Expansion
PT tests the extrinsic pathway, APTT the intrinsic, both the common
Expansion
| PT | APTT | Suggests |
|---|---|---|
| Prolonged | Normal | Factor VII deficiency, early warfarin, early liver disease, vitamin K deficiency |
| Normal | Prolonged | Haemophilia A (VIII) or B (IX), factor XI or XII, von Willebrand disease, heparin, lupus anticoagulant |
| Prolonged | Prolonged | Liver disease, DIC, massive transfusion, vitamin K deficiency, direct oral anticoagulants, factor X, V, II or fibrinogen deficiency |
| Normal | Normal | Platelet disorder, factor XIII deficiency, vascular cause, mild von Willebrand disease |
Which pathway
- PT measures the extrinsic and common pathway: factors VII, X, V, II, fibrinogen
- APTT measures the intrinsic and common pathway: XII, XI, IX, VIII, X, V, II, fibrinogen
- Thrombin time measures the conversion of fibrinogen to fibrin, and is prolonged by heparin, low or dysfunctional fibrinogen
Mixing study: mix the patient’s plasma 50:50 with normal plasma.
- Corrects: a factor deficiency, since the normal plasma supplies what is missing
- Does not correct: an inhibitor, such as a lupus anticoagulant or a factor VIII antibody
The lupus anticoagulant paradox is worth remembering: it prolongs the APTT in the test tube but causes thrombosis in the patient.
A normal clotting screen does not exclude a bleeding disorder. Von Willebrand disease, platelet function defects and factor XIII deficiency all present with a normal screen, so a convincing bleeding history warrants specific testing.