Laboratory test · Men’s health
Prothrombin Time (PT)
- Sample: venous blood
- Preparation: fasting recommended
Test price
- Sample
- venous blood
- Preparation
- fasting recommended
Paymentcash, cards and instalments
Need help choosing?Contact us and we will recommend tests for your circumstances. Contact and locations
Prothrombin time (PT) indicates how quickly blood clots through one part of the clotting system. It is most often used to monitor warfarin treatment, before operations and procedures, and in liver disease, vitamin K deficiency or a bleeding tendency. The result is also expressed as INR.
Who the test is for
- people taking warfarin or other vitamin K antagonists, to monitor treatment;
- people before surgery or a procedure when requested by a doctor, most often together with APTT and fibrinogen;
- people who bruise easily, have frequent nosebleeds or gum bleeding, heavy periods or prolonged bleeding after tooth extraction;
- people with liver disease, because PT reflects liver production of clotting factors;
- people with suspected vitamin K deficiency, for example due to poor fat absorption or prolonged antibiotic use.
What PT indicates
Blood clotting depends on proteins called clotting factors. PT measures the time needed for a clot to form in plasma and depends on factors II (prothrombin), V, VII and X and fibrinogen. The liver produces most of these factors, and factors II, VII and X require vitamin K. PT therefore becomes prolonged in liver disease, vitamin K deficiency and with medicines inhibiting vitamin K activity.
PT is reported as a ratio (a percentage in some laboratories) and as INR, the international normalised ratio. INR corrects reagent differences, allowing comparison between laboratories. It is important to understand the direction: when clotting takes longer, the ratio is lower and INR higher.
New oral anticoagulants (rivaroxaban, apixaban, edoxaban and dabigatran) are not monitored with PT or INR. They may affect results, so mention if you take them, but INR does not show how well they work.
Save with a panel
Test Prothrombin time (PT) is also included in these panels.
Preparation and sample
Blood is taken from a vein. Morning sampling after fasting is recommended because a fatty meal can make the sample cloudy and interfere with measurement.
- If you take warfarin, do not skip or change your dose because of blood sampling. If you normally take it in the morning, take it after sampling. State your dose and when you last took it.
- Also mention other anticoagulants, antibiotics or supplements because they may change INR.
How results are interpreted
The reference interval depends on the reagent and is printed on the report. In people not taking anticoagulants, INR is usually around 1. A doctor interprets results alongside your medicines and other findings.
INR during warfarin treatment
Warfarin deliberately prolongs clotting. For most patients, for example with atrial fibrillation or after venous thrombosis, the target INR is 2,0 to 3,0, and higher for some mechanical heart valves. Your doctor sets your target range. INR below target means less protection against clots, while above target means greater bleeding risk. Dietary changes, particularly the amount of vitamin K-rich leafy vegetables, alcohol, illness and many medicines affect INR, so only a doctor changes the dose.
Prolonged PT without warfarin
A low ratio and high INR in someone not taking warfarin may mean vitamin K deficiency, liver disease (such as cirrhosis), inherited or acquired deficiency of a clotting factor, or disseminated intravascular coagulation (DIC), a serious condition treated in hospital. A doctor interprets this alongside APTT, fibrinogen, blood count and liver tests.
Shortened PT
A high ratio and low INR usually have no clinical significance and do not mean a tendency towards thrombosis. Sometimes they result from sampling, so a doctor repeats testing if needed.
Urgent: if you take warfarin and notice blood in urine or stool, black stool, vomiting blood, bleeding that does not stop or a severe headache, contact a doctor or emergency care immediately.
How to arrange testing
You can have blood taken without an appointment or referral at one of our branches across Croatia (listed on the Contact page). Prices for all tests and panels are available in the service price list. Before procedures, PT is most often performed in the Coagulation Panel, which also includes APTT and fibrinogen.
Frequently asked questions
What is the difference between PT and INR?
They are the same test expressed in two ways. The ratio (or percentage) compares your clotting with normal plasma, while INR converts that result to make it comparable between laboratories. INR is used for warfarin monitoring.
What does low PT on the report mean?
A low ratio means blood clots more slowly. This is expected in people taking warfarin. Without it, a doctor establishes the cause; common causes are vitamin K deficiency and liver disease.
How often is INR checked?
Frequently at the start of warfarin treatment until the right dose is found. Once INR is stable, checks are less frequent, at intervals of several weeks. A doctor sets the schedule and shortens it when medicines or health change.
Should PT be tested before taking contraceptive pills?
Generally not. Before prescribing hormonal contraception, a doctor assesses thrombosis risk by discussing personal and family medical history and measuring blood pressure; PT is not routinely tested.