Thought I'd chime in here as I have been dealing with a similar issue and have communicated with pellicle about this as well.
Quick background-- my surgery was in November 2021. I live in the States, where we are unable to purchase our own Coagucheck device through typical means. The insurance company will purchase one for us after 90 days post-surgery. However knowing how anxious I'd be about my INR, I wanted a home device in place for right when I got home (I figured if I eventually qualified for the Coagucheck, it wouldn't hurt to have 2 devices).
So I purchased the Coagsense. Wound up being a very good purchase as my INR management from the clinic was suboptimal during the first couple of weeks, and my home machine allowed me to get my INR on target very quickly. In late November, I had a lab draw that was equal to what I was getting from my Coagsense. So that was reassuring.
Three weeks ago, the insurance company sent me the Coagucheck XS machine. First off, I find the Coagucheck easier to use, but that's beside the point. My first test on the Coagucheck was 2.9. I decided to check on the Coagsense right after that, and it was 2.2.
Over the next week or so, regular tests on both machines showed a discrepancy of 0.7, with the Coagsense being lower.
Last week I decided to test on both devices, then immediately go to the lab. The Coagucheck yielded 2.9, the Coagsense yielded 2.2. The lab draw was 2.9.
This led me to believe that the Coagsense started underestimating the INR relative to lab values somewhere between late November and early February.
I called Coagsense the other day. They were pretty helpful. He went through my technique with me, everything checked out. Long story short, they need more data. His first thought was that maybe the current strips were off (I switched to a new box of strips about a month ago). So they are sending me a new box of strips with a different lot number. The second thought was to get a couple more data points. So I'm going to test on both then go to the lab again next week to see if the Coagsense is truly the device that's off-base. It sounds like if it is, they will take the machine back and repair it.
Pellicle mentioned this calibration system they have. They have Hi control and Lo control strips, with a plasma reagent meant to test it. With this new box of strips, one of the control strips kept consistently failing. I wondered if that had something to do with it. The technician said no, that it was probably just the plasma test reagent, which is sensitive to temperature, was affected during shipment/storage. I asked repeatedly whether this could affect the accuracy of the device, and he said no. However he was a bit elusive when I asked what it was needed for...hinting that it was more of an FDA requirement and that I don't even need to use them when I get a new box of strips. Very weird.
Either way, I will be getting more data points and following up with them after I test with new strips and corroborate with the lab.