ProthrombinzeitINR

Prothrombin time (PT) and the international normalized ratio (INR) are laboratory measurements used to assess the blood coagulation system, particularly the extrinsic and common pathways. PT evaluates how long plasma takes to clot after the addition of tissue factor (thromboplastin) and calcium, reflecting the function of clotting factors I (fibrinogen), II (prothrombin), V, VII, and X. The test is commonly reported together with the INR, which standardizes PT results across different laboratories and reagent sensitivities.

The INR is calculated to allow comparison of results obtained with different thromboplastin products. The formula is INR = (PT_patient / PT_normal)^ISI, where PT_patient is the patient’s prothrombin time, PT_normal is a control value, and ISI is the International Sensitivity Index assigned to the thromboplastin reagent. The ISI accounts for reagent sensitivity and lab calibration.

Normal reference ranges vary by laboratory, but typical PT values are approximately 11 to 13.5 seconds, and normal INR values are about 0.8 to 1.2. In clinical practice, INR is routinely used to monitor anticoagulation therapy, especially with warfarin. Therapeutic targets commonly fall around INR 2.0–3.0 for many indications, with higher targets (for example 2.5–3.5) used for certain mechanical heart valves or specific conditions.

PT/INR can also aid in evaluating liver disease, vitamin K deficiency, and coagulation disorders. Results can be influenced by preanalytical factors (sample handling, anticoagulant concentration), reagent ISI, acute phase changes, and the use of direct-acting anticoagulants, which may affect interpretation.