Blood clotting test

Why do I need a blood clotting test?

The liver produces proteins that help your blood to clot. If your liver isn’t working well, your blood won’t clot as efficiently, so this test is helpful when assessing liver function or whether it is safe for you to have a planned procedure or surgery.

Below are some specific terms you might hear when doctors assess how well your blood clots:

Prothrombin time (PT)

This is the time it takes for your blood to clot. The prothrombin time will take longer if you are deficient in vitamin K.

International normalised ratio (INR)

As the chemicals used to perform the PT test may vary between laboratories, the INR is a calculation doctors use to allow for any differences found when results from different laboratories are compared.

Activated partial thromboplastin time ratio (APTR)

APTR is the time taken for thromboplastin to convert into thrombin, an enzyme which causes clotting by turning fibrinogen into fibrin (see below).


This is another substance in the blood that helps clotting by producing fibrin strands. Platelets stick to the fibrin strands to form a plug that prevents bleeding. Some medications, such as aspirin, clopidogrel or warfarin, can affect clotting.