The Prothrombin Time (PT) test measures the time from mixing patient plasma (with platelets removed) with tissue factor and tissue thromboplastin (which includes phospholipids), in the presence of calcium ions, until clot formation. It evaluates coagulation factors involved in the extrinsic and common pathways.

While the test result depends on the concentrations of the coagulation factors in those pathways, the sensitivity also varies depending on the type of tissue thromboplastin reagent used. Tissue thromboplastin can be derived from animal brain tissue or produced recombinantly. The sensitivity of tissue thromboplastin is expressed as the International Sensitivity Index (ISI). It is calibrated by comparing each reagent’s thromboplastin to WHO’s standard thromboplastin. Recombinant thromboplastin with ISI close to 1 is more sensitive.

Causes of prolonged PT: Deficiencies of Factor I, II, V, VII, or X, or the presence of inhibitors against these factors.

There are various ways to report PT results; INR is a method to correct for differences between reagents. It is especially recommended for patients on oral anticoagulants.

• PT INR < 1.1 : (in the absence of anticoagulant therapy) normal
• PT INR 1.1–1.9 : INR is elevated, further workup may be needed
• PT INR 2.0–3.0 : therapeutic range for warfarin anticoagulation
• PT INR > 3.0 : overly elevated INR, increased bleeding risk