In a patient with atrial fibrillation and suspected DVT who is admitted on a DOAC and cannot provide the drug history, which assay would best suggest the presence of dabigatran?

• A. Normal PT
• B. Abnormal aPTT
• C. Normal D-dimer levels
• D. Normal Anti-Xa

Answer: (B)

Dabigatran is a direct thrombin inhibitor, so its impact shows up most clearly on tests that measure steps involving thrombin. The aPTT commonly becomes prolonged when dabigatran is present, making an abnormal aPTT a useful clue to exposure when drug history isn’t available. PT, on the other hand, is not reliably affected by dabigatran and can be normal, so it’s not as helpful for suggesting its presence. D-dimer is a marker of fibrin degradation and reflects clot formation and breakdown rather than anticoagulant drug effects, so it won’t indicate dabigatran exposure. Anti-Xa assays are designed to detect factor Xa inhibitors and aren’t appropriate for dabigatran detection, so a normal anti-Xa doesn’t rule in or out dabigatran. The most sensitive test for dabigatran is the thrombin time, which would be markedly prolonged, but that test isn’t listed here, so among the given options an abnormal aPTT is the best available indicator of dabigatran presence.