PharmacologyTier 2Medical Sciences concept

Coagulation test sensitivity to direct oral anticoagulants (TCT/dabigatran, anti-Xa/rivaroxaban)

Core concept

Routine coagulation tests were designed for warfarin and heparin, and behave unpredictably with DOACs
Each DOAC has one screening test that is sensitive and one confirmatory quantitative assay

Drug	Most sensitive routine test	Quantitative assay
Dabigatran (direct IIa)	Thrombin clotting time (TCT/TT) - exquisitely sensitive	Dilute thrombin time (Hemoclot) or ecarin clotting time
Rivaroxaban (direct Xa)	PT/INR (reagent-dependent)	Drug-specific (calibrated) anti-Xa
Apixaban (direct Xa)	Often normal PT and APTT even at therapeutic levels	Drug-specific anti-Xa
Edoxaban	PT	Drug-specific anti-Xa

A normal thrombin time effectively excludes clinically significant dabigatran - the single most useful bedside rule
A normal PT and APTT does NOT exclude apixaban - the commonest and most dangerous misreading

APTT with dabigatran - prolonged but with a curvilinear, plateauing response; a normal APTT makes a high level unlikely, but the degree of prolongation does not quantify the level
PT/INR is insensitive to dabigatran and to apixaban; INR must never be used to assess DOAC anticoagulation
Drug-specific anti-Xa assays must be calibrated to the specific drug - a heparin-calibrated anti-Xa gives a qualitative answer only
Timing is everything - peak at 1-4 h, trough at 12-24 h; a level is uninterpretable without the dose time
False results on other tests caused by DOACs
- Lupus anticoagulant testing is invalid on a DOAC - false positives via dRVVT
- Falsely low antithrombin (Xa-based assay), protein C/S (clot-based assay), and factor assays
- Thrombophilia testing should not be performed on a DOAC
When a level actually matters
- Bleeding, urgent surgery, thrombolysis decision in stroke
- Suspected accumulation in AKI, extremes of weight, suspected overdose, or suspected non-adherence
Reversal is level-informed: idarucizumab for dabigatran, andexanet alfa or PCC for Xa inhibitors

Acute stroke on a DOAC - thrombolysis is contraindicated within 48 h of the last dose unless a DOAC-specific assay is normal (or dabigatran has been reversed with idarucizumab)
- A normal TT effectively clears a dabigatran patient for thrombolysis
Major bleeding: last dose time, renal function, DOAC-specific level, then reversal agent + supportive care + local haemostasis
Do not attempt to titrate a DOAC by any coagulation test - the doses are fixed, and monitoring is for exceptional circumstances only
Deranged PT/APTT in a patient on a DOAC is usually the drug, not a coagulopathy - repeat off drug before investigating
Heparin still needs APTT or anti-Xa; warfarin still needs INR - the DOAC exception applies only to DOACs

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