How often should International Normalized Ratio (INR) be checked?

From the Guidelines

For patients treated with warfarin, the INR should be determined at least weekly during initiation of anticoagulant therapy and at least monthly when anticoagulation (INR in range) is stable. This recommendation is based on the most recent and highest quality study, which emphasizes the importance of regular INR monitoring to minimize the risk of stroke and bleeding in patients with atrial fibrillation 1. The frequency of INR checks may vary depending on individual factors, such as age, other medications, and specific medical conditions. However, as a general guideline, patients newly starting warfarin should have their INR checked daily until stable, then 2-3 times weekly for 1-2 weeks, followed by weekly until consistently therapeutic for 2-3 weeks. Once stable, most patients require INR testing every 2-4 weeks. More frequent monitoring is needed during medication changes, illness, significant diet changes, or if readings are outside the target range (usually 2.0-3.0 for most conditions, or 2.5-3.5 for mechanical heart valves). Some patients with consistently stable INR values may extend to 6-8 week intervals under physician guidance. Home INR monitoring devices are available for patients requiring frequent testing. Regular monitoring is essential because warfarin has a narrow therapeutic window—too low increases clot risk while too high increases bleeding risk.

Key points to consider:

• The CHA2DS2-VASc score is recommended for assessment of stroke risk in patients with atrial fibrillation 1
• Selection of anticoagulant therapy should be based on the risk of thromboembolism, irrespective of whether the AF pattern is paroxysmal, persistent, or permanent 1
• Anticoagulant therapy should be individualized on the basis of shared decision-making after discussion of the absolute risks and relative risks of stroke and bleeding, as well as the patient’s values and preferences 1
• Reevaluation of the need for and choice of anticoagulant therapy at periodic intervals is recommended to reassess stroke and bleeding risks 1

From the FDA Drug Label

The PT should be determined daily after the administration of the initial dose until PT/INR results stabilize in the therapeutic range. Intervals between subsequent PT/INR determinations should be based upon the physician’s judgment of the patient’s reliability and response to warfarin sodium tablets in order to maintain the individual within the therapeutic range Acceptable intervals for PT/INR determinations are normally within the range of one to four weeks after a stable dosage has been determined

The frequency of checking INR should be:

• Daily after the administration of the initial dose until PT/INR results stabilize in the therapeutic range.
• At intervals of one to four weeks after a stable dosage has been determined, based on the physician's judgment of the patient's reliability and response to warfarin sodium tablets. 2

From the Research

Frequency of INR Checks

The frequency of INR checks depends on various factors, including the patient's condition, the stability of their INR values, and the presence of bleeding or thromboembolic complications.

• Initially, when warfarin therapy is initiated, INR should be monitored frequently, such as 2 to 4 times per week 3.
• As the patient's INR values stabilize, the interval between INR tests can be lengthened gradually, up to a maximum of 4 to 6 weeks 3.
• Patients with elevated INR values or those who experience bleeding or thromboembolic complications may require more frequent INR testing 3, 4, 5.

Factors Affecting INR Checks

Several factors can affect the frequency of INR checks, including:

• The patient's age, with elderly patients potentially requiring more frequent monitoring 3.
• The presence of bleeding or thromboembolic complications, which may require more frequent INR testing 4, 5.
• The patient's INR variability, with higher variability potentially requiring more frequent monitoring 5.
• The patient's time in therapeutic range (TTR), with lower TTR potentially requiring more frequent monitoring 5.

Laboratory Testing Beyond INR

Laboratory testing beyond INR, such as blood count tests, may also be important in patients treated with warfarin.

• Anaemia has been associated with an increased risk of bleeding and stroke/transient ischaemic attack (TIA) in patients with atrial fibrillation treated with warfarin 6.
• Repeatedly low haemoglobin results may indicate suboptimal management and increase the complication risk in anticoagulated patients 6.