Ever try to check one meter against another by testing with both at the same time and seeing if their results match up? Ever wonder why they might not?
The answer lies in the FDA’s definition of what constitutes “accurate” for any one meter. A meter is considered accurate enough if it’s within 20 percent of the true value that a lab would provide. Home meters may be calibrated slightly low or high, depending on the manufacturer, to ensure consistent results. So, if your blood glucose is really 100, a meter that reads 80 is close enough for the FDA. Another meter that says 120 is also close enough. However, if you compare them to each other, they’re forty mg/dl apart. You might think that one of them is right, and one is wrong, but that’s not the case. They’re both (just barely) right enough.
The potential differences get bigger as your glucose goes higher. Suppose your real blood glucose level is 300, as confirmed by a lab test. The first meter, which leans toward the low side, might give you a value of 240. That’s within 20 percent of 300, so it’s technically accurate. The second meter, which leans toward the high side, might give you a value of 360. That’s also within 20 percent of 300, so it too is technically accurate. If you compared them to each other and saw the 120-mg/dl difference between the two, you might conclude, wrongly, that one is right and the other is way off the mark. But in fact, your true value is neither 240 nor 360–your true value is 300. And that’s why it’s fundamentally futile to compare meter to meter. If you’re worried about your meter’s accuracy, test it against lab values.