How accurate are blood glucose monitors?

I was using my mom's One Touch Ultra to keep track of my glucose levels during my 3 hour oral glucose tolerance test. I was just wondering how accurate they are compared to the blood drawn by the lab. My last reading was a little funny (my 2 hours glucose was 145 and 3 hour was 149) so I tested again. It went... show more I was using my mom's One Touch Ultra to keep track of my glucose levels during my 3 hour oral glucose tolerance test. I was just wondering how accurate they are compared to the blood drawn by the lab. My last reading was a little funny (my 2 hours glucose was 145 and 3 hour was 149) so I tested again. It went from the 149 down to 127 (both done on same finger, left hand index finger) then back up to 137 (this time with right hand index finger). Then about 10 minutes later before I ate, I used the original hand from before but a different finger (left hand middle finger), and got a reading of 93!!! I'm super confused and don't know which to believe, so it's making me question my other reading that I took...
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