Most blood glucose meters store a certain number of readings in their memory, along with the date and time of each reading. Most also report either a 14-day or 30-day average of readings. An average is calculated by adding up all the numbers in a set and dividing the sum by the number of numbers in the set. For example, if you checked your blood glucose level 25 times in 14 days, your meter would tally all of your readings and divide the sum by 25 to get your 14-day average.
Blood glucose averages can be useful, but they can also be misleading. Compare the before-dinner blood glucose readings of two different people over four days:
Rhoda’s blood glucose before dinner:
Monday 158 mg/dl
Tuesday 178 mg/dl
Wednesday 174 mg/dl
Thursday 161 mg/dl
Raul’s blood glucose before dinner:
Monday 82 mg/dl
Tuesday 302 mg/dl
Wednesday 200 mg/dl
Thursday 87 mg/dl
Both Rhoda’s and Raul’s four-day average is 168 mg/dl, but there is a big difference in the patterns. Raul’s average gives no indication of the wide fluctuations in his blood glucose readings. By relying on the average, he’d miss the opportunity to correct the highs and lows.
Luckily Raul keeps a logbook, which reveals that he exercises before dinner on Monday and Thursday, so he’ll probably require a change in his medicine dose on exercise days. On Tuesday he had a very stressful business meeting over lunch in a restaurant. That explains the reading of 302 mg/dl.
Many people don’t check their blood glucose level if they suspect that the result will be high, so the average doesn’t tell them anything meaningful. Others only check before meals, then say, “How could my HbA1c be 8.8% when my average blood glucose is 148 mg/dl?” Your HbA1c (an indication of your average blood glucose over the previous 2–3 months) could be elevated because your after-meal readings are out of range. The average only reflects the blood glucose checks you did before meals.
An average can be useful if it helps to give you the big picture. A change in your average from 130 mg/dl in the summer to 170 mg/dl in the winter might reflect a decrease in exercise during the winter months. An average that slowly climbs or falls over time can alert you to look for changes in your blood glucose patterns.