Pre-analytical variability currently represents the most important source of errors that can lead to inaccurate laboratory results in monitoring of patients being treated with oral anticoagulant therapy. The volume of blood collected is critical for accurate coagulation results. The National Committee for Clinical Laboratory Standards (NCCLS) recommends a ratio of blood to anticoagulant volume of 9:1. However, investigators have published reports which suggest that a lower ratio may be acceptable. Unfortunately the recommendations of these reports were inconsistent. AIMS: The aim of this study was to determine the impact of tube fill volumes on prothrombin times and INR values both in healthy subjects and patients receiving oral anticoagulation therapy. METHODS: An experimental study was performed to compare INR values obtained by processing coagulation specimens containing different volumes of whole blood. The study group included 30 patients taking oral anticoagulation therapy and 15 healthy volunteers. Respectively 2.5ml, 3 ml, 3.5 ml, 4 ml and 4.5 ml of whole blood was drawn into tubes containing 3.2% (0.109M) sodium citrate. A prothrombin time test was performed on the platelet-poor plasma of each collection tube and the INR subsequently calculated. RESULTS: The INR values increased as total tube fill volumes decreased for both groups but this finding did not reach statistical significance in either group for the tube fill volumes studied. CONCLUSION: For blood specimens collected in 3.2% citrate anticoagulant using our reagent and instrument combination, a total tube fill volume of greater than 56% yielded reliable INR results.
Poster presented at the University of Pretoria, Health Sciences Faculty Day, 20 August 2008, Pretoria, South Africa.