Because strange things can happen to a person on any given day, I like to consider 10–14 days of data when deciding on the insulin-to-carbohydrate ratio for a given meal. This table shows what 12 days of data might look like. Based on the information in the table, I would assign an insulin-to-carbohydrate ratio of 1 unit per 12 grams of carbohydrate for this person’s breakfast. A ratio of greater than 1:12 tends to produce a blood glucose rise; less than 1:12 tends to produce a drop. When used, 1:12 held the blood glucose fairly steady, with lunch readings within 30 mg/dl of breakfast readings. I would throw out the data on 6/3 due to the low reading prior to breakfast. I would also throw out the data on 6/8; it is inconsistent with every other result, and the meal was much larger than usual.
Fine-tuning insulin-to-carbohydrate formulas can be a challenging proposition, so the more detailed your records, the better. You might discover a variety of factors that have a subtle influence on your blood glucose levels. Look for variations by day of the week, work or school schedules, time of the month, physical or recreational activities, changes in pump infusion sets or insulin vials or cartridges, injection or infusion sites, dining in versus eating out, and even social engagements.
Betty, for instance, had high readings every Sunday at lunchtime but normal readings the rest of the week. The reason? Church, most likely. Betty is very passionate about prayer. The lack of movement in church (she sits for several hours) coupled with the adrenaline surge she gets from the service is likely producing a consistent blood glucose rise. The solution: Use her usual 1:10 breakfast formula during the week, but increase it to 1:6 on Sundays.
Dan was experiencing very inconsistent blood glucose levels prior to dinner despite having the same lunch each day and using a consistent 1:15 bolus ratio at lunchtime. In reviewing his records, he found that most of his dinnertime lows were preceded by morning workouts; most of his dinnertime highs were preceded by no workout. The solution: Use 1:10 at lunch after sedentary mornings, but decrease it to 1:20 following morning exercise.
Given the complexities of determining bolus formulas, it is worth having a second set of eyes look over your records. Don’t hesitate to ask your physician or diabetes educator to review your data and help you to form reasonable conclusions.