Prothrombin Time (PT)

Prothrombin Time: PTMeasures the function of the Extrinsic Pathway. Sensitive to Factors II, IV, V, VII, X.PT reagent contains Calcium ions and Thromboplastin from brain tissue (Rabbit).Thromboplastin (Tissue Factor) protein-lipid complex found in tissues outside blood vessels.

When is it ordered?Used to monitor oral anticoagulant therapy (Warfarin / Coumadin).When a patient who is not taking anti-coagulant drugs has signs or symptoms of a bleeding disorderWhen a patient is to undergo an invasive medical procedure, such as surgery, to ensure normal clotting ability.

PT Reagent CalibrationReagents are calibrated against standard PT reagent established by the WHO.ISI = International Sensitivity Index.ISI is assigned by the manufacturer for each lot of reagent using reference material from WHO.The lower the ISI the more sensitive the ReagentISI of 1.8 to 2.4 = Low sensitivity ISI of 1.4 to 1.8 = Average sensitivity ISI 1.0 to 1.4 = High Sensitivity

PT: INR ValuesINR = International Normalized Ratio.MNP = Mean Normal Plasma.

INR = (PT / MNP)ISI

An INR of 1.0 means that the patient PT is normal.An INR greater than 1.0 means the clotting time is elevated.

Expected PT ValuesMean Normal Plasma = 10 to 14 seconds.Mean Normal Plasma value varies with PT sensitivity. A high sensitivity (Low ISI) PT will give a high normal PT value (13 to 15 seconds).Oral anticoagulant monitoring = Target INR of 2.0 to 3.0.INR of greater than 5 or 5.5 = unacceptable high risk of bleeding.

Performing a PT testPre-warm PT reagent and sample to 37 oCAdd 100 L sample to tubeAdd 200 L of PT reagent to tubeStart timerRecord time to clot in secondsCalculate INR