CoaguChek® XS versus standard laboratory prothrombin time for anticoagulant monitoring in patients with antiphospholipid syndrome

Introduction: The standard of care for thrombotic antiphospholipid syndrome (APS) is anticoagulation with vitamin K antagonists (VKAs). Prothrombin time, and its corresponding international normalized ratio (INR), is the laboratory test routinely performed to assess anticoagulation. Self-management of VKA therapy using point-of-care (POC) devices seems to be an attractive option.

Purpose/objective: To evaluate the accuracy of a POC device (CoaguChek XS) in APS patients by comparing it with venous laboratory INR. Furthermore, we analyzed whether other clinical and laboratory features could interfere with the CoaguChek XS results.

Patients and methods: This is a single-center cross-sectional study with 94 APS patients from a tertiary rheumatology clinic performed from August 2014 to March 2015. The comparison between CoaguChek XS and venous laboratory INR results was evaluated using the coefficient of determination (r) followed by the Bland-Altman test. A paired t-test was also applied. A difference of up to ±0.5 INR unit between the two systems was considered clinically acceptable.

Results: The mean CoaguChek-INR was 2.94 ± 1.41 and venous laboratory INR was 2.43±0.86, with a correlation coefficient (r) of 0.95. Categorizing INR values in ranges (INR <2, INR 2-3, INR 3-4, and INR >4), we found that the INR >4 group presented a lower correlation (r = 0.64) compared to the other ranges (p < 0.05). Although both methods were highly correlated, CoaguChek XS showed higher values than the venous laboratory INR, with an increased average of 0.42 ± 0.54. Therefore, we proposed a simple linear regression model to predict the venous laboratory INR values, using results obtained from CoaguChek XS. A difference ≤0.5 INR unit between the two systems was observed in 57.4% of patients, and the aPL profile did not influence the results.

Conclusion: Although CoaguChek XS and venous laboratory INR demonstrated a good linear correlation in the group of INR ≤4, extra caution should be taken in APS patients, since a reasonable proportion of patients can present differences in INR results that are not acceptable. We do not recommend routine POC in APS patients.