Fingerstick blood sample for PT/INR testing
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This article waslast modified on April 9, 2018.

The U.S. Food and Drug Administration (FDA) has published new information on the agency’s website for people who take warfarin (a blood thinner) and use portable, battery-operated INR test meters to monitor their therapy. This update was prompted by reports that some INR test meters may have generated inaccurate results. The FDA regulates INR test meters and test strips, and the agency wants to make sure all people who use these home-testing devices, regardless of make or model, are using them safely.

Warfarin is a drug that helps inhibit the formation of harmful blood clots. Healthcare practitioners may prescribe it for people who have atrial fibrillation (AFib or AF) or have experienced deep vein thrombosis (DVT) or pulmonary embolism (PE), for example. The goal of warfarin therapy is to maintain a balance between preventing clots and avoiding excessive bleeding.

It’s very important for people who are taking warfarin to closely monitor their therapy with an INR (International Normalized Ratio) calculated from a prothrombin time (PT), a test that measures the time it takes for a person’s blood to clot. An INR below the target range suggests the level of medication may be too low, and the person is still at risk of developing harmful blood clots, while an INR above the target range suggests the level of medication is too high, and the person is at increased risk of excessive bleeding. The INR can be used by a healthcare practitioner to adjust a patient's warfarin dose to get the INR into the desired range. A patient’s target INR and testing frequency is determined by his or her healthcare practitioner.

In addition to PT/INR tests done in laboratories, many people use portable INR meters to conveniently monitor their response to warfarin from home. INR test meters may also be used by healthcare practitioners at the point of care, such as an outpatient care setting. Using a small drop of blood from a fingerstick, the meter reads the test strip and measures how long it takes the blood to clot, and displays the results as an INR on its screen.

Earlier this year, the FDA received Medical Device Reports (MDRs) of adverse events about INR test meters that may have generated inaccurate results. In response, the agency posted updates and new information to their website about warfarin INR test meters for the patients and caregivers who use these meters daily. These updates include cautions and tips for patients using INR test meters at home, such as:

  • Carefully reading all instructions that come with the meter
  • The proper way to collect blood from a fingerstick
  • How often to use the meter
  • When to be concerned about the results as well as when to contact a healthcare practitioner
  • What factors can affect results
  • How to make sure the device is working properly

The website also has updated content for healthcare practitioners who use INR meters in clinical settings, including information on medical conditions that could affect test results, recommendations for confirming meter results with tests performed in laboratories, as well as how to report problems with INR test meters to the FDA.

For complete details, visit the FDA webpage Warfarin INR Test Meters

However convenient and helpful INR meters may be, it’s important to know that there can be “tradeoffs” between the ease of home tests and the quality of tests done by medical or laboratory professionals. This knowledge can help protect a person from incorrect, inaccurate, or incomplete test results that could be harmful. (For more, read the feature article With Home Testing, Consumers Take Charge of Their Health)

View Sources

(9 March 2018) FDA, Medical Devices. Warfarin INR Test Meters. Available online at www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/InVitroDiagnostics/WarfarinINRTestMeters/default.htm. Accessed on March 11, 2018.

(13 February 2018) MCN Healthcare, Stay Alert! Warfarin INR Test Meters. Available online at www.mcnstayalert.com/stayalert/alert/ag9zfm1jbi1zdGF5YWxlcnRyFQsSCEFsZXJ0X3YyGICAwNymiqUIDA/warfarin-inr-test-meters. Accessed on March 11, 2018.

(12 February 2018) FDA Email Alert. The FDA added new information about warfarin INR test meters for patients and caregivers using INR test meters. Available online at http://s2027422842.t.en25.com/e/es?s=2027422842&e=50663&elqTrackId=B1F0B909CCF90C71B9C490C37BFE6647&elq=a3e04d664f204a30bfaebe29e1942c53&elqaid=2409&elqat=1. Accessed on March 11, 2018.

(August 2017) AHRQ. Your Guide to Preventing and Treating Blood Clots. Available online at www.ahrq.gov/patients-consumers/prevention/disease/bloodclots.html. Accessed on March 11, 2018.