MINIMUM SPECIMEN VOLUME REQUIREMENTS FOR ROUTINE COAGULATION-TESTING - DEPENDENCE ON CITRATE CONCENTRATION

We evaluated the effect of sample volume and citrate concentration on results of routine coagulation assays (prothrombin time [PT] and activ ated partial thromboplastin time [APTT]). The study was performed on s amples obtained from healthy persons and patients receiving oral antic oagulant therapy. Standard evacuated tubes (3.2% and 3.8% sodium citra te) were filled to varying total sample volumes ranging from 3.0 to 5. 0 mL, and results of routine coagulation tests were compared. Underfil ling may significantly affect the APTT and PT resulting in artifactual prolongation of results. This effect is most pronounced in samples dr awn into 3.8% citrate. By using 3.8% citrate, there is a statistically significant difference in the results of PT assays in the samples les s than 80% filled compared with those that are 100% filled. For APTT a ssays performed on samples drawn into 3.8% citrate, a statistical diff erence occurred at less than 90% filled. This effect was less pronounc ed when samples were drawn into 3.2% sodium citrate. Ve found no stati stically significant difference in PT results from a 3.2% citrate tube between fill volumes of 60% and 100% and none for APTT results betwee n fill volumes of 70% and 100%. This study further supports the recomm endation to use 3.2% sodium citrate concentration, because 60% of the optimum filled volume for PT and 70% of the optimum filled volume for APTT are acceptable.