Dex keeps reading REALLY low when I am not that low?

I am on a dexcom trial… all 3 days now though, when I am in the 80’s the thing keeps telling me I am in the 50’s… I’ve even triggered the <55 alarm quite a few times when a fingerstick shows I’m in the mid 80’s… and I wasn’t dropping, so I’m fairly certain I wasn’t low and missed it. Several times now, my #'s have been steady on fingersticks (in the 80’s and 90’s) but keep fluctuating up and down on the dex, repeatedly triggering the low alarms when I am definitely NOT that low. I’ve calibrated a few times when it has said I am this low (and my BG has been stable for hours… no HUGE changes) and it still keeps reading really low.

Am I doing something wrong? Could this be partially because it’s a borrowed dexcom? I was kind of assuming that may be the case, and it’s basing the readings on past calibrations from other people, but that’s really just speculation on my part… and it’s speculation based on some “high” readings the first day when I was only in the low 200’s (and steady, definitely not spiking to 400 and back down, because I didn’t consume nearly enough carbs to get that high).

Disabling the low alarm doesn’t disable the <55 alarm, and if it’s going off when I’m a good 30+ points higher that would really discourage me from wanting to wear it full time. I am not going to correct for a fingerstick in the 80’s or 90’s when I know I am not dropping, KWIM? I just don’t get why dex keeps suggesting I should be much lower.

I certainly don’t FEEL like I’m low when the alarms go off either… which makes sense, because a fingerstick shows that I’m not low, and repeated fingersticks every 10-15 minutes don’t show that my BG is changing much at all.

On the DEXCom your lows will read lower and your HI’s will read higher than blood readings. My understanding is that when your sugar is low the body draws it into the blood from the tissue that the the dexcom is in therfore making the dexcom reading lower. And it does the reverse thing when your sugar is high. Your body sort of spills it out of the blood some how and more of it ends up in the tissues and then your dexcom will read higher. It was tricky. It is best used to see the trends.


Interesting… the lower readings alone aren’t the problem, it’s the fixed hypo-alert that can’t be changed. I keep tripping it when I’m not that low, and it’s getting annoying!

Calibration is much trickier than it should be. I got very frustrated with the dex not ‘taking’ my bg input. These days I only calibrate when prompted by the dex. Calibrating less works out way better for me. I go through great effort to achieve a good calibration because the readings can only be accurate as the calibration. I delay the calibration until my bg is in the range that I am interested in: 80-120. I also wait until my bg is stable or trending down by 1 point or 2 between readings. Then I wash my fingers and do whatever else I know about getting a good bg reading. If the bg reading is within 10 points of the dex then I use that number. If the bg reading is off by more then I do another bg test. I enter the reading that is closest to the dex readout.

The dex behavior that you describe happened to me only once, with my sensor #11. Not all sensors are created equal. Don’t give up. The dex changed my life for the better. I cannot imagine giving up my dex.

#1) …because it’s borrowed?
No. Dexcom doesn’t totally ignore older calibration entry values, from previous Sensors, but it gives MUCH more weight to new ones. After a few days of new entries, the old data doesn’t do squat in the Seven+. (If you have borrowed an OLD, NON-PLUS Seven, do everyone a favor: give it a cleanup in the dishwasher, so it can’t frustrate anyone else. The Dexcom Receiver is not water tight, and will drown.)

#2) … various issues with the <55 alarm…
Although they drive you crazy, these are side effects of the real problem. Don’t let them distract you from…

#3) I’ve had several periods of time in which my fingersticks are nearly “flat-line” at about 90 mg/dL, but Dexcom readings fly around like crazy and set off alarms
This is the problem, and here’s the order of steps I’d use in trying to solve it.

First, on the borrowed Dexcom: During June and July, Dexcom shipped many 7-plus Sensors which passed the internal QA testing procedures but FAILED to perform properly in the hands of customers. I didn’t have any bad Sensors and never called them to complain. But, in mid-August, they cold-called me ON THEIR OWN to make sure that I hadn’t had any Sensor problems.

But you’re not on their customer list. You got no such phone call. If the Sensors aren’t from a brand new order, you might be using bad ones.

Second: Site selection. Many people have bad results with the FDA-approved “frontal abdomen” location described in the manual, but because Dexcom didn’t submit tests to obtain approval for alternate sites, they can’t describe them. (If YOU bring up the subject, then they can talk about it, but they can’t volunteer such information. It’s the law.) For almost everyone, the best site is an “Off-Label” usage. Most men do better with love handles (straight down from our armpits, NOT in our frontal abs). Most women get better results with upper butt cheeks: A couple inches above contact with the chair when you sit down, but below the bony pelvic girdle. Thisis also the most successful site with kids. Try there, unless that’s where you always put your infusion sets…

Third: Stay far, far away from current AND older infusion sites. Dexcom says that 3" away is adequate for even a current site, and one inch away from old pump sites, but I need far greater distances.

What a coincidence! Today at 5pm the reading on my dex dropped from 80 to LOW in 15 minutes. It stayed at LOW for 2 hours. Then it crept up. I was 80 when it showed LOW. Just now at 9:40pm the dex showed 86. I measured my bg twice: 154 and 156. When I entered the data into the dex the readout went up to 110 on the first entry and to 150 on the second entry. My confidence in this sensor is close to zero. This is my first 1/2010 sensor. I read a lot of bad things about this batch on CWD. I am prepared for a bad ending. I hope dexcom gets a handle on these problems.

I woke up to ??? and found out that there were no readouts for the entire night. The ??? did not go away after another 30 minutes. I pulled the sensor. Need to call in for a replacement. This sensor was good for less than 2 days.

I just received my Dex in the mail on Friday so I’ve only had my sensor on and reading glucose since about 6:30pm on Friday BUT my sensor is also a 1/2010 sensor and has yet to be more than 15 points away from my glucose monitor. Maybe I just got lucky but not all of these sensors are problematic.

Not exactly, but you’re close:

Dexcom’s calibration formulas try to match STEADY-STATE bG readings as closely as they can. (Of course they do-- why on earth would the Dex designers intentionally make it’s read-outs too high at high bG readings?) But you’re exactly right about these two factors during periods of bG change:

All sources of Blood glucose (liver, intestines, stomach, IV, and mouth lining) hit the bloodstream first, and “soak” into the ISF which Dexcom actually measures later. But nearly all of the relevant glucose consumption consists of cells drawing glucose from ISF, a local insulin-mediated transfer. It isn’t seen in the bloodstream until capillary walls begin to back-fill the unbalanced insulin densities.

Quick aside: there's a few major glucose consumers which DO draw insulin directly from the blood, and two of these are the biggest energy consumers in the body: Liver and Brain. But we don't stick Sensors into our brains, livers, kidneys, or spinal cords, so we'll ignore them.

But you left out another extremely important factor: the delay across the Dexcom membrane, refreshing enough of the ISF inside the Sensor wire (and making contact with Glucose Oxidase to create the readings! In rising bG, this delay adds to the delay from Blood to ISF. But in falling bG, this delay counter-acts the “ISF falls faster than bG” effect, often causing Dex to track bG very closely.

YMMV! But for me, rising bG shows about 20 minutes late on Dexcom while moderately falling bG matches tracks bG just as well as flat-line readings! (“moderately falling” taken to mean less than 2 mg/dL, neither kind of downward arrow being shown.) And the 2-3 mg/dL downward rate is pretty close too, only about 5-10 minutes behind bG.

I’m not sure if Dexcom is having problems with their receivers; I know they are having problems with their sensors. I also don’t know if there are tricks for calibration that we should be following (Dex Rep. told us to calibrate in the 80s, and also to try to get a 200ish calibration as this will help Dex figure out her blood sugar ranges). First, there is a 20 percent degree of error with the Dexcom; in addition blood glucometers can have a 30 percent degree of error, so you can see why, at times, Dexcom may be “off.” If the glucometer is off 30 percent and Dex is also off 20 percent at the same time… The above has happened to us quite frequently. What I do is temporarily change the low alarm to 100 so Dex will stop beeping all the time. I also calibrate every time the reading is too far off. Dex tech support told me to do this each and every time I feel there is too much variation. Do this for a week. Then download and support will look at info and trouble-shoot for you.

Reading in the 50’s while 80/90’s is pretty much in range…I mean, Dex is good but no CGM is going to be 100% all of the time.

One thing that I learned from a Dex rep when I was having issues once - when readings are off you can try calibrating, wait 15mins and calibrate again, do this 3 times or so - watching to see if it starts to read better.

If this doesnt help call Dex - make sure you tell them you did this bc they will ask you to do it - they will generally send you a replacement sensor BUT like I said above, 30-40points isnt bad as far as the range goes, super annoying with the alarms I know AND I know Dex generally does better, at least for me.

Yesterday I started a new sensor because the previous sensor produced no reading for the entire night and showed ??? in the morning. I calibrated the new sensor when my bg was going up. BIG MISTAKE. After a couple of hours the readout was LOW and stayed LOW for a couple of hours. I was 80. Looking back I should have calibrated then. I stuck it out and calibrated when the dex asked me too. At that time the dex showed 80 and I was 150. I thought to myself: “Another bad sensor”. Today this sensor worked like a charm. I tested 4 times during the day and it was never off by more than 5 points. I only test between calibrations when I have reason to distrust the dex. Looks like my body and the sensor made friends. I am crossing my fingers that I will get 2 weeks out of this one.

I’ve had some pretty good #s, within 5-10 points of my meter…but then I’ve had some 50-80 points off also. They generally tend to run low with Dex. The moving trends always seem good though. Knowing I’m climbing quickly, or more importantly, dropping quickly is more important than the actual # to me. I dosed once using info from Dex without confirmatilon from my Ping and ended up low. I’ll not do that again. “Trust but verify” is my new motto with Dex.

It’s the older Seven, not a Seven plus. The sensor had an expiration of 1/2010 on it, so I don’t think it was from the “bad” batch.

When my BG is “normal”, like around 85 or so, and steady, Dex constantly reads in the 60’s (or lower). It seems to consistently read about 20-40pts lower than I really am… whether I am 160, or 80, it reads lower. It has had moments though, where it’s exceptionally accurate, almost surprisingly so… it tends to read more correctly with highs now than it was for the first few days (the first day I had it, I was “high” for a while according to dex, which would be >400, but I was never anywhere close to that high).

I have gotten LOTS of ???, mostly overnight, which I think may be from moving around or sleeping on the sensor, but any time my BG is changing rapidly, Dex seems to just give up.

The sensor is the back of my arm… I haven’t infused insulin there in I don’t know how long :slight_smile:

I know it’s not totally “out of range”, it’s just annoying because I keep tripping the hypo alarm when I’m not low, and not going low. Otherwise I really wouldn’t be bothered by it being off :slight_smile:

whoops with my “aside”: I meant to say that liver and brain directly draw GLUCOSE from blood.

Hey Sarah, I’m “all over the computer” today, and can respond quickly to any new posts this afternoon/early evening (I don’t know your timezone). First thing: is this (at least) a second Sensor? Never keep hoping for a bad one to “come around” after it’s had an entire day to warm up-- rip it and throw it away, it WON’T get better.

As Helmut says, shipped-in-August Sensors with “Expiration 1/2010” are showing a high rate of almost-immediately failures. I’ll SWAG that to be a strong possibility.

Some people have good luck with “back-of-the-arm” triceps Sensors, but I’ll SWAG that a younger woman might be better off on the butt cheek. (Your photo suggests an age much younger than the pre-menopausal 40s/50s when this build-up begins to occur.)

And finally, per above: Drop the old “Seven” and it’s transmitter (black in color) into a pot of boiling water. Leave it there, gently simmering, until the bubbles stop ;). Unlike the other companies, Dexcom offers a genuine 30 day, money-back trial period, and I must again recommend, strongly, that you conduct your “trial” with the FAR-BETTER current version of the product.

My own results with the original “Seven” were so poor, that following my “upgrade”, that I continued using the old original 3-day model until I couldn’t buy any more of it’s Sensors. That was nearly a year year of leaving the “Seven” on the shelf, unused, because for me, it SUCKED. Based on my experience, you should avoid the original Seven.

Reading in the 50's while 80/90's is pretty much in range....
No. Dexcom accuracy starts REALLY going to heck under 60 mg/dL (e.g., might say "58" when you're only "48", might blow a "below 55!" alarm when you're really at 66 mg/dL) -- but it should not EVER be sounding "below 55!" alarms when you're in the 90s. (Well, OK, the mathematical possibility of such reports from a properly working Sensor *is* well above 0%, but it's not at all frequent. Under proper conditions, with a properly operating Dexcom, I'll SWAG each "below 55!" to have less than a 5% chance of being sung-out so badly off the mark, when "true" bG is actually above 90 mg/dL.)

There’s a number of posts in this Thread about calibration, and I have minor disagreements. For your consideration, here’s my own opinions/rules. (There’s six of them; as Helmut says there’s LOTS of considerations in doing a really good job.) Remember, I am not a licensed medical professional of any kind, and these are entirely unqualified personal opinions from a LAY PERSON: You use them entirely at your own risk, unless explicitly approved by a member of your QUALIFIED Medical Treatment, or by Dexcom. Dexcom in fact disagrees with me on some of my opinions!

(1) Never, ever calibrate during a period when bG is RISING, even a little bit. Dexcom readings are much more “late” on rising bG than falling bG. (Calibration during a time of “moderately” falling bG, a fall rate of less than 1 mg/dL per minute, is OK.)
detail: This is because the two delays of “First, higher bG soaks into ISF”; and “Then, glucose crosses the Dexcom membrane in sufficient quantity to show the change within it’s glucose oxidase” are additive, making Dexcom readings both “late” AND “later”. But in falling bG, insulin drives glucose across cellular membranes FIRST, and mostly from ISF. Then the drop in bG, “backfilling” from blood into into ISF, begins happening after ISF glucose had already fallen to a lower level. In this case, the delay across the Dexcom Sensor’s surface and the the delay between ISF and bG glucose help the Dexcom to look more like bG, with less “delay”. (The two delays are counter-balancing each other.) The “success” of this counter-balancing effect varies with each person, the rate of bG drop, and the “freshness” of ISF concentrations surrounding your site. (If you’re exercising, with a high heart rate, ISF is getting “refreshed” to match blood more quickly.) For me, in normal daytime activity, they’re almost exactly a tie-- moderately falling bG matches has the Dexcom readings matching my One-Touch fingerstick with no delay at all, with nearly the same variance as steady-state readings. Lucky me! But YMMV, And even I have “late” Dexcom readings on falls when I’m sleeping or otherwise immobile; my ISF becomes more “stale”.

(Whether ISF is a better indication than bG for monitoring cellular glucose starvation is an interesting question; I think it that it IS. So, in falling bG, it isn’t that Dexcom is “right” in almost matching the fingerstick value-- at the cellar level, you’re hurting. The almost-matching bG and Dexcom values are BOTH late in reporting the problem.

(2) Never, ever calibrate below 75-80 mg/dL.Just like other glucose-oxidase based measurement devices (including nearly all fingerstick strips and meters), Dexcom just doesn’t have a high enough raw data value to work from, accuracy becomes much worse. It’s much smarter to give it calibration values dispersed over the range 80-120 mg/dL and let it estimate lower values methematically-- entering “bad” data really screws it up, and even though the fingerstick is done carefully, Dexcom’s internal loss of accuracy makes the data "BAD!"for calibration purposes.

Think for a moment about the fingerstick accuracy standard "20%, or 20 mg/dL for bG readings below 100 mg/dL": At 100 mg/dL, that's a standard deviation of 20%, but at 50 mg/dL, that's a standard deviation of 40%. At 20 mg/dL, the standard deviation is as wide as the value of the reading! CGMS devices have the same challenge, but with the extra challenges of "it's not really capillary blood; and "ISF becomes more stale or fresh for many reasons; and most of all, "The Sensor isn't fresh out of the bottle, it's already polluted by previous sampling!" Because of these challenges, CMGS accuracy at low-normal and Hypo readings falls of much more badly than fingersticks. (3) Be sure to enter calibrations across your full range of "typical values". For optimal results, IMO, you want to have your recent calibration set include a reading from "normal/slighty high' bG (100-110 mg/dL); a "slightly low" reading (just above 80 mg/dL, a bit low but NOT breaking my rule #3); and a "high reading", right around the value of your High Alarm setpoint.
Only read this if you're interested in all the nasty mathematical and chemical issues behind my recommendation; if you don't LIKE math, it could hurt your eyeballs. ;) Think of CGMS calibration curves as a straight-line graph. (In fact, Minimed's microcode clearly is a straight-line graph, but because Dexcom treats their "ISIG-like" raw data value as proprietary data, I don't know. I have seen enough Dexcom raw data to know that the behavior is vastly different than Minimed's, with much higher "background" levels and, overall, a much smaller "slope" for a given bG change. It might truly be a curve, and not just a straight line; I've don't have enough raw data numbers to say. And even if I DID have an adequate data set to amke this determination, I would respect their choice of keeping the raw data private.) Now back to the gory details: Obviously, if you enter your "startup" values, for example, "105" and "103" mg/dL, your CGMS has good calibration data for readings right around that value--

But the “slope” of the curve? It’s got absolutely nothing to work from, it’s an almost total guess! In order to define a line, as Minimed would, or ANY kind of “curve” at all, you need to enter at least one good calibration value at a significantly different bG.

When my initial entries for a “fresh startup” are that “good” (grrrrr, hardly ever happens), I actually eat two sugar tabs to FORCE my bG up to about 120-130, in order to add a “high bG” calibration value right away. (Glucose tabs because they work fast and consistently, with no unpredictable “tail” of rising bG after about 30 minutes.) Remember rule #1 first, and be absolutely sure that the rise is Finished before entering a value!) Then I’d correct aggressively, enough insulin to push my bG down to 70 mg/dL at +3 hours, right after making that entry.

(My motto is “insulin works slow, backfilling works fast”. While wearing a CGMS Sensor you trust, it’s smart to correct aggressively-- if you absolutely, positively won’t fail to keep your eye on your readings. Just temporarily raise the adjustable low alarm setpoint, so that you catch the intended fall with plenty of time for “backfill” to reach your bloodstream well before your ISF and actually reach Hypo levels. (In this case, I’d set the low alarm to it’s max value, 100 mg/dL.) Obviously, having targeted 70 mg/dL at only +3 hours, and “risking” a sub-50 reading before the correction were to finish, l would be eating considerable “backfill” UNLESS the following hours ended up with VERY unexpected behavior.

Don’t do these aggressive corrections unless you are totally, absolutely certain to catch the drop at a realtively high bG reading, with plenty of time left to digest “backfill” before going hypo! If my initial calibration had been on the high side, I’d correct and wait for the drop to occur AND level off to less than 1 mg/dL, with no “backfill” food on board, before doing an entry. Typically, 4-5 hours later.

For me, a “perfect calibration set” has a recent number from the 80-85 mg/dL range, a number around 110-115, and a number around 140. (which is actually well above my “high alarm” setpoint of 120 mg/dL, but my Dexcom is set to blow a lot of High Alarms. 140 readings aren’t at all ucommon for me.)For Jan’s child, a “perfect calibration set” will need a number or two well above that, at least 180-200 mg/dL. And MAYBE even a bit more than that, although extreme numbers (250+) should not IMO ever be used: such extreme numbers distort the curve a lot, and in such a bad Hyperglycemic situation, you should be using a sequence of fingerstick tests anyway.

If you think about it, this already implied my next Rule. But for clarity, I’ll state it anyway:

(4) It’s not the RAW NUMBER of calibrations per day; it’s the quality of the “recent calibration set”.
IMO, Anyone who says things like “You should never calibrate, except when asked for”, or “Dexcom gets confused by too many calibrations”, before having been told (or seen from your data upload, or graph) what the recent calibration values actually were, is making a recommendation without adequate background information. (And therefore, very likely to be making a BAD recommendation.)

There is only ONE way in which extra calibrations become a bad thing-- that’s when new, redundant readings push the “weight” of other readings down via being “old” – and those deeply-discounted bG readings were at non-redundant bG levels, necessary to set a good curve.

I recommend that you want to have at least two “high quality” calibrations during the preceding 12 hours, separated by at least 30 mg/dL from each other. (“High Quality” requires proper handwashing and fingerstick technique, PLUS not taken within 15 minutes rising from bed, PLUS not taken during rising bG.) That’s not merely twice the rate of Dexcom’s default schedule, it adds the “significantly different” requirement necessary to set a proper “curve”, rather than merely provide redundant information about a pretty-much identical bG- you might need even more than 2 fingerpokes per twelve hours, in order to get a non-redundant reading on board.

This leads directly to my rule 5 (like Rule #4, an obvious corollary from the preceding Rule):
(4) Start-Up days should usually be given have EVEN MORE pokes.
Since start-up bG readings are taken within just a few minutes of each other, they’re gonna be at pretty much the same reading: The real reason why Dexcom asks for two, I think, is to insist on making you give yourself a sort of “double-check” on your technique and strips; if they’re AREN’T close, something’s wrong. I feel that it’s always wrong to leave Dexcom running for twelve hours without getting that third, different data point into the initial calibration calculation.

But in the case of a true FIRST TIME start-up, (which is probably a majority of start-ups for everyone here except me), you also flying blind due to the actual break-in behavior of a newly-injected Sensor. First day accuracy is always bad, and needs even more calibrations.

Why? Several reasons. First, for almost everyone, there is at least a small reaction to the chemical presence of the Sensor; after the initial response, this "stabilizes" over many hours. Second, for absolutely everyone, unless you're dead ;) , there is a reaction to the physical damage of the violent "punch-in" of the wire. This also calms down" over time. Your body cells are probably also reacting to ultra-tiny manufacturing variances (scratches) on the wire surface itself. (Please do understand that I'm talking about scratches which are MUCH smaller than the intended holes for ISF contact; most of these are within the EDGES of those same holes. Rather quickly, in my theory, your body clogs up these scratches in a strong reaction which then subsides, leaving holes at sizes which behave more consistently after the counterattack is finished. Finally, the internal Glucose Oxidase reagent has a "break-in" period too, consisting of both "initial fill" and surface property issues-- this "break-in" is NOT completely finished in only two hours.
My last Rule might surprise many of you, until I explain it (in terms of the previous Rules): (6) Never, EVER plug in a One-Touch-Ultra "calibration cable". Why? it's obvious, actually: Your meter is full of readings taken during rising bG; and readings taken during too-fast falling bG; and maybe an occasional "that was weird, I'll wash my hand and check it again... yep, I sure messed up on that first try!" Dexcom has NO IDEA which readings are "good" for calibration, and which readings are "bad" for calibration. And remember-- every redundant calibration, taken within about 24 hours of it's almost identical predecessor, helps to push down the priority of every non-redundant predecessor. If there's a whole lot of these nearly identical bG readings within the last day, good for you (YAY, you're a flat-liner! Unlike an ER pt., that's a good thing when you're wearing Dexcom.)

Having too many recent and redundant calibration entires in the receiver will cause even a BRAND-NEW entry with better variance to be discounted; there’s simply too many, everything gets discounted. When you control bG data entry, as I recommend, you simply skip entering 3rth 4th and etc. “nearly-redundant” values which you’ve obtained via fingerstick in the last 12 hours. If you plug in the meter, though, you’re in a world of hurt, calibration-wise, for values far from the “redundant” fingerstick bGs.

Throw those cables away, I say. Even if you’re insurance provides OTU strips and meters, throw out the "One Touch Ultra (or Ultra-II) Calibration Cables. Always enter calibration values from the menu.

Thanks Rick, great post. I definitely did not follow Rule #4. Rule #4 makes a lot of sense if DexCom adjusts for the slope.

durn: I exceeded my “15 minutes to edit” period, I was going to strip out ALL of that nested block-quote stuff, putting my asides into italics like I did in rule #1.

Sorry for resulting difficulty in reading. This has turned out to be one of my most “Major” posts of “secret Dexcom tricks” ever. As Helmut has already suggested, I might find a bit of time to try to duplicate it’s content into an easier-to-find, easier-to-read (and maybe easier-to-download) Internet Location sometime soon.