Laboratory control of oral anticoagulant therapy: preservation of prothrombin time specimens using a polypropylene collection system

Previous studies have shown a marked time and temperature dependent shorten ing of the prothrombin time (PT) when brood is exposed to borosilicate (gla ss) or siliconized borosilicate tubes. Current recommendations are that sam ples for PT estimation should be tested within 2 h of collection. in this s tudy using polypropylene collection tubes, blood obtained from 30 patients on oral anticoagulant therapy showed no significant change in International Normalized Ratio (INR) value after 24 h storage - either at 4 degrees C or room temperature, After 48 h, changes in INR values from refrigerated samp les were still clinically insignificant. After 48 h storage at room tempera ture, however, a minority of samples showed an increase in INR value which may be of clinical importance. The range of INRs studied was 1.0-9.1. In a second evaluation, replicate specimens from 22 orally anticoagulated patien ts with INRs ranging from 1.0 to 9.6 showed no significant change after 24 h at either temperature even when samples had been subjected to 30 min of g entle agitation prior to storage and analysis. Overall, the results indicat e that when polypropylene collection tubes are used, prothrombin time speci mens can be successfully preserved for up to 24 h at room temperature or up to 48 h when refrigerated.