ProthrombinzeitPT

Prothrombin time (PT) is a laboratory test that measures how long plasma takes to clot after the addition of tissue factor (thromboplastin) and calcium. In clinical practice, the test is often reported together with the International Normalized Ratio (INR). The PT primarily assesses the extrinsic coagulation pathway and the common pathway of coagulation.

Specimens are usually drawn as venous blood into citrate anticoagulant tubes and processed to obtain plasma. The test uses a thromboplastin reagent containing tissue factor and phospholipids, along with calcium, to trigger the coagulation cascade. The result is reported as a clotting time in seconds and, when standardized, as an INR.

Normal PT values vary by reagent and instrument, but typical reference ranges are about 11 to 13.5 seconds for PT, and an INR of 0.8 to 1.2 for people not on anticoagulants. The INR adjusts PT results to account for differences in reagents and equipment, enabling comparison across laboratories.

Clinical use includes monitoring anticoagulation therapy with vitamin K antagonists such as warfarin (target INR commonly 2.0–3.0 for many indications), assessing bleeding or clotting risk in liver disease, vitamin K deficiency, and evaluating preoperative bleeding risk. A prolonged PT suggests deficiencies in factors II, V, VII, or X, or impaired synthesis, while a shortened PT is uncommon.

Interpretation must consider reagents, patient factors, and medications. PT can be affected by liver disease, vitamin K status, pregnancy, and anticoagulants. Direct oral anticoagulants may alter PT in a reagent-dependent way and are not routinely used to monitor these drugs. Additional tests, such as an aPTT or specific factor assays, may be used for broader evaluation.