External quality assessment of the manual tilt tube technique for prothrombin time testing: a report from the IFCC-SSC/ISTH Working Group on the Standardization of PT/INR.

Objectives: Detailed technical instructions have been made for harmonization of the prothrombin time (PT) test using the manual tilt tube technique (MTT). The MTT has been proposed as the reference measurement procedure for PT and international normalized ratio (INR). An external quality assessment (EQA) scheme has been developed specifically for calibration laboratories performing the harmonized MTT. Here we report the results of the first 10 surveys of this new EQA scheme and investigate whether there is improvement in performance over time and in comparison with previous studies.

Methods: Four deep-frozen plasma samples with different PT levels were dispatched to 4 European laboratories. PT's were determined by eight operators. All operators used the same PT reagent (recombinant human). Various measures of PT variation were defined, i.e. within-operator, within-survey, within-run, between-operator, and between-survey coefficient of variation. Between-operator variation (CV_S) was calculated from the each operator's mean PT.

Results: The median within-operator variation of all operators varied from 1.3 to 2.3 %. Some operators improved their performance, others did not. Between-operator CV (CV_S) ranged from 1.0 to 2.2 %. Overall, the between-operator and within-operator variation using the harmonized MTT was lower than in a previously published multicentre calibration study. Overall, the within-operator variation was low and did not change significantly over time.

Conclusions: within-operator and between-operator variation of the PT measured with the harmonized MTT were low when compared with previous studies. The results suggest that the average within-operator variation of the eight operators in this study is as low as possible.