When you get a cut or bruise your blood will clot to stop you bleeding. Your liver makes many of the important proteins, called clotting factors, that help with this. So if your liver is damaged it can take longer for your blood to clot.
Clotting tests can help doctors to understand if your liver is able to make all the clotting factors properly.
It is important to know if you have a problem with clotting. Especially if you are going to have surgery and might bleed a lot.
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Having a clotting test
One sample of blood will be taken, usually from your arm.
Some foods, drinks and medicines can change clotting test results. If you are having an aPTT test you should not eat any very fatty foods before your test.
Let your medical team know about all medicines you are taking. Including over the counter and herbal medicines and supplements. Also let them know if you drink alcohol or have changed your diet recently.
There are several different tests that can check on clotting. You can find more details about them in the drop down boxes below.
Prothrombin time (PT)
This test looks at how long it takes for a blood sample to clot. The prothrombin time can be longer (prolonged) if you have some types of liver condition.
The test result is usually looked at alongside the results of an activated partial thromboplastin time test (aPTT).
International normalised ratio (INR)
The international normalised ratio is very similar to the prothrombin time. But the test is carried out in a very strictly controlled way.
This means that it should not matter which laboratory does the test, the results would always come out the same.
Activated partial thromboplastin time (aPTT)
Also called: Partial Thromboplastin Time, PTT, Kaolin Cephalin Clotting Time, KCCT
This test also looks at clotting time. But it looks at the actions of different clotting factors to those looked at in a PT or INR test.
What do my results mean?
Clotting tests look at how long it takes for your blood to clot. If you have liver disease it can take longer than normal. This is called “prolonged clotting”.
The aPTT test looks at the effect of some of your clotting factors. The PT and INR tests look at a slightly different set of clotting factors.
In some cases of liver disease, clotting may be normal for an aPTT test but prolonged for a PT or INR test. Or it may be prolonged for all tests.
However, lots of things can cause prolonged clotting. A clotting test on its own cannot diagnose liver disease. The results must be looked at alongside your symptoms, history, and other test results.
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This content was last reviewed: July 2025
Our expert reviewers:
We would like to thank everyone who helped with creating and reviewing this page. Including Professor Stephen Ryder, consultant hepatologist at Nottingham University Hospitals NHS trust and Dr Laura-Jane Armstrong, Associate GP, Avonside Health Centre. And all our patient reviewers.
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Everyone’s experience of liver disease will be different. Always talk to your specialist medical team for personal advice.
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