My New Friend Dexcom

No skiing today, let it snow.

Interstitial fluid.

Always looking behind can be dangerous.

Jamba Juice Test: Dexcom vs Ultra (click for larger graph).

4/16/06: Altitude, Interstitial, and Jamba Juice

Unfortunately, my skiing plans didn't work out thanks to a snowstorm that shut down most of Lake Tahoe's resorts. Even though I don't have any skiing stories to share, I have noticed some interesting things with Dexcom over the past few days. Just before leaving for the mountains, I inserted, initialized, and calibrated a new sensor. Throughout the four hour drive to Tahoe I had great readings. I went to bed a little high, about 200, and gave a correction bolus.

Shortly after falling asleep, I was woken up by Dexcom's "HIGH" alert which meant I was over 200. It had only been 15 minutes since I gave the correction bolus, so I ignored the alert and went back to sleep. Dexcom continued to alert, waking me up every 45 to 60 minutes, for the rest of the night. Finally, I woke up at 6:45am to another "HIGH" alert and Dexcom was also asking for a calibration. A finger stick with the Ultra read 127, which is normal. A few minutes after calibrating the new test, Dexcom alerted, once again, this time asking to remove the sensor. Frustrated at my lack of sleep, the inaccurate results, and the waste of a new sensor, I looked at the nine hour trend graph to look for an explanation.

The graph looked like a random array of dots ranging from 120 to 250. I must have had a "noisy" site, but it didn't make sense that I had great readings up until bedtime. I wonder if the "noise" was caused by my body adjusting to Lake Tahoe's 6,500 ft elevation? Changes in altitude strangely affect my BG's and I haven't been able to figure out a pattern. At altitude, sometimes I go high, while other times I'm extremely sensitive to insulin. I initialized a new sensor and received good readings for the rest of the weekend.

Over the weekend, my grandma called to share that she really enjoyed reading my blog about Dexcom. When I was first diagnosed with diabetes, 16 years ago, Grandma attended the 3-day "diabetes boot camp" with me and my family. She was surprised how much diabetes technology had improved since my diagnosis. Grandma also asked me for a definition of interstitial fluid. I was a little surprised by her question, but even more surprised at my lack of a good answer.

After some thorough research, I found that interstitial fluid is a water solvent containing amino acids, fatty acids, enzymes, hormones, neurotransmitters, salts, cellular waste products, and glucose. Interstitial fluid is found throughout the body, bathes the cells of the body's tissues, delivers materials to cells, and removes metabolic waste. Studies have shown that the glucose in interstitial fluid can be used to estimate the glucose in blood plasma. The Dexcom sensor measures interstitial glucose every five minutes through a tiny platinum electrode (about the size of a human hair) under the skin.

Even though there is a strong relationship between interstitial and blood glucose values, these values are not the same, which is why you shouldn't dose insulin or eat carbs without doing a finger stick. When I first put on Dexcom, the rep said I would probably see a ten minute lag between sensor and finger stick BG readings. I have occasionally noticed discrepancies between readings, usually during or after intense exercise, when my BG values are changing rapidly. These experiences have reinforced the importance of not dosing insulin solely based on Dexcom readings.

Due to the ten minute delay, dosing on sensor readings would be just like riding my bike while looking behind, over my shoulder (see picture). Always looking behind gives a lot of information about the past, but gives no information about the present. Without current information, I could run into unexpected obstacles or dangers, especially when conditions are changing quickly. Whether riding bikes or managing diabetes, I need to know where I currently am, in order to make good decisions for the future.

Without consistent data, I couldn't really determine my time delay between sensor and finger stick readings. So, I thought I would put my Dexcom to another test… the Jamba Juice Test. I went to a local Jamba Juice and ordered a regular sized "Mighty Cherry Charger" smoothie which had 103gm of carbs. I recorded Dexcom and finger stick BG readings every ten minutes over three hours. Before taking my first sip I was a little low, so I gave insulin (5.2 units) after my BG started to rise (about 15 minutes later). I expected to see a 5 to 10 minute delay between readings.

Eighteen or so finger sticks later (leaving painful, swollen fingertips), the graphed comparison data (left) surprisingly revealed that the Dexcom and finger stick readings were almost identical! During this test, there really wasn't a time delay between readings. Even more interesting and amazing, was that the test was taken during my sixth day of wearing the same sensor (I figured out how to extend the sensor life and Dexcom has applied to the FDA for seven day sensor use). Very accurate results on the sixth day of sensor use increased the level of trust between me and Dexcom significantly.

I'm looking forward to more comparison tests, especially with exercise, but I need to wait and give my fingertips a bit of a break.


I do not work for nor am I compensated by Dexcom in any way. I'm writing this story because I think continuous glucose technology is interesting and exciting. Also, I am not a health care professional and do not give medical advice. I will share my experiences, but please check with your health care team before making any changes to your diabetes or health management.