Medical Disclaimer: your physician/provider will give you the instructions and dosage needed for the insulin administration. The following information is for educational purposes and is not a prescription for any specific patient. Incorrect insulin dosage can cause severe hypoglycemia with dangerous outcomes.
Understanding The Terms Related to The Insulin Administration
About 40-50% of the daily insulin dose is used during the intervals between meals and at night (fasting states.) This amount of insulin is usually constant.
This dose of insulin is used for two purposes:
- Reducing blood glucose levels that are higher than the goal/target level.
- Disposing of the new blood glucose that will become available after eating carbohydrates/food.
High Blood Sugar Correction Amount
This is the amount of bolus insulin needed to correct high blood sugar. Generally, 1 unit of rapid-acting bolus insulin is needed to reduce the blood glucose level by 50 mg/dl. This drop can range from 15-100 mg/dl depending upon the individual, their state, time of the day, etc.
Insulin to Carbohydrate Ratio
This is the amount of carbohydrates that will be reduced by 1 unit of insulin. Generally, 1 unit of rapid-acting insulin will dispose of 12-15 grams of carbohydrates.
Keep in mind that the amount of carbohydrate disposed of by 1 unit of insulin can vary. Some reasons for this variation are the following:
- Different individuals respond differently to insulin.
- Person’s health. For example fever, infections, etc.
- Time of day, for example early in the morning our bodies produce hormones that increase blood glucose levels. This is called the dawn phenomenon. Usually, our natural insulin counters this effect, however, in diabetics this correction may not occur leading to high fasting glucose levels.
- Blood glucose rebound as a result of insulin administration that causes hypoglycemia. This can occur in the morning after the administered insulin has caused hypoglycemia leading to the release of stress hormones that in turn work to release more glucose in the blood. This is called the Somogyi phenomenon.
Calculate the amount of bolus insulin needed for the carbohydrates in the meal
This is the insulin needed to dispose of the carbohydrates that you will eat in your meal.
Assumption: your insulin sensitivity index is: 15gm per unit. Warning, this index is different for different people. You will have to determine your sensitivity index by repeatedly checking your blood glucose levels before meals, administering insulin, taking meals, and then checking the blood glucose levels.
Let’s say you will eat 1 cup of white rice. It has 45 grams of carbohydrates. (Reference: If you eat a cup of cooked long grain rice, you will be eating 45 grams of carbohydrates.)
You will need 3 units (45/15) of bolus insulin (rapid-acting insulin) to dispose of this one cup of white rice.
- A naan has 80 carbohydrates (Reference). Bolus insulin units needed is 5.33 (80/15)
- A small chapati has 15 grams of carbohydrates. (Reference: A small, 6-inch chapati contains 71 calories, 3 grams of protein, 0.4 gram of fat and 15 grams of carbohydrates, including 2 grams of fiber.) Bolus insulin unit needed is 1 (15/15)
Calculate the amount of bolus insulin needed to correct the excess blood sugar
- Your pre-meal goal/target blood glucose level is 120 mg/dl
- The carbohydrate correction factor is 50 mg/dl
- (Pre-meal blood glucose) – (target pre-meal blood glucose level) = excessive blood glucose levels
- Additional bolus insulin units = (excess blood glucose levels)/(carbohydrate correction factor)
Before your meal, check your blood glucose level. Let’s say it is 270 mg/dl.
Thus, 270 mg/dl – 120 mg/dl = 150 mg/dl
This means your pre-meal blood glucose level is 150 mg/dl above the target level. We have to add bolus insulin (rapid-acting insulin) to correct this excessive blood glucose as well. The correction factor of 50 mg/dl means that we will have to administer 1 extra unit of bolus insulin for each 50 mg/dl of excess blood glucose.
(Excess glucose)/(insulin correction factor) = additional bolus insulin units needed
150/50 = 3 units of additional bolus insulin to correct for this excessive 150 mg/dl blood glucose
The total bolus insulin (rapid-acting insulin) needed is 3 units.
For this patient who is going to eat two chapatis (30 grams carbohydrates) and has pre-meal blood glucose level of 270 mg/dl the bolus insulin levels will be 2 units (for chapatis) + 3 units (for the excessive pre-meal blood glucose level) = 5 total units.
Calculating The Basal Insulin Dose
First, we need to calculate the total daily insulin dose needed by the body. Then we will take 40-50% of this amount as the basal insulin (long-acting) amount needed. The rest of the insulin will be given via the bolus (rapid-acting) dosages (as calculated above.)
- Total daily insulin requirement = (total body weight in pounds) / 4
- Total daily insulin requirement = (total body weight in kilograms) x 0.55
Let’s work out an example
Let’s say the patient’s weight is 216 pounds or 98 kilograms. Let’s calculate the total insulin required for this patient:
- Calculating with pounds: Total daily insulin required = 217 lbs / 4 = 54.25 units
- Calculating with kilograms: Total daily insulin required = 98 kg x 0.55 = 53.9 units
The total daily insulin required by this patient is 54 units. (Warning do not administer this amount. Only 40% of this amount is needed in basal insulin.)
Calculating the daily required basal (long-acting) insulin dose
Daily basal (long-acting) insulin dose required = 40-50% of the total daily insulin required.
- 40% = 54 units x 0.4 = 21.6 units of the basal (long-acting) insulin is required.
- 50% = 54 units x 0.5 = 27 units of the basal (long-acting) insulin is required.
Final Calculations for the whole day and one meal (mentioned above)
For the patient in the examples above, the required insulin dosages are as follows:
- Basal (long-acting) insulin: 21 to 27 units
- Bolus (rapid-acting) insulin: 5 units needed when he/she was planning to take 2 chapatis and had a pre-meal blood glucose level of 270 mg/dl with a goal/target level of 120 mg/dl
Reference: This UCSF article on the insulin dose calculation is excellent, though a little confusing to read.