Protimenow - Along with the others, I'm so sorry about your recent stroke, and I hope you recover quickly and completely.
PEM - I've only been home testing with my Inratio2 meter for about 5 months, and have had only one occasion where I have both a meter test and a lab test taken from blood drawn within an hour or two of the meter home test. On that occasion, the home test result was 3.4 and the lab test result was 2.8
I just assumed the truth was somewhere in the middle, around 3.1, or that the lab test was probably wrong since the blood wasn't actually tested until the next day after shipment from the doctor's office to the lab.
But, your formula of home-test-raised-to-the-power-of-0.85 works perfectly for the 3.4/2.8 home/lab difference on the one data point I can add from my Inratio2 meter.
This, along with Protimenow's experience makes me a bit nervous too. If your formula is true for my Inratio2 meter as well, then to stay within a lab-draw reading range of 2.5-3.5, I should strive for a home-test range of 2.9-4.4
I have no problem with the 2.9 side, but if my home reading is 4.4, I usually consider that well out of range and adjust my warfarin accordingly.
To be within range under both test types, it would appear I should strive for a home test range of 2.9 to 3.5 on my home meter. From my history so far, this tighter home-test target range will be difficult, if not impossible for me to achieve.
I would also be interested in hearing from others as to whether their Inratio2 meters show the same offset from lab-draw INR tests.
Another test I plan to perform when I do my next home test, is to make two different tests with my meter within 1/2 hour of each other just to test the repeatability of the meter itself with no other variables (same test strip lot number).
When I get a new batch of test strips, and have two with different lot numbers, I'll check the repeatability again by running two tests within 1/2 hour of each other using strips from different lots.
I'll post with my results after these tests, but I would be interested to know if anyone has already done such tests and what their results were.
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UPDATE to Post:
I just found the following from the FDA website after a bit of googling:
http://www.accessdata.fda.gov/cdrh_docs/reviews/K092987.pdf
Although there is no date on the report, it does make for some interesting reading.
In the "analytical performance" part of the report, because Biosite/Alere is using the venous blood draw as the basis for the purported accuracy of the meter & test strip readings, it would seem that "Ground Truth" would be the venous blood draw method test results.
The other item that caught my eye is that there can be a +/- 0.5 INR offset, with a +/- 30% relative bias of the test strips within the therapeutic range for INR values of 2.0 to 4.5. I am making an assumption that the absolute offset and relative bias of the test strips is what is calibrated out when the test strip lot code number is entered into the Inratio2 meter for a given manufacturing lot.
The report seems to say that there is a residual overall accuracy level of about +/- 8.5% So, for a nominal reading of INR=3.0, this gives an allowed uncertainty of +/- 0.25 on the INR reading.
From PEM's results, and my own single point comparison with a blood draw test, it would seem that these accuracy results may not always be met. Perhaps some other factors are at work to skew the results, but I plan to get a few more test comparisons to blood draw tests for my own meter if I can.
Protimenow - Along with the others, I'm so sorry about your recent stroke, and I hope you recover quickly and completely.
PEM - I've only been home testing with my Inratio2 meter for about 5 months, and have had only one occasion where I have both a meter test and a lab test taken from blood drawn within an hour or two of the meter home test. On that occasion, the home test result was 3.4 and the lab test result was 2.8
I just assumed the truth was somewhere in the middle, around 3.1, or that the lab test was probably wrong since the blood wasn't actually tested until the next day after shipment from the doctor's office to the lab.
But, your formula of home-test-raised-to-the-power-of-0.85 works perfectly for the 3.4/2.8 home/lab difference on the one data point I can add from my Inratio2 meter.
This, along with Protimenow's experience makes me a bit nervous too. If your formula is true for my Inratio2 meter as well, then to stay within a lab-draw reading range of 2.5-3.5, I should strive for a home-test range of 2.9-4.4
I have no problem with the 2.9 side, but if my home reading is 4.4, I usually consider that well out of range and adjust my warfarin accordingly.
To be within range under both test types, it would appear I should strive for a home test range of 2.9 to 3.5 on my home meter. From my history so far, this tighter home-test target range will be difficult, if not impossible for me to achieve.
I would also be interested in hearing from others as to whether their Inratio2 meters show the same offset from lab-draw INR tests.
Another test I plan to perform when I do my next home test, is to make two different tests with my meter within 1/2 hour of each other just to test the repeatability of the meter itself with no other variables (same test strip lot number).
When I get a new batch of test strips, and have two with different lot numbers, I'll check the repeatability again by running two tests within 1/2 hour of each other using strips from different lots.
I'll post with my results after these tests, but I would be interested to know if anyone has already done such tests and what their results were.
---------------------
UPDATE to Post:
I just found the following from the FDA website after a bit of googling:
http://www.accessdata.fda.gov/cdrh_docs/reviews/K092987.pdf
Although there is no date on the report, it does make for some interesting reading.
In the "analytical performance" part of the report, because Biosite/Alere is using the venous blood draw as the basis for the purported accuracy of the meter & test strip readings, it would seem that "Ground Truth" would be the venous blood draw method test results.
The other item that caught my eye is that there can be a +/- 0.5 INR offset, with a +/- 30% relative bias of the test strips within the therapeutic range for INR values of 2.0 to 4.5. I am making an assumption that the absolute offset and relative bias of the test strips is what is calibrated out when the test strip lot code number is entered into the Inratio2 meter for a given manufacturing lot.
The report seems to say that there is a residual overall accuracy level of about +/- 8.5% So, for a nominal reading of INR=3.0, this gives an allowed uncertainty of +/- 0.25 on the INR reading.
From PEM's results, and my own single point comparison with a blood draw test, it would seem that these accuracy results may not always be met. Perhaps some other factors are at work to skew the results, but I plan to get a few more test comparisons to blood draw tests for my own meter if I can.
Very interesting - thanks for the data point!
I have tested across InRatio machines (they sent me a new one and so I did a comparison before sending the old one back), across test strip lots, and within lots. All were consistent within 0.2. They seem very self-consistent and also reliably predict the lab result. So the question remains, if we can't operate in the narrow range you defined (2.9 - 3.5 on the home meter), then how do we ascertain which result is the unbiased, ground truth result - the lab or the meter?
Toward that end, I had an interesting conversation with a subject matter expert from Quest Diagnostics yesterday. Among the enlightening things she had to say was that there is another test called the chromogenic factor 10 assay. It exists to measure anticoagulation for people with the Lupus anticoagulation factor, who cannot be reliably tested using standard methods. So the idea she proposed was that I could have the factor 10 test done to try to determine whether the lab result or the home meter result is correct. I am waiting to find out if the factor 10 test actually produces an "equivalent INR" number or just measures whether or not someone is in range. Also, I suspect the therapeutic range used in the factor 10 test, which is 11% to 42% corresponds to an INR range of 2.0 to 3.0, which is more typical than the mech valve range of 2.5 to 3.5 that we try to stick to.
Another interesting bit of information from the Quest SME is that the reagents used to measure INR are assigned ISI values based on their responsiveness to "gold standard" patient blood samples. In other words, they use an antiquated method called the "tube tilt test" to visually time the onset of clotting. This procedure is meticulously used to generate blood samples that correspond to INRs of 1.0, 1.5, 2.0, 2.5, and 3.0. The reagent manufacturers then use these predetermined samples to generate their sensitivity indices (ISI values). What's notable here is that the gold standard patient samples only go up to an INR of 3.0. That could underlie the observation that above an INR of 3.0, home meters and lab tests become less reliable and less consistent with each other. One rationale, consistent with my conversion model, is that the relationship is roughly linear up to 3.0, but that nonlinearities in the relationship become more pronounced after that. So if labs or meters use linear extrapolation above 3.0, they may under or over-predict.
Thanks a lot for the FDA info. I don't understand it yet, so I will have to carefully reread your posting
Best,
pem