Prothrombin Time (PT) w/INR and Partial Thromboplastin Time (PTT) Blood Test

The prothrombin time (PT) w/INR test measures the length of time it takes for a blood clot to form in a sample of blood. A partial Thromboplastin Time (PTT) test is ordered when someone has unexplained bleeding or clotting. A PT PTT is a blood test usually used as a starting place when investigating the cause of a bleeding or thrombotic episode.

Description

A blood clot forms following the successful and timely completion of a sequence of events known as the coagulation cascade, in which coagulation factors are activated one after the other. In order for the process to work effectively, an adequate quantity of each coagulation factor must be present, and each must function properly. An insufficient quantity can result in prolonged bleeding, while excess quantity can result in too much clotting. The PT test evaluates factors I, II, V, VII and X, while the PTT test evaluates factors I, II, V, VIII, IX, X, XI and XII, as well as prekallikrein (PK) and high molecular weight kininogen (HK). The combination of both tests examines how the factors work together, and evaluation of both results together can help diagnose certain conditions.

The PT and PTT Blood Tests are useful in the diagnosis of excessive, unexplained bleeding in patients who are not taking blood-thinning medications. These bleeding disorders include conditions such as nosebleeds, bruising, heavy menstrual periods, blood in the stool and/or urine, and bleeding gums, among others. The tests are also sometimes ordered for patients prior to surgery to ensure normal clotting ability.

A “prolonged” result means that a blood clot is taking longer than normal to form. There are a variety of causes for this, including coagulation/clotting factor deficiency/dysfunction, vitamin K deficiency, liver disease, inhibition by certain antibodies, and others. 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); presence of lupus anticoagulant

• PT normal, PTT normal or slightly prolonged: Normal clotting ability, mild deficiencies in other coagulation factors, mild form of von Willebrand disease

See also Walk-In Lab’s PT/INR Blood Test.