Extrinsic measured by prothrombin time, intrinsic by APTT
Mnemonic
“1972” for the vitamin K dependent factors: X, IX, VII, II, plus proteins C and S.
“Play tennis outside (PT) or inside (APTT)”:
- PT measures the extrinsic (outside) pathway, factor VII, and is the one affected first by warfarin and by liver disease
- APTT measures the intrinsic (inside) pathway, factors XII, XI, IX, VIII
“The intrinsic pathway counts down 12, 11, 9, 8”, and both converge on the common pathway: X, V, II (prothrombin), I (fibrinogen).
Two consequences follow directly: haemophilia A (VIII) and B (IX) prolong the APTT only, and warfarin prolongs the PT first because factor VII has the shortest half life at about 6 hours. That short half life is also why warfarin is transiently procoagulant at the start, since protein C falls before the others.
Expansion
- Extrinsic pathway: tissue factor plus factor VII. Measured by the prothrombin time. The physiological initiator in vivo
- Intrinsic pathway: factors XII, XI, IX and VIII. Measured by the APTT
- Common pathway: factors X, V, II (prothrombin), I (fibrinogen) and XIII
Interpreting abnormal tests:
| PT | APTT | Suggests |
|---|---|---|
| Long | Normal | Factor VII deficiency, early warfarin, early liver disease |
| Normal | Long | Haemophilia A or B, von Willebrand disease, heparin, lupus anticoagulant |
| Long | Long | Common pathway defect, severe liver disease, DIC, vitamin K deficiency, direct oral anticoagulants |
Vitamin K dependent factors are II, VII, IX and X, plus proteins C and S. Factor VII has the shortest half-life, which is why the PT rises first in liver disease and warfarin initiation.
Note that the classic cascade is a laboratory model; in vivo, tissue factor and factor VIIa initiate coagulation and then amplify it through the intrinsic factors, which is why factor XII deficiency prolongs the APTT without causing bleeding.