Abstract Number: PB0335
Meeting: ISTH 2021 Congress
Background: Many patients are still treated with vitamin K-antagonists (VKA). Laboratory monitoring of VKA therapy depends on determination of the Prothrombin Time (PT) and calculation of the International Normalized Ratio (INR). A Reference Measurement System (RMS) is required to achieve traceability and longitudinal accuracy of PT/INR results. The RMS depends on the establishment of a Reference Measurement Procedure (RMP). A harmonized WHO Manual Tilt Tube Technique (MTT) has been proposed as RMP for the PT/INR. The harmonized MTT was developed in two workshops in which operators of three WHO calibration laboratories participated.
Aims: Setting up Ring Trials to assess the performance of the harmonized MTT by laboratories involved in calibration of PT/INR systems.
Methods: Two lyophilized control plasma samples (level 1 and level 2) were sent together with the WHO International Standards for thromboplastin (RBT/16 and rTF/16) to three calibration laboratories. The laboratories were instructed to analyze the control plasmas by 2 operators on 5 days in duplicate. Analysis of variance (ANOVA) was performed on the PT results.
Results: Mean PT results and within-operator coefficients of variation (CV) are shown in Table 1. The differences between the operators were statistically significant (P<0.001). The PT results obtained by operators in laboratory #3 were longer than those in the other laboratories, suggesting a systematic error. Transformation of PT results to INR is shown in Table 2. The INR results of laboratory #3 were lower than those in the other laboratories.
| Operator number | Lab number | RBT/16, level 1 | RBT/16, level 1 | RBT/16, level 2 | RBT/16, level 2 | rTF/16, level 1 | rTF/16, level 1 | rTF/16, level 2 | rTF/16, level 2 |
| Mean PT (s) | CV (%) | Mean PT (s) | CV (%) | Mean PT (s) | CV (%) | Mean PT (s) | CV (%) | ||
| 1 | 1 | 20.45 | 0.8 | 44.73 | 0.8 | 13.42 | 1.8 | 34.45 | 0.7 |
| 2 | 1 | 20.64 | 1.4 | 45.71 | 1.0 | 13.53 | 2.0 | 34.24 | 3.0 |
| 3 | 2 | 20.28 | 3.4 | 45.34 | 1.7 | 14.03 | 3.5 | 34.33 | 2.5 |
| 4 | 2 | 20.46 | 2.4 | 45.31 | 2.5 | 14.06 | 1.1 | 33.84 | 2.6 |
| 5 | 3 | 23.16 | 4.1 | 48.89 | 1.9 | 16.00 | 2.7 | 36.71 | 2.0 |
| 6 | 3 | 24.15 | 4.2 | 48.76 | 2.2 | 16.82 | 3.5 | 39.06 | 3.8 |
PT results obtained in Ring Trial
| Control sample | Reagent | Mean INR, Operator #1 | Mean INR, Operator #2 | Mean INR Operator #3 | Mean INR, Operator #4 | Mean INR, Operator #5 | Mean INR, Operator #6 | Between-operator CV (%) |
| Level 2 | RBT/16 | 2.57 | 2.62 | 2.64 | 2.61 | 2.47 | 2.34 | 4.6 |
| Level 2 | rTF/16 | 2.85 | 2.80 | 2.70 | 2.65 | 2.51 | 2.55 | 5.1 |
Mean INR and between-operator coefficient of variation obtained in Ring Trial
Conclusions: Despite harmonization of the MTT procedure, there were important differences in PT and INR results between calibration laboratories. Further efforts should be made to reduce these differences, e.g. by training of the operators in communal workshops. Regular Ring Trials should be continued for calibration laboratories using the harmonized MTT.
To cite this abstract in AMA style:
van den Besselaar A, van Rijn C, Cobbaert C. Ring Trial for Quality Assurance of the Manual Tilt Tube Technique for Prothrombin Time and INR Determinations in WHO Calibration Laboratories [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 2). https://abstracts.isth.org/abstract/ring-trial-for-quality-assurance-of-the-manual-tilt-tube-technique-for-prothrombin-time-and-inr-determinations-in-who-calibration-laboratories/. Accessed November 28, 2023.« Back to ISTH 2021 Congress
ISTH Congress Abstracts - https://abstracts.isth.org/abstract/ring-trial-for-quality-assurance-of-the-manual-tilt-tube-technique-for-prothrombin-time-and-inr-determinations-in-who-calibration-laboratories/