Prothrombin Time – 005199

The thromboplastin reagent used by LabCorp consists of recombinant tissue factor mixed with synthetic phospholipid. Recombinant tissue factor is free from contamination with coagulation factors that can be found in tissue factor extracted from other sources. This serves to increase the PT assay’s sensitivity for factor efficiencies. Prothrombin time results are reported in seconds and are also converted to international normalized ratio (INR) values. The INR serves to normalize results obtained from different laboratories for the variable responsiveness of different thromboplastin reagents. Each thromboplastin is assigned an activity Index (ISI) based on comparison to an international reference thromboplastin from the World Health Organization. The formula for calculating the INR is

INR = [patient’s results / normal patient average]^(ISI)