Prothrombin time determination. The lack of need for a discard tube and 24-hour stability

The National Committee for Clinical Laboratory Standards (NCCLS) recommends that all coagulation studies be done on a specimen from the tube drawn second or later. For patients receiving long-term anticoagulant therapy, this may require that the first tube of blood drawn be discarded for each prothrombin evaluation. In a prospective study we compared the prothrombin times (PTs) as international normalized ratios (INRs) from a series of three tubes obtained from 241 patients receiving consistent dosages of oral anticoagulant therapy to determine the need for discarding the first tube drawn, as well as the stability of PT determinations over a 24-hour period. Tube one was treated as the discard tube. Tubes one and two were analyzed within the laboratory's standard 4-hour time frame, while tube three was kept stoppered at room temperature, centrifuged a half hour before PT determination, and analyzed after a 24-hour delay. Comparisons of the INRs were made in four ranges comprising 1.2 to 2.0, 2.1 to 3.5, 3.6 to 5.9, and 6.0 or more. Most INR comparisons were less than the 10% maximum variance listed as acceptable by the NCCLS. A comparison of INR results between tube two and tube one showed a statistically significant difference only for the INR range of 6.0 or more. The comparison of the 24-hour specimen with tube one showed statistically significant differences in paired t testing for the first three INR cohorts. However, the 95% confidence intervals demonstrated that these mean differences were probably too small to be clinically significant. For the fourth cohort (INR > or = 6.0) the mean difference was not significantly different on paired t testing, but the 95% confidence interval was larger at -0.07 to 0.839. In this sample of outpatients receiving consistent dosages of oral anticoagulant therapy the use of a discard tube seemed unnecessary, and the 24-hour stability of PT determinations was documented.