Aims-To examine the reliability of international normalised ratio (INR
) determination on samples stored as whole blood for up to two days at
room temperature. Methods-The INR of 40 patients receiving oral antic
oagulants was determined on fresh blood and on samples stored far 24 a
nd 48 hours, using five locally calibrated prothrombin time systems. T
hese incorporated Manchester reagent, Recombiplastin, IL PT Fibrinogen
HS Plus, Manchester combined capillary prothrombin rime reagent, and
a freeze dried in-house reference rabbit brain thromboplastin, RBT 101
0. In addition, factors II, V,VII, and X mere determined on samples ob
tained from 18 of these patients before and after incubation at room t
emperature. Results-The INR of the samples changed by differing amount
s during storage, depending on which system was employed. Although the
mean change after 24 hours storage was relatively small, there were i
ndividual samples that changed by > 0.5 LNR with all systems. These ch
anges mould lead to adjustment in dosage of certain patients. After 48
hours these effects were greater with all systems except that employi
ng Recombiplastin. There were only small reductions in the measured fa
ctors by 48 hours. Conclusions-After storage of samples for only 24 ho
urs, some patients' INR changed sufficiently to affect dosage, In view
of these observations, the practice of storing whole blood samples fo
r INR determination cannot be recommended.