Warfarin INR Test Meters

An INR test meter is a portable, battery-operated meter, used to monitor patient response to warfarin. The meter has a screen that displays results, and an opening for meter-specific test strips. A test strip is inserted into the meter, and then a lancet (a medical pricking needle) is used to obtain blood which is applied to the test strip. The meter reads the test strip, measures how long it takes the blood to clot, and provides the result based on a standardized calculation in the form of the International Normalized Ratio, or INR. The INR is used by patients and physicians to determine if warfarin dosing is appropriate.

INR test meters and test strips are medical devices regulated by the U.S. Food and Drug Administration. The FDA wants to ensure patients and health care providers use these devices safely. Please read on for more information.

What is Warfarin?

Warfarin (also known by the brand names Coumadin and Jantoven) is a blood thinner prescribed to prevent and treat blood clots. Warfarin therapy may be prescribed for patients with certain types of irregular heartbeat, blood clots in the legs or lungs, and patients who have certain medical device implants such as artificial heart valves.

A patient’s warfarin dose depends on many factors, including the patient’s age, overall health and genetic makeup. The dose may also need to be adjusted based on a patient’s diet and current medications.

Monitoring Warfarin

Warfarin must be monitored to ensure it is working effectively and being used safely. Achieving the correct warfarin dosage can be difficult but is extremely important. If the dose of warfarin is too low, the patient is at risk of developing harmful blood clots. If the dose of warfarin is too high, the patient may be at risk of serious bleeding. It can be monitored by drawing blood from a vein and sending the blood to an accredited laboratory to test, or it can be monitored by testing blood from a fingerstick with an INR test meter outside of a laboratory. INR test meters may be prescribed to patients for use in the home, and they may also be used by health care providers at the point of care, such as a health care setting.

How often an INR test meter is used to monitor warfarin, and the results a patient should expect, will be based on recommendations from the patient’s health care provider. An INR target range is set by a health care provider. It is typically between 2.0 and 3.0 for basic blood-thinning needs, though the range may vary based on a patient’s specific conditions. An INR above the patient-specific target range may increase the risk of bleeding, while an INR below the target range may increase the risk of developing a blood clot.
Patients should work with their health care provider to develop a plan for achieving and maintaining a target INR; recognize when there is a problem; and know when to contact their health care provider.

The information and resources on this website are intended to provide useful information for patients and caregivers who are using INR test meters in the home and for health care providers using INR test meters in a clinical setting.

Home Users - Patients and Caregivers: For more information and tips on using an INR test meter to monitor warfarin safely and effectively, visit Tips for Patients and Caregivers Using INR Test Meters at Home.

Health Care Providers: For more information and tips on using an INR test meter to monitor warfarin safely and effectively, visit Tips for Health care Providers Using INR Test Meters in a Clinical Setting.

How to Report a Problem

All health care providers, patients and caregivers, are strongly encouraged to voluntarily report INR test meter problems directly to the FDA through MedWatch, the FDA’s voluntary reporting program. Problems should be reported whenever one suspects there is an issue with an INR test meter such as a malfunction or incorrect result, or that the meter caused or contributed to a serious injury or death. Hospitals, nursing homes, coagulation clinics, and outpatient facilities should follow the reporting procedures established by their facilities. INR test meter users should also report these problems to the manufacturer using the phone number listed on your meter’s instructions.

When submitting a report through MedWatch, please include as much of the following information as possible:

• Brand of the test meter involved (and the model or catalog number, if known)
• Lot number of the test strips involved
• Date of the event
• Whether you are the patient, a family member or a health care provider
• Whether the manufacturer was notified about the problem
• Patient's age
• Description of patient’s symptoms before and after the incorrect test result
• Detailed description of the event, such as:
  • The initial INR test meter result
  • Whether the blood sample was collected according to the manufacturer’s recommendations
  • If the test was repeated by the laboratory and you are reporting an incorrect result, include:
    • The laboratory test result
    • The time period between the repeated testing (e.g., 10 minutes later, or several hours later)
    • What conditions changed between the two INR tests (for example, using a different finger, using a lab test or different INR test meter, taking medication or eating leafy greens between INR tests)
    • Test results prior to the incorrect test result

What types of problems should be reported?

The following are examples of some of the problems that may arise and should be reported:

• Test strip or test meter defects
• Quality control problems (liquid control solutions, test strip controls)
• INR test meter fails to perform as described in the labeling
• Discrepant test results (such as falsely low INR results as compared to a lab test) that caused or contributed to an incorrect warfarin or vitamin K dosage
• Difficulty understanding the INR test meter or test strip instructions
• Difficulty using the meter
• INR test meter stops working unexpectedly
• Unexplained bleeding, any other problems that compromise patient health or safety, or death

Related Resources:

• The FDA Warns Against Use of Previously Owned Test Strips or Test Strips Not Authorized for Sale in the United States: FDA Safety Communication
• Public Workshop - Point-of-care Prothrombin Time/International Normalized Ratio Devices for Monitoring Warfarin Therapy
• Transcript of FDA Press Conference on Warfarin