Paving the way for establishing a reference measurement system for standardization of plasma prothrombin time: Harmonizing the manual tilt tube method

Abstract

Background International normalized ratio (INR) is traceable to World Health Organization (WHO) International Standards for thromboplastins. International Standards must be used with a manual tilt tube technique (MTT) for prothrombin time (PT) determination. An important part of the total variability of INR is due to poor harmonization of MTT across WHO reference laboratories. Objectives To determine the origins of PT differences between operators performing MTT and to develop a harmonized MTT. Methods Two workshops were held where WHO reference laboratory operators could compare their PTs using MTT and the same equipment. A harmonized MTT was used by seven operators in the second workshop. Results Differences have been observed in tilting frequency and in the height of pipetting plasma in the test tube. At the beginning of the first workshop, the tilting cycle time varied between 1.1 and 2.7 seconds. The mean PT of normal plasma obtained by pipetting plasma at the top of the tube was 14.3 seconds but was 12.9 seconds when plasma was pipetted at the bottom of the tube. When using the harmonized MTT for WHO International Standard rTF/16, the differences between operators were not greater than 1.1 seconds in normal plasma, and not greater than 1.3 seconds in patient plasma with average INR of 3.0. INR between‐operator coefficient of variation was 2.3%. Conclusion Application of a harmonized MTT in three reference laboratories resulted in substantial reduction of between‐operator variation of PT and INR. The harmonized MTT is proposed as Candidate Reference Measurement Procedure.

Bias and uncertainty of the International Normalized Ratio determined with a whole blood point-of-care prothrombin time test device by comparison to a new International Standard for thromboplastin

“…The operator who performed the reference lot validations against rTF/16 in the present study, participated also in a workshop on the manual tilt tube technique and was identified as operator no. 3 [10]. It is reassuring that this operator's INR determination was very close to the mean INR of all operators in the workshop [10].…”

“…3 [10]. It is reassuring that this operator's INR determination was very close to the mean INR of all operators in the workshop [10].…”

“…Part of the scatter is due to analytical imprecision of both PT measurements with the International Standard rTF/16 and Coa-guChek XS PT measurements. The imprecision of the PT measured with rTF/16 and the harmonized manual tilt tube technique depends on the skill of the operator [10]. The imprecision of the CoaguChek XS measurements in venous and capillary blood is approximately 2.5% and 4.0%, respectively [11].…”

“…Bias and uncertainty in the INR determined with a non-harmonized MTT and the International Standard may be propagated to commercial PT systems. Harmonization of the MTT is an essential requirement for establishing a reference measurement system of the plasma prothrombin time [10]. In the future, an internationally recognized Reference Measurement Procedure (RMP) for the MTT has to be established as part of a Reference Measurement System (RMS).…”

Effect of the reaction temperature on the prothrombin time and the apparent International Normalized Ratio determined with International Standards for thromboplastins

“…In the present study, we assessed the influence of the reaction temperature on the PT determined with the current international standards for rabbit thromboplastin (RBT/16) and recombinant human thromboplastin (rTF/16). The proposed reference measurement procedure (RMP) for the PT is the manual tilt tube technique 6 . In the present study, a manual hook technique has been used.…”