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Effect of HCT on POC INR

George received this question from the ASCLS Consumer Web Forum via Bonnie Jo Taylor, MHS, MT(ASCP)SH.

I have hemochromatosis, too much iron in my blood. I’ve also had blood clots in my lungs. I peroidically undergo phlebotomy treatments to lower iron count. Will this alter the results of my INR finger prick test results?

Here is George’s edited response, please send your follow-up comments:

Hello, and thank you for your question. The short answer is no, your periodic phlebotomies do not affect your finger stick INR accuracy and you can rely on the results from the instrument.

For a little more detail, the key measurement that may affect your INR is the hematocrit (HCT). We use the HCT in conjunction with hemoglobin and red blood cell count to diagnose and treat anemia. The normal HCT in adult males is 40–54%, with results below 40% indicating anemia. The most commonly used finger stick INR instruments are calibrated to provide accurate INRs when the HCT is between 25% and 55%. Phlebotomy temporarily reduces the HCT, however it is unlikely to cause the HCT to dip below 25%.

The results from finger stick INR instruments are validated by comparing them to the INR results from the central laboratory, where we employ a plasma-based method that is unaffected by the HCT. If you have cause to doubt your finger stick INR results, you may request the INR be performed in your institution’s central laboratory using venous blood. Results from finger stick instruments are generally stable and reproducible, and should compare with central laboratory INR results within 10%.

Though high HCT results are not necessarily found in hemochromatosis, people taking Coumadin whose HCT results are over 55%, a condition called polycythemia, should have their INR tested in the central laboratory, as the finger stick instruments are not calibrated to give accurate results for people with HCTs over 55%.

Again, thank you for your question, please feel free to email your follow-up questions or comments, and best wishes for your successful treatment.

van den Besselaar AM, Witteveen E, van der Meer FJ. Influence of haematocrit on international normalised ratio (INR) differences between a whole blood point-of-care coagulation monitor and reference prothrombin time in plasma. Thromb Haemost. 2008;100:1181–4.

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