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  1. Warfarin INR Test Meters

Information for Health Care Providers Using INR Test Meters in a Clinical Setting

The FDA has received Medical Device Reporting (MDRs) of adverse events with INR test meters that may have generated inaccurate results. To minimize the potential for false results, we recommend the following:

Carefully follow all instructions provided with the INR test meter. Important sections in the manufacturer’s instructions include the following:

  • Proper sample collection techniques
  • Required operating conditions
  • The use of quality controls
  • Interpretation of error codes
  • Product limitations and specifications

The instructions will include a toll-free phone number to contact the manufacturer if you have questions about how to use the meter.

Use proper procedures to obtain a blood sample, as this can impact INR test meter results. To obtain an appropriate blood sample:

  • Ensure finger is not too cold by increasing blood flow
    • Massage the hand and the base of the fingers
    • Have the patient hold their hand under their arm
    • Wash the hand with warm water
  • Clean and dry the finger before pricking
  • Do not squeeze or “milk” the finger
  • Switch to another spot on the same finger, or switch to another finger if you are unable to get a sufficient amount of blood

Consider how your patient’s medical conditions could impact test results. INR test meters use sample matrices and clot detection technologies that are different from laboratory INR tests. Elements of blood associated with certain medical conditions can cause inaccurate INR test meter results. The technology used by INR test meters may be affected by:

  • Hematocrit
  • Acute phase reactants
  • Antiphospholipid antibodies (Lupus anticoagulants)
  • Medical conditions such as:
    • Anemia
    • Renal failure
    • Malignancies
    • Infections
    • Inflammatory conditions

We recommend using a laboratory test for confirmation when INR results are above 4.5. When the INR is greater than 4.5, the discrepancies between the INR test meter results and the laboratory test results are higher than those at lower INR ranges.

We recommend you routinely compare in-office INR test meters with a laboratory test. When comparing, you should try to use the same laboratory test for consistency since the test results may vary.

Encourage your patients to have their home-use INR test meter routinely compared to a laboratory test to evaluate device performance.

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