure to correct suggests a specific inhibitor, and correction only after 1–2 hours of incubation suggests a slow-acting inhibitor such as a lupus anticoagulant, which predisposes to thrombosis rather than bleeding. Factor assays confirm and quantify.
Hereditary thrombophilias include factor V Leiden (activated protein C resistance), the prothrombin G20210A variant, and deficiencies of antithrombin, protein C, and protein S. Test only when the result will change management, and avoid testing during acute thrombosis or anticoagulation. Antiphospholipid syndrome requires clinical criteria plus persistent antibodies (anticardiolipin, anti-β2-glycoprotein I, or lupus anticoagulant) at least 12 weeks apart.
Monitoring: unfractionated heparin by aPTT or anti-Xa; low-molecular-weight heparin generally by anti-Xa in renal impairment, pregnancy, or obesity; warfarin by INR. Direct oral anticoagulants need no routine monitoring but variably affect PT and aPTT and can cause false-positive lupus anticoagulant testing. DIC shows thrombocytopenia, prolonged PT/aPTT, low fibrinogen, and high D-dimer.
Trap: an underfilled citrate tube or a heparin-contaminated line draw mimics a factor deficiency; repeat the sample before diagnosing hemophilia.
Rule of thumb: immediate correction on mixing means deficiency; no correction means inhibitor; correction after incubation means lupus anticoagulant.