home test vs lab test

[tcerame]

During my last home health visit my INR was at 8. The nurse retested it and again got an 8.0 on her home testing machine. She then drew a blood sample and took it to a lab to have it run. They got a 5.4. Our insurance said they would approve a home testing unit, but has anyone else had any problems with the two results being so different. I know that both results are too high and my coumadin was adjusted for it. But i worry about the difference between the home result and the lab result. Thanks, Tom

 

[Ross]

Tom the reason for that is because of how high your INR was. The machines are not very accurate at all past an INR 5.0. In fact, the INRatio meter will only go as high as 7.5. They are calibrated more for the ball park figures of 2 to 4. If I'm wrong, I'm sure I'll be corrected. When your INR is in the correct range, you should have only a .3 to .6 spread in the values of a draw vs machine.

 

[Johnny Stephens]

I have thrown 5+ two times (in the rollercoaster days right after surgery), and both times I was given the same explanation -- the accuracy of the machine falls off sharply above 5.0. Both times, the lab came back high as well, but with markedly different numbers.

 

[Blanche]

Variance at higher INRs

Variance at higher INRs

One reason that the home monitors AND the lab tests can be less accurate at the higher INR levels has much to do with the way that the INR is calculated. INR (International Normalized Ratio) is a mathematical equation that uses the sensitivity of the reagent (ISI) as the exponent in the equation, which causes a problem with test results at higher levels. I have always maintained that in the long run the home monitors are more accurate than the labs because the monitors use a constant ISI, where the labs' ISIs can change with each new batch of reagents. When Al was comparing his machine to two labs, we found that the ISIs for both labs were not only different, they changed frequently. The ISI of the Protime monitor is 1, which would give a more accurate reading.

Perhaps someone with an engineering or mathematics background will come on and give a more cogent explanation of the relationship of the ISI (the exponent in the equation). In the meantime, the following article does cover this in a comprehensive manner. This article is also in the Reference Section of this board.
http://www.wwnurse.com/Nurse-zine/Articles/coagulation.shtml

Kind regards,
Blanche