What is the purpose of this test?
Order this Prothrombin Time (PT) w/ INR and Partial Thromboplastin Time (PTT) Blood Test, which helps detect and diagnose a bleeding or excessive clotting disorder. The Prothrombin Time (PT) test assesses an individual's blood clotting ability. The results of the PT are used to calculate the international normalized ratio (INR), which is particularly important for monitoring individuals taking the blood-thinning medication warfarin (Coumadin®).
The PT measures the time a clot takes to form in a blood sample after reagents are added. It is often accompanied by partial thromboplastin time (PTT), and together, they evaluate the quantity and function of coagulation factors that are crucial to proper blood clot formation.
When an injury occurs, the body initiates the clotting process called hemostasis. This involves a series of sequential chemical reactions known as the coagulation cascade, where clotting factors activate in a specific order, resulting in clot formation. Normal clotting requires sufficient and correctly functioning coagulation factors. Insufficient factors may lead to excessive bleeding, while excessive factors can cause excessive clotting.
During a laboratory test, two pathways initiate clotting - the extrinsic and intrinsic pathways. These merge into a common path to complete the clotting process. The PT test evaluates how well all coagulation factors in the extrinsic and common pathways work together.
The PT w/ INR is conducted alongside a PTT, which evaluates clotting factors in the intrinsic and common pathways. These include factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen), as well as prekallikrein (PK) and high molecular weight kininogen (HK).
What do the PT w/INR and PTT results indicate?
The PT and PTT evaluate the ability to produce a clot within a reasonable time. The test results will be prolonged if any of these factors are deficient or not functioning correctly. The normal range for PT values in healthy individuals varies as the reagents used to perform the PT test differ from one lab to another. A general interpretation of PT & PTT Blood Test results can be summarized as follows:
- PT prolonged, PTT prolonged: Dysfunctional or insufficient coagulation factor I, II, V, or X; acute disseminated intravascular coagulation (DIC) (abnormal activation of coagulation factors); severe liver disease.
- PT prolonged, PTT normal: Dysfunctional or insufficient coagulation factor VII, chronic low-grade DIC, insufficient vitamin K, liver disease, warfarin (anticoagulant) therapy.
- PT normal, PTT prolonged: Dysfunctional or insufficient coagulation factor VIII, IX, or XI; severe von Willebrand disease (hereditary bleeding disorder); lupus anticoagulant.
- PT normal, PTT normal or slightly prolonged: Normal clotting ability, mild deficiencies in other coagulation factors, a mild form of von Willebrand disease.
To standardize results across different laboratories in the U.S. and the world, a World Health Organization (WHO) committee recommends the use of the Internationalized Normalized Ratio (INR), calculated based on the PT test result, for people taking the blood-thinning medication warfarin (Coumadin®). Warfarin is prescribed for people with conditions such as deep vein thrombosis (DVT) and some cardiovascular diseases (CVD) like atrial fibrillation to prevent inappropriate clotting.
The INR adjusts for changes in PT reagents and enables results from different laboratories to be compared. Most laboratories report PT and INR values whenever a PT test is performed. However, the INR is only relevant for individuals taking the blood-thinning medication warfarin.
When should I order a Prothrombin Time (PT) w/ INR and Partial Thromboplastin Time (PTT) Blood Test?
Individuals may order this test if they have experienced symptoms related to a bleeding or excessive clotting disorder. These bleeding disorders include:
- Nosebleeds
- Bruising
- Heavy menstrual periods
- Blood in the stool or urine
- Bleeding gums
This test is sometimes ordered for patients before surgery to ensure normal clotting ability.
See also Walk-In Lab’s PT/INR Blood Test.
