title: Clinically relevant differences in prothrombin time and INR values related to blood sample collection in plastic vs glass tubes.
abstract: 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.
authors.affiliation: Department of Laboratory Medicine, University of California San Francisco, USA.
authors.forename: Eberhard W
authors.initials: EW
authors.lastname: Fiebig
authors.forename: Joan E
authors.initials: JE
authors.lastname: Etzell
authors.forename: Valerie L
authors.initials: VL
authors.lastname: Ng
mesh_terms: D001800:Blood Specimen Collection / Q000295:instrumentation*; D005898:Glass; D006801:Humans; D019934:International Normalized Ratio / Q000379:methods; D010969:Plastics; D011517:Prothrombin Time / Q000379:methods
chemical_list: D010969:Plastics