Diagnostic Equity Case: Varying INR

This question came from a patient to the Diagnostic Equity Service on 12-9-2025:

I notice my INR keeps changing even though I take the same warfarin dose. Could this be a lab issue or a metabolism issue? Thank you for what you do.

Abby Carroll, MS, MLS (ASCP)cm answered: In most cases, INR variability is attributed to metabolism and day-to-day biological factors. Several factors can temporarily impact the INR result, including illness, dietary changes, and the use of antibiotics. Your INR can change even when you take the same warfarin dose because your body’s response to the medicine naturally shifts from day to day. Things like what you eat (especially how much vitamin K is in your meals), being sick, drinking alcohol, starting or stopping medications, including herbal supplements, or even changes in your activity level can all affect it. Lab errors are possible but rare; differences between labs or how the sample was taken can sometimes cause odd results. Most of the time, though, INR changes occur because your body is reacting slightly differently, not necessarily due to a laboratory issue.

One of the best ways to manage warfarin is to be consistent with your diet, medication schedule, and lifestyle. You can ask your healthcare providers whether any of your medications or supplements might be affecting your INR, if changes in your diet—especially foods high in vitamin K (like leafy greens)—could be playing a role, and whether using the same lab each time would help reduce testing differences. If you have any concerns about an unexpected change in your INR, please discuss this with your provider as well.

Click here to learn more about the Diagnostic Equity Service, managed by Tamara Lobban Jones. The service is an evidence-based resource that helps interpret laboratory test results, recognize meaningful changes, and understand when follow-up may be appropriate. The service includes…

• Interpreting blood test results and understanding what they measure.
• Recognizing when results warrant professional follow-up.
• How to use test results to support informed choices about health and wellness.

  Developed in collaboration with medical laboratory scientists to ensure accurate, evidence-based, and clearly communicated information.