There are specific ways to calculate the RATIO of insulin per carbohydrate serving required on an individual basis. People with type 1 diabetes need to test this by monitoring blood glucose responses. With time they can become very familiar with their carbohydrate intake so that the process of adjusting insulin doses becomes second nature.
Steps to successful carbohydrate counting:
Identify foods containing carbohydrate
Calculate TOTAL carbohydrate content of meal/snack and convert into number of CPs
Consider factors that may influence blood glucose response (e.g. fat, glycaemic index)
Calculate insulin required
Consider pre-meal blood glucose (is a correction required?) and consider planned exercise/activity.
Give insulin bolus
Record blood glucose response
Where do you find carbohydrate?
• Cereal derived starch products: breakfast cereals, grains, bread, rice, pasta, couscous, flour based products [pastry, biscuits, cakes], thickening agents [eg. cornflour]
• Vegetable starch: potato, legumes [lentils, beans, peas]
• Fructose: fruit, fruit juice
• Lactose foods: milk, yoghurt, ice cream, custard
• Sucrose (added sugar): table sugar, chocolate, sweets and soft drinks
The CP (Carbohydrate Portion) System
A CP = 10g of carbohydrate
A CP contains approximately 10g carbohydrate. Add up the CP portions in a meal or snack using the CP list or by adding up the total grammes of carbohydrate and dividing by 10.
The insulin needed per CP is determined in conjunction with the health care professional by using the ‘50 rule’(see below) and by monitoring blood sugars responses.
On average 1 CP (10g carbohydrate) can raise blood sugars by 2-3mmol/l but the effect is individual.
Extra insulin can be given to correct high blood glucose levels above target in addition to that needed for the carbohydrate eaten. The corrective dose is worked out individually using the 100 Rule.
100 rule: Divide 100 by the Total Daily Insulin Dose
For Example: As before TDD is calculated as 26 units. 100 26 = 3.8 mmol/l
We would say that this person can expect a unit of insulin (fast acting) to reduce their blood sugars by 4mmol/l.
This is useful if the patient misjudges the amount of carbohydrate in a meal, they can correct at the next one. If frequent corrective doses are given, there may need to be a change to the insulin: CP ratio or background insulin.
Calculating the insulin dosesThe amount of insulin required for carbohydrate is described as units per CP. Typical range is ½ -3 units per CP.
This is worked out on an individual basis and takes into account insulin sensitivity. It is usual for most patients to start on a 1 unit per CP ratio and then adjust according to blood sugar responses. Another way of determining the ratio or for circumstances where it seems that a different ratio is needed is the '50 Rule'.
50 Rule: Divide Average Total Daily Insulin Dosage (meal & background insulin) by 50 to give units of insulin per CP
Example: If a persons total daily insulin dose = 26 units
26 / 50 = 0.52 say 0.5 or ½ (round down insulin doses to begin with). So this person could try a ½ unit per CP
To test the bolus ratio it is useful to check the blood glucose responses before the next meal. When testing responses, it is important that the carbohydrate is calculated correctly and that the patient hasn’t exercised or experienced a hypo on that day.
For a more comprehensive guide see attachment