Wildly out-of-line Sensor readings are thrown away, showing “???” instead (exactly as Helmut describes). But also, readings which thought to be “untrustable” due to calibration curve problems also provoke periods of “???”, and lengthy periods of “???” usually occur from this problem. (The following is based an actual experience-- although not an employee, I’ve SEEN some raw data data across such periods, via support, with one of my “old seven” Sensors, and my suggestions may have contributed to some of the improvements which have led to less frequent “???” in the newer model.)
There’s also the unfortunate possibility of just-plain-bad Sensors, it seems that a whole bunch have been escaping from Q/A and causing problems for paying customers in the wild over the last few months :((
The old “seven”, before the “plus”, was much worse, it spewed out extended periods of “???” much more frequently. It actually wasn’t the raw readings, it was the calibration curve going bad over time. The “Plus” doesn’t do this so much, but if you haven’t given it some quality, well-dispersed data points work with, even the “Plus” is prone to frustrating episodes of “???”.
If the Sensor isn’t failing from old age, or from sliding around do to loosening tape, then there’s two ways to solve the problem . The recovery method differs, depending on whether the trouble is occuring on a genuinely “fresh” Sensor, or is developing with one which is already at least two days old.
On day one and day two: If the Sensor isn’t “blood poisoned” or Defective, you can probably solve the problem by simply entering more more calibration finger poke readings, to create a better curve. I think that start-up NEEDS more finger-pokes than Dexcom actually asks for, AND they need to have a good “dispersion”. Just one poke, done many hours after the first pair, at nearly the same bG reading, is durn near useless–your graph CAN’T be good at other readings, because Dexcom doesn’t have any data to work from. Being a bit conservative, it’s programmed to show lots of “???” instead of really RISKY guesses. In this case, it just needs more calibration data from which it can work.
Try to give it good values, at least 30 mg/dL apart from each other. (An optimal set would include a reading in the middle of your range AND one near your “high alarm setting” AND one around 80 mg/dL. Don’t enter a calibration at bG below 75-80 mg/dL, the raw data voltage at these lower glucose levels becomes too small, too hard to distinguish from background noise. And NEVER enter calibration readings during rising bG, see below.
Later days: Do a full restart, so that the calibration isn’t being “distorted” by a bunch of old, stale readings. (That’s the cause of “recoverable” ??? problems.) Try this full restart only once, then give up. If the Sensor didn’t come right up showing numbers, then it is worn out, get rid of it. After it starts, it is allowed to go “???” a couple of times, before you enter your “dispersed” calibration entries. But after it’s had quality finger-poke data entered on both sides of the range which it should be showing, another “???” should invoke the trash can.
Never, ever enter calibration readings during rising bG-- that actually DAMAGES your calibration curve, I think. You need to do your entries during level or slightly dropping bG periods. (“Slightly dropping” means horizontal arrow, but the graph shows a downward rate of less than 1 mg/dL per minute). The two initial finger pokes in a start-up really only provided ONE point on the curve, Dexcom only asked for two as double-check on your technique and strips. (Dex is basically gonna use the average, unless bG is rising or falling rapidly at the end of your two-hour warmup.) IMO, you always need more data points (nicely dispersed) to set the “curve” properly.
So, do not wait for the twelve-hour “enter calibration now” timer to expire! Create a different, valid bG reading sooner, and enter it. If I’m a bit “high” initial startup entries, I treat them and wait for insulin to pull bG down by 20-30 mg/dL (for me, about 3 hours) and enter that data right away. If I started out “good”, then I’ll push my bG up via glucose tabs on purpose, up into the 120-140 mg/dL range. (Glucose tabs are perfect for this because they do their job in 20 minutes and then they’re GONE, leaving flat bG. Do NOT eat normal food and enter a calibration while bG is still floating upwards, and expect that reading to “help”. If you bG was continuing to float upwards after the finger-poke, it won’t help at all.)
If you’ve got an old-style “Seven”, do yourself a favor: Order the upgrade.