Prothrombin Time (PT/INR)
The prothrombin time (PT) measures how long it takes a sample of your blood to form a clot. It is usually reported together with the INR (International Normalised Ratio), a standardised figure that allows results from different laboratories to be compared. The test tells doctors whether your blood clots too slowly, which can cause bleeding, or whether anticoagulant treatment is well controlled.
At YGOPH, the PT/INR is often requested before a caesarean section or other surgery, for women with heavy bleeding during or after childbirth, in severe pre-eclampsia, for patients with liver disease and for newborns who bleed unexpectedly. It is also used to adjust the dose of oral anticoagulants such as warfarin or acenocoumarol.
What is the prothrombin time?
Blood clotting depends on a chain of proteins called clotting factors, many of which are made in the liver with the help of vitamin K. The PT test adds a reagent to your plasma and measures, in seconds, how long it takes a clot to form. The INR converts this time into a ratio: in a healthy person not taking anticoagulants it is around 1.0, and a higher value means the blood takes longer to clot.
- Before surgery, including caesarean section, when there is a risk of bleeding
- Heavy bleeding in pregnancy or after childbirth
- Severe pre-eclampsia or HELLP syndrome
- Liver disease or jaundice
- Bleeding in a newborn, which may be due to vitamin K deficiency
- Monitoring treatment with warfarin or acenocoumarol
Good to know
PT and other clotting tests
The PT is often combined with the activated partial thromboplastin time (aPTT), the platelet count and fibrinogen. Together these tests give a fuller picture of clotting and help doctors detect disseminated intravascular coagulation (DIC), a serious complication of severe haemorrhage, infection or placental abruption.
Monitoring anticoagulant treatment
People taking warfarin or acenocoumarol, for example after a blood clot or a heart valve replacement, need regular INR checks. The target is often between 2 and 3, but your doctor will set the right range for you. Tell your doctor about any new medicine, including antibiotics and traditional remedies, because many interact with anticoagulants.
Understanding your results
A PT or INR above the normal range means the blood clots more slowly than usual. This may be caused by anticoagulant treatment, vitamin K deficiency, liver disease or the consumption of clotting factors during severe bleeding. Your doctor will interpret the result alongside your symptoms and other tests.