Prothrombin time: one tube or two.

This study compares the prothrombin times (PTs) and calculated international normalized ratios (INRs) from first and second evacuation blood tubes to determine the clinical importance of using a second tube specimen for protime coagulation studies. The National Committee for Clinical Laboratory Standards (NCCLS) currently recommends that all coagulation studies be done on a second or later drawn blood tube. For patients on long-term anticoagulation therapy, this often requires that first blood tubes be drawn and discarded at each prothrombin evaluation. In this prospective study, a first and second evacuation blood tube was drawn from 343 outpatients who had a physician-ordered prothrombin time test performed. There was no statistically significant difference in the paired PT or calculated INR from any of the first and second tubes. The average difference in the INR from tube 1 to tube 2 was 2% (standard deviation [SD] 1.1%). In this sample of outpatients, the use of a second tube for PT testing was not clinically justified.