Prothrombin time (PT) and international normalised ratio (INR)

What is being analysed?

Prothrombin Time (PT) is a blood test used to assess the time it takes for blood to clot. It measures the activity of certain clotting factors in the blood, primarily factors I, II, V, VII, and X. PT is often used to monitor patients taking anticoagulant medications such as warfarin.

What does the result mean?

The result of a PT test is usually reported in seconds. A longer PT indicates that blood is taking longer to clot, which may indicate a deficiency in one or more clotting factors. Conversely, a shorter PT may suggest a higher-than-normal level of clotting factors or an excess of blood-clotting substances.

Why do the analysis?

PT analysis is crucial for assessing the overall coagulation ability of a patient's blood. It helps diagnose bleeding disorders, monitor the effectiveness of anticoagulant therapy, evaluate liver function, and assess the risk of excessive bleeding during surgery.

When to do the analysis?

PT analysis is performed when a patient is suspected of having a bleeding disorder, is on anticoagulant therapy, or when monitoring liver function. Additionally, it is done before surgeries to assess the risk of excessive bleeding.

What sample is required?

A small sample of blood is required for PT analysis. It is usually drawn from a vein in the arm using a needle.

Is any type of prior preparation necessary?

No special preparation is usually required for a PT test. However, patients taking anticoagulant medications may need to adjust their dosage before the test, as directed by their doctor.

How is it used?

PT results are often interpreted in conjunction with the International Normalized Ratio (INR). The INR standardizes PT results, allowing for comparison between different laboratories and different methods of testing. The INR is calculated using the patient's PT result and a set of calibration standards.

Normal values

The normal range for PT varies depending on the laboratory and the specific reagents used in the test. However, in general, a normal PT is between 11 and 13.5 seconds.

Altered values:

Altered PT values may indicate various conditions:

Prolonged PT may suggest a deficiency in clotting factors, liver disease, vitamin K deficiency, or the presence of anticoagulant medications. Shortened PT may indicate excessive clotting activity, which could be due to high levels of clotting factors or the presence of prothrombotic conditions.

Prothrombin Time (PT) and International Normalized Ratio (INR) Values

Normal Range 

Interpretation

11 - 13.5 sec

Normal

>13.5 sec

Prolonged, possible clotting disorder or medication effect

<11 sec

Shortened, possible excess clotting activity