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.
More details on what is tested
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.
Support for you
The British Liver Trust offers a range of support for people with a liver condition and those closest to them. This includes support groups and an online forum. Support for you.
You can also call or email our helpline. Our friendly team of specialist liver nurses cannot give you a diagnosis or interpret test results. But they can answer questions, offer support, or just listen.
Call the helpline on 0800 652 7330 or find out more here.
We also have lots of information about living with a liver condition:
- Liver blood tests (LFT)
- Tests for liver conditions
- Take our free online screener
- Liver conditions A to Z
- Useful words glossary
- Living with a liver condition
- Eating, drinking and keeping active
- Mental health
Special thanks
We would like to thank everyone with lived experience of liver disease tests who helped with these pages. Including, Sejal Patel, and Jennifer Voller.
Also, Professor Stephen Ryder, consultant hepatologist at Nottingham University Hospitals NHS trust and Dr Laura-Jane Armstrong, Associate GP, Avonside Health Centre, for reviewing this page.
Published: July 2025
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