Ml. Brigden et al., PROTHROMBIN TIME DETERMINATION - THE LACK OF NEED FOR A DISCARD TUBE AND 24-HOUR STABILITY, American journal of clinical pathology, 108(4), 1997, pp. 422-426
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.