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Vitamin K and the prothrombin time
A 58-year-old man with obstructive jaundice from a pancreatic head mass has easy bruising. Prothrombin time is 22 seconds (control 13) and aPTT is mildly prolonged. After three daily doses of intravenous vitamin K, the prothrombin time returns to 13 seconds. What does this response indicate?
- correctA. Vitamin K deficiency from fat malabsorption, with intact hepatocyte function
- B. Inherited factor IX deficiency
- C. Disseminated intravascular coagulation
- D. Hepatocellular failure with impaired clotting factor synthesis
Explanation
Key fact
Correction of the prothrombin time by vitamin K proves the liver cells still work. In hepatocellular failure it does not correct.
Vitamin K deficiency from fat malabsorption, with intact hepatocyte function. The liver could make the factors all along; it lacked the cofactor.
How it happens
- Bile is needed to absorb the fat-soluble vitamins, including K.
- Obstructive jaundice blocks bile flow, so vitamin K absorption fails.
- Vitamin K is the cofactor for gamma-carboxylation of factors II, VII, IX and X.
- Uncarboxylated factors cannot bind calcium and are inactive.
- Factor VII has the shortest half-life, so the prothrombin time lengthens first.
- Replacing the vitamin parenterally bypasses the gut, and a working liver corrects within a day or two.
Correction proves the factory works and only the raw material was missing
Why not the others
- B. Inherited factor IX deficiency — is haemophilia B: a lifelong bleeding history, an isolated prolonged aPTT with a normal prothrombin time, and no response to vitamin K.
- C. Disseminated intravascular coagulation — consumes fibrinogen and platelets and raises D-dimer. It does not correct with vitamin K.
- D. Hepatocellular failure with impaired clotting factor synthesis — would not correct with vitamin K, because the problem is the factory, not the raw material. That is exactly why this test is done.
Remember
Give vitamin K and watch the prothrombin time. Corrects means malabsorption; does not correct means the liver has failed.
Go deeper
Vitamin K also carboxylates the anticoagulant proteins C and S, which is why starting warfarin without heparin cover can cause a transient prothrombotic state and skin necrosis.
Warfarin works by inhibiting vitamin K epoxide reductase, blocking regeneration of the reduced vitamin.
Newborns are given vitamin K at birth because breast milk contains very little and the gut is not yet colonised by vitamin K-producing bacteria.
Harper's Illustrated Biochemistry, 33rd ed. (McGraw Hill, 2025); Harrison's Principles of Internal Medicine, 22nd ed. (McGraw Hill, 2025).