Clinically Relevant Differences in Prothrombin Time and INR Values Related to Blood Sample Collection in Plastic vs Glass Tubes

We compared prothrombin times (PTs) and international normalized ratios (INRs) for blood samples drawn into plastic vs glass collection tubes. We collected 60 venous blood samples into 4.5-mL glass and 2 plastic tubes (2.7 and 3.5 mL). An additional 153 samples, including 63 from warfarin-anticoagulated patients, were collected only in glass and 2.7-mL plastic tubes. The PTs and INRs were determined following routine laboratory procedures. A subset of 35 frozen aliquot samples was analyzed with a different instrument-reagent combination. The PTs and INRs for samples in plastic tubes were significantly lower than for samples in glass tubes. The mean INR differences increased with INR magnitude from approximately –0.1 (INR, 1.5) to –0.7 (INR, 4.5). Of the plastic tube INRs, 50% were more than 10% lower than INRs from samples collected in glass tubes. Therapeutic monitoring based on plastic-tube INRs could result in higher doses of warfarin. Clinical laboratories have gradually replaced glass blood collection tubes with plastic tubes in an effort to eliminate potential sharps injuries and biohazardous exposures associated with breakage of glass tubes. In recent years, the transition to plastic tubes has accelerated as regulatory agencies have enforced their use and put laboratories on notice to discontinue use of glass tubes as soon as feasible. Coagulation testing is known to be sensitive to a variety of preanalytic variables related to properties of the sample collection tube, including anticoagulant concentration, 1-4 filling level with blood sample, 5-7 and the composition of the tube itself. 8-11 Previous studies reported significant differences in thrombin time 8 and prothrombin time (PT) test results 9,11 for blood sam