Sir, Dabigatran recently became the first oral anticoagulant other than warfarin to be approved in the United States and Europe for atrial fibrillation and venous thromboembolism, and additionally in Europe, for orthopedic surgery thromboprophylaxis. Its use as an alternative to warfarin is increasing [1]. It rapidly, reversibly and competitively inhibits both free and clot-bound thrombin.

Dabigatran is typically administered in a fixed-dose fashion without routine coagulation monitoring because of its predictable pharmacokinetic profile. However, laboratory assessment of the anticoagulant effect may be useful in the setting of major bleeding, renal failure, preoperatively, overdose, assessing patient compliance, over-or underweight patients, or other scenarios [2]. In addition, hypercoagulation testing is sometimes requested by clinicians while a patient is taking dabigatran. Therefore, it is essential to determine which tests may be erroneous due to dabigatran, to ensure proper diagnosis and management. Prothrombin time (PT) is thought to be too insensitive and thrombin time too sensitive for monitoring dabigatran, while the activated partial thromboplastin time (aPTT) can be helpful but may be insensitive with some reagents and may not discriminate therapeutic from supratherapeutic levels [3–5]. We therefore established a therapeutic range with an in-laboratory prepared diluted plasma thrombin time (dpTT)[6]. We also demonstrate that dabigatran can cause false-positive lupus anticoagulant testing, falsely increased (falsely normal) activated protein C resistance ratios (APCR), falsely elevated (falsely normal) antithrombin (AT) activity and protein S activity levels, falsely reduced fibrinogen at high concentrations of dabigatran, with no effect on chromogenic protein C activity. To our knowledge, this is the first study examining the effect of dabigatran on lupus anticoagulant testing using the popular hexagonal phase confirmatory method as well as the PTT-LA (aPTT-based) lupus anticoagulant screen. This also appears to