Blood and interstitial fluid glucose profiles

I made a test to familiarize with my glucose profiles and to check how precise Medtronic’s assumptions are in my case. I made the test during weekend when I did not perform any intensive physical activities and was able to measure my blood glucose every 15 minutes in an environment with room temperature. Glucose profiles in both blood and interstitial fluid are shown on Fig. Profiles 1.

Fig. Profiles 1.    Comparison of glucose values measured by glucose meter and sensor

I calibrated the sensor at 8:46 with glucose reading 4.2 mmol/l and collected blood and interstitial glucose values for 11 hours between 9:30 (Time = 0) and 20:30 (Time = 660).

This profile comparison shows that:

I performed regression analysis considering both above mentioned facts with following results:

Glucose profile according to coefficients calculated by regression analysis is compared with glucose profiles in both blood and interstitial fluid on Fig. Profiles 2.

Fig. Profiles 2.    Results of regression analysis

The Fig. Profiles 2. shows much better match between blood glucose profile and interstitial glucose profile based on formula with additional constant than without it. The match is very good except areas of minimum and maximum glucose values. The green profile of interstitial glucose calculated from regression analysis is shifted to the left by 12 minutes to show the match with red blood glucose profile. This shift is applied by regression analysis and cannot be achieved in reality. ISIG values will always be delayed after blood glucose values, in case of this test by approximately 12 minutes as shown on Fig. Profiles 3.

Fig. Profiles 3.    Lag between blood and interstitial glucose profiles

Further analysis shows that difference between blood glucose values measured by meter and interstitial glucose values displayed by Paradigm pump depends on:

It is possible to estimate the difference between blood and interstitial fluid glucose levels related to blood glucose reading used for calibration and value of ADD constant based on equations {Principles 1}
       SG = ISIG x CAL
and {Profiles 1}
       SG = ISIG x CAL + ADD
Correction graphs are shown on Fig. Profiles 4., Fig. Profiles 5. and Fig. Profiles 6. Horizontal axis corresponds to glucose readings displayed by pump, calculated according to equation {Principles 1}, while vertical axis corresponds to glucose values calculated according to equation {Profiles 1} that corresponds much better to blood glucose values. Because correction depends on value of ADD constant and blood glucose reading used for calibration, each figure contain multiple lines, each for specific value of blood reading used for calibration (Fig. Profiles 4. and Fig. Profiles 5) or specific value of ADD constant (Fig. Profiles 6). Respective parameter values are described in the figure legend and printed next to the line.

Fig. Profiles 4.    Corrected glucose values in low glucose range based on the blood glucose reading used for calibration, value of ADD constant = - 2.5 mmol/l

Fig. Profiles 5.    Corrected glucose values in high glucose range based on the blood glucose reading used for calibration, value of ADD constant = - 2.5 mmol/l

Fig. Profiles 6.    Corrected glucose values based on the value of ADD constant, calibration with blood glucose level of 5.0 mmol/l