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Prothrombin time
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Prothrombin time, abbreviated PT, is one of the blood tests ordered routinely to check how well clotting is working, that is, the blood's ability to form a clot.
The trick is in the tube. It contains sodium citrate, which chelates calcium, meaning it traps the calcium and takes it out of circulation. Without calcium, the coagulation cascade, the chain of reactions that forms a clot, cannot get started, and the sample sits still until it reaches the lab. There, calcium is added back and the lab times how long it takes from the moment clotting starts until a clot appears. Calcium is taken away to stop the reaction and given back to time it.
That time gets longer when pieces are missing. It happens with vitamin K deficiency, because the body needs vitamin K to make several clotting factors, and it happens with vitamin K antagonist medications, which deliberately inhibit factors II, VII, IX and X.
In brief
Prothrombin time is a routine blood test that evaluates clotting status.
Citrate in the tube chelates calcium and halts the cascade; in the lab calcium is added back and the time until a clot forms is measured.
Calcium is taken away to stop the reaction and given back to time it.
It is prolonged by vitamin K deficiency and by vitamin K antagonists, which inhibit factors II, VII, IX and X.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingProthrombin Time, StatPearlsPublished
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