Prolonged PT and APTT: Factor Deficiency vs Inhibitor
A prolonged PT or APTT begins with a checklist: repeat on a correctly filled citrate tube (double-spun, platelet poor).
Pattern localization
- Prolonged PT only: low factor VII — early warfarin effect, vitamin K deficiency, or liver disease; factor VII has the shortest half-life.
- Prolonged APTT only: factors VIII, IX, XI, XII, also heparin contamination and lupus anticoagulant; factor VIII is the classic cause (classic hemophilia A).
- Both prolonged: common pathway factors II, V, X, or fibrinogen — DIC, liver disease, vitamin K deficiency, warfarin, or a combined deficiency.
Mixing study
A 1:1 mix with pooled normal plasma corrects a factor deficiency immediately; persistent prolongation indicates an inhibitor. Incubate for 1-2 hours to uncover time-dependent inhibitors (factor VIII inhibitors).
Factor assays
If the mix corrects, factor assays localize the deficiency. A negative mixing (non-correcting APTT) on a heparin-contaminated sample shows a reversed pattern with an elevated thrombin time; run a lupus anticoagulant panel when no factor-specific inhibitor is found.
Quick flags
Thrombin time distinguishes fibrinogen (prolonged TT and reptilase) from heparin effect (prolonged TT, normal reptilase). Platelets, D-dimer, and fibrinogen screen for DIC.