PROTHROMBIN TIME DETERMINATION - THE LACK OF NEED FOR A DISCARD TUBE AND 24-HOUR STABILITY

The National Committee for Clinical Laboratory Standards (NCCLS) recom mends 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 disca rded for each prothrombin evaluation. In a prospective study we compar ed the prothrombin times (PTs) as international normalized ratios (INR s) from a series of three tubes obtained from 241 patients receiving c onsistent dosages of oral anticoagulant therapy to determine the need for discarding the first tube drawn, as well as the stability of PT de terminations over a 24-hour period. Tube one was treated as the discar d tube. Tubes one and two were analyzed within the laboratory's standa rd 4-hour time frame, while tube three was kept stoppered at room temp erature, centrifuged a half hour before PT determination, and analyzed after a 24-hour delay. Comparisons of the INRs were made in four rang es comprising 1.2 to 2.0, 2.1 to 3.5, 3.6 to 5.9, and 6.0 or more. Mos t INR comparisons were less than the 10% maximum variance listed as ac ceptable by the NCCLS. A comparison of INR results between tube two an d 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 test ing for the first three INR cohorts. However, the 95% confidence inter vals demonstrated that these mean differences were probably too small to be clinically significant. For the fourth cohort (INR greater than or equal to 6.0) the mean difference was not significantly different o n paired t testing, but the 95% confidence interval was larger at -0.0 7 to 0.839. In this sample of outpatients receiving consistent dosages of oral anticoagulant therapy the use of a discard tube seemed unnece ssary, and the 24-hour stability of PT determinations was documented.