Assessing rapid acting insulin doses for type 1

The aim of insulin therapy for someone with type 1 diabetes is to mimic the production of insulin from the pancreas in a person without diabetes.

For someone without diabetes, the pancreas produces a steady amount of insulin over each 24 hour period, which we call basal insulin.

The pancreas also produces extra insulin when food is eaten to bring the blood glucose levels back into the ideal range. This is referred to as the bolus insulin.

Basal and bolus insulin

The diagram below represents the basal and bolus insulin profiles. This can be mimicked for someone with diabetes who is on multiple daily injections or an insulin pump.

In type 1 diabetes, the bolus insulin given at meal times and significant snacks is a rapid acting insulin. It is given to cover the carbohydrate load of the meals and snacks, and used to bring above target blood glucose levels back into the target range. Ideally, this is a rapid acting insulin. The options available in Australia are Novorapid, Humalog, Apidra and Fiasp.

Novorapid, Humalog and Apidra start working 10-20 minutes after injecting, they have their maximum effect in 1-2 hours after injecting, and their action continues to lower blood glucose for 3 to 5 hours.

They are best injected 10-20 minutes before a meal.

Fiasp is another rapid acting insulin. In fact, it is called an ultra rapid acting insulin as its onset is shorter at 5-15 minutes, its peak action is 0.5-1.5 hours and it has a similar duration time of 3-5 hours. It is recommended Fiasp be injected just before a meal or at the time eating commenced.

As stated, the bolus dose has two components:

  1. the insulin to cover the carbohydrate content of meals of snacks, which is determined by an insulin to carb ratio
  2. additional insulin to bring an above target blood glucose level back into the target range which is determined by an insulin sensitivity factor (also called a correction factor).

When assessing insulin doses for type 1 diabetes it is best to look at the basal insulin first by doing basal tests. Once the basal insulin is determined, the bolus insulin can be assessed by looking at the insulin to carb ratios at each meal, and assessing the insulin sensitivity factor separately.

It sounds complicated but your diabetes team can help you. In many cases, they observe if the meal insulin to carb ratios and insulin sensitivity factors are correct from the records you bring to clinic visits.

Assessing bolus doses

You can also assess bolus doses.

The insulin to carb ratio is assessed by looking for patterns over several days for each meal. For example, to asses a breakfast insulin to carb ratio, choose a meal where the carb content of the meal is easy to count, and don’t eat additional food between breakfast and lunch.

The blood glucose level before breakfast should be in the middle or upper target range or just above it, and no additional insulin should be given to correct the blood glucose level. The insulin to carb ratio is correct if there is a pattern over several days where the blood glucose before lunch was less than or equal to 2mmol/L higher or lower than the pre-breakfast blood glucose level.

This process can then be repeated for lunch and dinner.

The insulin sensitivity factor can be assessed by looking at how the correction dose works over time. For example, if your blood glucose level was 10mmol/L and you gave a correction dose of 1 unit of insulin, you can observe how much 1 unit of insulin tends to lower the blood glucose.

Talk with your diabetes team for specific guidance on your insulin dosing. For general enquiries call the NDSS Helpline on 1800 637 700.

By Helen d’Emden

Advanced Accredited Practising Dietitian, Credentialled Diabetes Educator

Join our community of over 33,000 people living with diabetes