A Few Days on the MM Glucose Sensor

By Mary Jean Renstrom

On Wednesday, Dec. 1, 1999 I began a 72 hour test of the new MiniMed Continuous Glucose Sensor. The sensor reads glucose in interstitial fluid through a small cannula inserted under the skin, similar to an infusion set. The sensor is connected with a small electrical cable to an electronic device that you clip to your belt or put in a pocket. The device is about the same size as an insulin pump. It was comfortable to wear, most of the time I didn't even know it was there.
Each time I tested my bg with my meter, I entered the results into the sensor as a calibration check. If the numbers had been vastly different, the sensor would have alarmed. My meter readings are the little squares on the graphs. I was also to enter an "event" any time I ate, gave myself insulin, or exercised. These events are recorded as symbols at the bottom of the daily graphs.
The data needs a bit of explanation. It would make more sense if you could see my food log, but since you can't, I'll fill in a few gaps:
12-1-99: Everything worked great for this day. The coefficient of correlation between my meter and the sensor was 96%.

12-2-99: I ate a high fat lunch at 1:00. I used a dual wave bolus with half of the insulin immediately and the other half spread over one hour. I decided not to correct for the high bg at 3:00, knowing that the insulin from my square wave had not finished peaking yet. Looks like it worked.

12-3-99: On this day, I decided to do a fasting check of my basal rates after breakfast. I had some problems with hypoglycemia. At 9:00 a.m. and 1:30 p.m., I ate 9 grams of glucose to correct the lows. The graph indicates that my bg went below 40 (the lowest level the sensor measures) several times. However, I don't think my bg was this low. The coefficient of correlation between my meter and the sensor for this day was 69%.

12-4-99: I ended the trial at 9:00 a.m. I did bolus and eat breakfast at 8:00 a.m., but forgot to enter those facts into the sensor.

Here's the summary data showing the meter readings I entered and the correlation between the meter readings and the sensor data, etc...

Data Summary

Optimal Accuracy Criteria Sensor Meter

Date # Paired
Sensor/Meter
Readings Correlation
Coefficient
[r] Mean
Absolute
Error (%) Number
of
Readings Average
(mg/dL) SD
(mg/dL) Sensor
Range
(mg/dL) Number
of
Readings Average
(mg/dL) SD
(mg/dL) Meter
Range
(mg/dL)

12-01-99 7 0.96 8 167 126 31 76-184 7 136 35 101-191

12-02-99 8 0.86 19 288 130 51 40-238 8 146 39 104-221

12-03-99 x 7 0.69 25 288 65 32 40-178 7 92 28 54-141

12-04-99 x 1 n/a 0 107 78 35 40-162 1 78 0 78-78

All Days 23 0.88 16 850 101 50 40-238 23 123 42 54-221

x: This day does not satisfy the criteria for optimal accuracy as indicated by the shaded entries in the summary table.
Please use your clinical judgement in evaluating the graph.

c: The calibration slope for this day is outside the allowable range of 2 to 10 or no paired sensor/meter data
are available. As a result, no sensor plot is provided.

Overall, it was a valuable experience. I was surprised to see the peaks and valleys between my meter readings. Most importantly, I have discovered that I have nocturnal hypoglycemia, and I sleep right through it. I'll be adjusting my basal rates to cover this.
Mary Jean Renstrom

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Data Summary
	Optimal Accuracy Criteria			Sensor				Meter
Date	# Paired Sensor/Meter Readings	Correlation Coefficient [r]	Mean Absolute Error (%)	Number of Readings	Average (mg/dL)	SD (mg/dL)	Sensor Range (mg/dL)	Number of Readings	Average (mg/dL)	SD (mg/dL)	Meter Range (mg/dL)
12-01-99	7	0.96	8	167	126	31	76-184	7	136	35	101-191
12-02-99	8	0.86	19	288	130	51	40-238	8	146	39	104-221
12-03-99 x	7	0.69	25	288	65	32	40-178	7	92	28	54-141
12-04-99 x	1	n/a	0	107	78	35	40-162	1	78	0	78-78
All Days	23	0.88	16	850	101	50	40-238	23	123	42	54-221
x: This day does not satisfy the criteria for optimal accuracy as indicated by the shaded entries in the summary table. Please use your clinical judgement in evaluating the graph.
c: The calibration slope for this day is outside the allowable range of 2 to 10 or no paired sensor/meter data are available. As a result, no sensor plot is provided.