Discrepancy of the international normalized ratio observed in the external quality assessment survey: a cause for concern

Interlaboratory variability in the measurement of direct oral anticoagulants: results from the external quality assessment scheme

Essentials Tests for direct oral anticoagulants (DOACs) are not widely applied. These tests are perceived to be difficult to run and subjected to large between-lab variation. We carried out proficiency testing surveys for DOAC testing in Italy. Interlab variability was small and similar to that of the international normalised ratio.

SUMMARY

Background Tests for direct oral anticoagulants (DOACs) are not widely available. The perception that they are difficult to perform and are subject to large between-laboratory variation makes their implementation difficult. Aims We carried out proficiency-testing surveys for DOACs within the activity of the external quality-assessment scheme of the Italian Federation of Thrombosis Centers. Design Participants were provided with coded freeze-dried plasmas without or with graded concentrations of the three main DOACs, and asked to measure prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time and DOAC concentrations with dedicated tests. The results were centralized for statistical analysis. Results and conclusions All participants (n = 235) reported results for PT and APTT, and approximately one-third reported results for DOAC concentration. PT and APTT showed variable responsiveness to DOACs: PT was more responsive to rivaroxaban than to dabigatran or apixaban. APTT was more responsive to dabigatran than to rivaroxaban or apixaban. The thrombin time ratio (test/normal) was close to unity for plasmas without dabigatran, and was high (i.e. 7.6-fold or 15.4-fold longer than the plasma free from the drug) for plasmas containing dabigatran at low (i.e. 42 ng mL-1 ) or high (i.e. 182 ng mL-1 ) concentration. Dedicated tests were responsive to the respective drugs, and their interlaboratory variability was relatively small (overall coefficients of variation of 8.7%, 8.4% or 10.3% for dabigatran, rivaroxaban and apixaban, respectively) and was comparable to that observed within the same survey for the International Normalized Ratio (i.e. 11.4%). In conclusion, tests for DOAC measurement performed reasonably well in a national quality-control scheme. Regulatory authorities should urgently issue recommendations on their use, and clinical laboratories should make them available.

Variability between laboratories performing coagulation tests with identical platforms: a nationwide evaluation study

Background

While the assessment of analytical precision within medical laboratories has received much attention in scientific enquiry, the degree of as well as the sources causing variation between them remains incompletely understood. In this study, we quantified the variance components when performing coagulation tests with identical analytical platforms in different laboratories and computed intraclass correlations coefficients (ICC) for each coagulation test.

Methods

Data from eight laboratories measuring fibrinogen twice in twenty healthy subjects with one out of 3 different platforms and single measurements of prothrombin time (PT), and coagulation factors II, V, VII, VIII, IX, X, XI and XIII were analysed. By platform, the variance components of (i) the subjects, (ii) the laboratory and the technician and (iii) the total variance were obtained for fibrinogen as well as (i) and (iii) for the remaining factors using ANOVA.

Results

The variability for fibrinogen measurements within a laboratory ranged from 0.02 to 0.04, the variability between laboratories ranged from 0.006 to 0.097. The ICC for fibrinogen ranged from 0.37 to 0.66 and from 0.19 to 0.80 for PT between the platforms. For the remaining factors the ICC’s ranged from 0.04 (FII) to 0.93 (FVIII).

Conclusions

Variance components that could be attributed to technicians or laboratory procedures were substantial, led to disappointingly low intraclass correlation coefficients for several factors and were pronounced for some of the platforms. Our findings call for sustained efforts to raise the level of standardization of structures and procedures involved in the quantification of coagulation factors.