From the Guidelines
INR monitoring is not required or recommended when using Direct Oral Anticoagulants (DOACs). Unlike warfarin, which requires regular INR monitoring to ensure therapeutic anticoagulation levels, DOACs have predictable pharmacokinetics and pharmacodynamics that allow for fixed dosing without routine coagulation monitoring, as stated in the ehra practical guide on the use of new oral anticoagulants in patients with non-valvular atrial fibrillation: executive summary 1. This is one of the main advantages of DOACs over warfarin therapy.
When considering the role of INR in DOAC use, it's essential to note that:
- INR is not a reliable measure of the anticoagulant effect of DOACs, and point of care tests to assess INR should not be used in patients on NOACs, as per 1.
- Specialized coagulation tests, such as anti-Xa assays for factor Xa inhibitors or diluted thrombin time/ecarin clotting time for dabigatran, may be used in specific clinical scenarios like suspected DOAC overdose, emergency surgery, or bleeding complications.
- The activated partial thromboplastin time (aPTT) and prothrombin time (PT) may provide a qualitative assessment of the presence of dabigatran and factor Xa inhibitors, respectively, but are not sensitive for quantitative assessment, as noted in 1.
Key points to consider:
- DOACs do not require routine monitoring of coagulation, and the dose or dosing intervals should not be altered in response to changes in laboratory coagulation parameters, according to 1.
- The time delay between intake and blood sampling should be carefully recorded when biological monitoring is performed, as emphasized in 1.
- Quantitative tests for DTI and FXa inhibitors exist, but their use is not routinely recommended due to limited availability and lack of data on cut-off values for safe surgery, as stated in 1.
From the Research
Role of International Normalized Ratio (INR) in Direct Oral Anticoagulant (DOAC) Therapy
- The International Normalized Ratio (INR) is a test used to measure blood clotting, but its role is limited when using Direct Oral Anticoagulants (DOACs) 2.
- DOACs, such as dabigatran, rivaroxaban, apixaban, edoxaban, and betrixaban, have predictable pharmacokinetic and pharmacodynamic responses at a fixed dose and do not require monitoring, including INR 2.
- However, some studies have reported that DOACs, particularly direct factor Xa inhibitors, can cause elevations in INR, although the clinical significance is limited 3.
- Apixaban, a type of DOAC, has been associated with a notable increase in INR in hospitalized patients, with 84.5% of patients having an elevated INR on day 1 of therapy 3.
- The effect of DOACs on INR can vary depending on the specific DOAC and the reagent used in the laboratory test, making it important for clinicians to understand the association between DOACs and INR to avoid inappropriate interpretation of routine coagulation assays 3.
Clinical Implications of INR in DOAC Therapy
- Although INR is not a reliable marker of DOAC activity, it can still be useful in certain clinical situations, such as in patients with renal impairment or those taking multiple medications 4, 5.
- Clinicians should be aware of the potential for INR elevations when using DOACs and take this into account when interpreting laboratory results 3.
- The use of DOACs has transformed the landscape of antithrombotic therapy, but there is still uncertainty about when they should or should not be used, and INR may play a role in certain clinical scenarios 6.