A blood clot forms following the successful and timely completion of a sequence of events known as the coagulation cascade, in which coagulation factors are activated one after the other. In order for the process to work effectively, an adequate quantity of each coagulation factor must be present, and each must function properly. An insufficient quantity can result in prolonged bleeding, while excess quantity can result in too much clotting. The PT/INR Blood Test evaluates factors I, II, V, VII and X. If any factor is dysfunctional, a prolonged result will be the outcome, meaning a clot is taking too long to form.
A PT/INR Blood Test is often used to monitor patients taking warfarin, a blood-thinning medication, to ensure the dosage is providing an appropriate balance between preventing clotting and causing excessive bleeding. Periodic tests may be ordered to assess the efficacy of the prescription.
Different reagents are used to perform PT tests in different laboratories. As a result, what is considered a normal range will fluctuate. The Internationalized Normalized Ratio, or INR, is a calculation developed by a World Health Organization (WHO) committee to standardize results and enable them to be compared with results from other laboratories. The PT/INR Blood Test reports both PT and INR values, although the INR value is more applicable for patients on warfarin; it is used to adjust dosage to get the PT in the appropriate range.
To diagnose excessive or unexplained bleeding and bruising, the PT & PTT Blood Tests are more commonly ordered.