Lab test guide
Prothrombin time (PT): a clotting screen explained
The prothrombin time, or PT, measures in seconds how long it takes a blood sample to clot through the extrinsic pathway. It is often paired with the APTT as a first-line clotting screen before surgery or when someone bruises or bleeds unusually, and it is one of the more sensitive laboratory markers of serious liver disease. Its standardised cousin, the INR, is covered separately.
Last checked: 2026-10-04
Key facts
At a glance.
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What it measures
How long blood takes to clot, reported in seconds, which depends on the method used by the laboratory.
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Paired test
It is commonly requested with the APTT as a routine coagulation screen.
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Liver link
The PT is one of the more sensitive tests for serious liver disease because the liver makes clotting factors.
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Sample tube
Blood goes into a sodium citrate tube that must be filled exactly to the mark or the result may be inaccurate.
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Prolonged PT
Means blood is taking longer to clot; if you are not on an anticoagulant, further testing is needed to find out why.
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Preparation
None; if you take anticoagulants, the sample should be taken before the daily dose.
What the PT measures
Clotting is a cascade in which a series of proteins, the coagulation factors, activate one another until a fibrin clot forms. The prothrombin time tests part of that chain. In the laboratory the plasma is mixed with a reagent that triggers clotting and the time to form a clot is measured. Lab Tests Online UK says the result depends on the method used and is measured in seconds.
Because the reagents differ, a PT can vary slightly from laboratory to laboratory. That is why the same PT is also converted to an international normalised ratio when monitoring warfarin, a calculation that makes the value comparable everywhere.
Why it is ordered
Lab Tests Online UK lists several reasons: to help diagnose a bleeding disorder, to check clotting efficiency before surgery, and to help estimate the severity of liver disease. The PT is commonly requested along with the APTT as a routine coagulation screen when someone has unexplained bleeding or easy bruising or before an operation.
It can also be used in people not on anticoagulants to look for liver disease or vitamin K deficiency, and before invasive procedures to be sure clotting is adequate. In a person on warfarin the INR derived from it is the key monitoring number.
Preparation and sample handling
No preparation is required. If you are receiving anticoagulant therapy, the specimen should be collected before the daily dose. Blood goes into a sodium citrate tube to prevent clotting before testing. Lab Tests Online UK stresses that the blood should be taken quickly and smoothly and that the tube must be filled exactly to the designated mark, or the result may be inaccurate.
Some substances you consume can interfere: various foods, alcohol and many medications. Antibiotics and painkillers can increase the PT, while vitamin K in a multivitamin or liquid nutrition supplement can decrease it. Tell the team about everything you take.
Reading PT with the APTT
Comparing the PT and APTT narrows down where in the cascade a problem lies. Lab Tests Online UK provides the following guide, which a clinician applies alongside symptoms and other results.
| PT | APTT | Possible conditions |
|---|---|---|
| Prolonged | Normal | Liver disease, decreased vitamin K, decreased or defective factor VII |
| Normal | Prolonged | Decreased or defective factor VIII, IX, XI or XII, von Willebrand disease, lupus anticoagulant |
| Prolonged | Prolonged | Decreased or defective factor I, II, V or X, liver disease, disseminated intravascular coagulation |
| Normal | Normal | Decreased platelet function, low platelets, factor XIII deficiency, mild deficiencies or mild von Willebrand disease |
What a prolonged PT can mean
A longer PT means blood is taking longer to form a clot. If you are not taking anticoagulant drugs, additional testing may be needed to determine the cause. Common explanations are liver disease, low vitamin K from poor absorption or diet, and the effect of medicines. In an unwell person with a prolonged PT and APTT, doctors also think about conditions that consume clotting factors.
A prolonged PT does not by itself mean you will bleed. Doctors combine it with symptoms, the APTT, a platelet count and fibrinogen before judging risk or deciding on treatment such as vitamin K.
Next steps
Depending on the pattern the clinician may repeat the test, request clotting factor assays, liver function tests, a platelet count, fibrinogen or tests for lupus anticoagulant. If you are about to have surgery, the surgical and anaesthetic team will decide how to manage an abnormal result.
Seek urgent help for heavy bleeding that does not stop, black stools, vomiting blood or a head injury while your clotting is known to be abnormal; call 999 or go to A&E.
Frequently asked
Questions people ask.
Quick, plain-English answers to the questions we hear most.
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What is the difference between PT and INR?
They are essentially the same test. The INR is a standardised version of the PT that gives the same result in every laboratory and is used to monitor warfarin.
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Do I need to fast before a PT test?
No. If you take anticoagulants, the sample should be taken before the daily dose.
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Why is the PT often done with the APTT?
Comparing the two helps locate which part of the clotting system may be affected, as in the pattern table above.
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Can liver disease prolong the PT?
Yes. The PT is one of the more sensitive tests for serious liver disease because the liver makes clotting factors.
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Can food or drink affect the result?
Various foods, alcohol and many medicines can interfere. Vitamin K supplements can decrease the PT, while antibiotics and painkillers can increase it.
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Why must the tube be filled to the mark?
The anticoagulant in the tube is measured to match the blood volume. An under- or over-filled tube can make the result inaccurate.
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Does a prolonged PT mean I am at risk of bleeding?
Not necessarily. Clinicians combine it with other tests and your symptoms to judge the real risk.
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Can PT be measured at home?
Point of care meters measure the INR from a finger-prick drop of blood in clinics, pharmacies and for some patients at home, but accuracy can be harder to guarantee outside a laboratory.
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What treatment is used for a prolonged PT?
It depends on the cause. Examples include treating the underlying liver or absorption problem or giving vitamin K; the clinician decides after further tests.
Where this comes from
Sources and checking.
This guide is for information, not medical advice.
Your GP or consultant knows your history and can tell you which parts apply to you. If your symptoms are severe, rapidly worsening, or come with red-flag features, seek urgent medical care. Figures and availability change, so treat them as a guide and confirm with the provider.
Published 2026-10-04, next review 2027-10-04.
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Lab Tests Online UK, Prothrombin time and international normalised ratio (PT/INR) (https://labtestsonline.org.uk/tests/prothrombin-time-and-international-normalised-ratio-ptinr), retrieved October 2026
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NHS, Warfarin (https://www.nhs.uk/conditions/warfarin/), retrieved October 2026