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Prothrombin Time – Definition, Normal Values and INR

Prothrombin time (PT) is a blood coagulation test that measures how quickly blood clots. It is used to diagnose clotting disorders and to monitor anticoagulant therapy.

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Things worth knowing about "Prothrombin Time"

Prothrombin time (PT) is a blood coagulation test that measures how quickly blood clots. It is used to diagnose clotting disorders and to monitor anticoagulant therapy.

What Is Prothrombin Time?

Prothrombin time (abbreviated PT) is a laboratory test that measures the time it takes for blood to form a clot. It is one of the most important parameters for assessing blood coagulation and is routinely used in clinical diagnostics. In many countries, the result is also expressed as the INR (International Normalized Ratio), which allows for standardized comparison across different laboratories.

The prothrombin time specifically evaluates the extrinsic coagulation pathway, which is triggered by tissue factors outside the blood vessel. The coagulation factors involved include factor I (fibrinogen), factor II (prothrombin), factor V, factor VII, and factor X.

How Does the Test Work?

A blood sample is taken from the patient. In the laboratory, tissue thromboplastin (tissue factor) and calcium are added to the blood plasma. The time until a clot forms is then recorded in seconds.

In healthy adults, prothrombin time typically ranges between 11 and 15 seconds.
Results may also be expressed as a percentage (Quick value), with normal values ranging from 70% to 130%.
The INR value is the internationally standardized ratio, with a normal value of approximately 1.0 in healthy individuals.

When Is Prothrombin Time Measured?

Prothrombin time is used in a variety of clinical situations, including:

Coagulation disorders: Suspected inherited or acquired deficiencies in clotting factors.
Liver disease: Since the liver produces many coagulation factors, conditions such as cirrhosis or hepatitis can prolong the PT.
Vitamin K deficiency: Vitamin K is essential for the synthesis of several clotting factors. A deficiency leads to a prolonged PT.
Monitoring anticoagulant therapy: Patients taking vitamin K antagonists such as warfarin or phenprocoumon require regular PT/INR monitoring.
Preoperative assessment: Blood clotting is routinely evaluated before surgical procedures.

Interpretation of Results

Prolonged Prothrombin Time

A prolonged prothrombin time (low Quick value, elevated INR) may indicate the following conditions:

Liver disease (e.g., liver cirrhosis, acute liver failure)
Vitamin K deficiency (e.g., due to malnutrition, malabsorption, or medication)
Use of anticoagulant medications
Inherited deficiency of coagulation factors (e.g., factor VII deficiency)
Disseminated intravascular coagulation (DIC)

Shortened Prothrombin Time

A shortened prothrombin time is generally less clinically significant but may suggest an increased tendency toward clot formation (thrombophilia).

INR and Therapeutic Range

The INR is the internationally standardized expression of prothrombin time. In healthy individuals, the INR is approximately 1.0. Patients receiving anticoagulation therapy -- for example, for atrial fibrillation, pulmonary embolism, or mechanical heart valves -- are typically maintained within a target INR range of 2.0 to 3.5, depending on the clinical indication.

References

Kasper DL et al. - Harrison's Principles of Internal Medicine, 20th edition, McGraw-Hill Education, 2018.
Longo DL et al. - Haematology: Basic Principles and Practice, Elsevier, 2018.
World Health Organization (WHO) - WHO Expert Committee on Biological Standardization: Guidelines for thromboplastins and plasma used to control oral anticoagulant therapy. WHO Technical Report Series, No. 889, 1999.

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