Updated: Jul 14
By Amy Rush, APD CDE
Over the past few editions of clinic chat, we have looked at how to test and adjust basal insulin, insulin-to-carbohydrate ratios and insulin sensitivity factors. This month we look at active insulin time and target blood glucose. Like insulin-to-carbohydrate ratios and insulin sensitivity factors, these are programmed into your pump or bolus calculator if you use these devices, and will impact the insulin bolus advice they offer.
Target blood glucose
Insulin pumps and bolus calculators will calculate the amount of insulin you need to bring blood glucose levels back to a certain target after eating or correcting for a high blood glucose level. Your ‘target blood glucose’ may be a specific number, such as 6mmol/L, or a target range, such as 5.5-6.5mmol/L.
The target set in your insulin pump or bolus calculator is important. You might have different target blood glucose levels for different times of the day. For example, during the day while you can actively monitor and manage your blood glucose levels, you may be happy to aim for 5mmol/L. For night-time corrections, however, some people feel safer aiming for a slightly higher target.
Active insulin time
Active insulin time (also called duration of insulin action) is the amount of time a bolus of rapid-acting insulin actively lowers the blood glucose level for. When delivered, a rapid-acting insulin dose starts working in around 15-20 minutes, reaches its peak around 90 minutes, and will continue to lower blood glucose for several hours. If another dose of rapid-acting insulin is taken too soon after a previous dose, there may be too much insulin circulating in the blood. This is called ‘insulin stacking’ and can cause hypoglycaemia.
How does active insulin impact the dose my bolus calculator recommends?
Giving a bolus when blood glucose levels are high (a correction bolus)
When blood glucose levels are high, the pump or bolus calculator will calculate the correction dose required to bring blood glucose down to the target blood glucose level taking active insulin into account. Any active insulin still working in the body from a previous insulin dose will be subtracted from the correction bolus. For example, if the recommended correction bolus is 2 units, but you have 0.5 units of active insulin remaining in your system from your last dose, your bolus calculator will recommend you take 1.5 units to stop you from stacking your insulin.
Giving a carbohydrate bolus
If you’re using a bolus calculator or pump calculator to bolus for a meal or snack, you need to know exactly what your system does with active insulin. Some insulin pump/bolus calculators will take active insulin into account if your blood glucose is below target, subtracting it from your bolus so you don’t end up with a stacked dose which could send you even lower. Some do not do this, and will simply deliver the amount of insulin that corresponds to the amount of carbohydrate you input into the system. If you are unsure of how your pump or bolus calculator deals with active insulin under certain conditions, consult the device manual, contact the pump manufacturer or check in with your diabetes educator.
Calculating active insulin time
Your active insulin time (AIT) is important, and knowing it can help you optimize the accuracy of your bolus calculation devices. AIT starts when you give your rapid-acting-insulin bolus and ends when it is no longer lowering your blood glucose levels. AIT varies considerably between individuals, however most people will find their insulin pump or bolus calculator AIT has been factory set or set by their health care professional anywhere between 2-6 hours. If you don’t know your AIT, try this simple test to determine how accurate the settings in your devices are for your body:
What you need
A blood glucose monitor, a continuous glucose monitor or a Freestyle Libre.
Your usual rapid-acting insulin.
When to test
The best time to test AIT is when you meet the following criteria:
Your blood glucose level is at least 3 mmol/L above your target blood glucose level (this might happen if you didn’t take quite enough insulin to cover the carbohydrates at your last meal)
You have not eaten or taken rapid-acting insulin for at least 4 hours.
You are willing to not eat for the next 3-6 hours – if you use a continuous or flash glucose monitor a good time to test might be after an early dinner, so you can see what happens through the night.
Many other factors can impact blood glucose levels and can affect the results of your AIT test. Avoid testing at the following times:
Immediately before or after exercise
If you are unwell or highly stressed
If you are female, at the start of your menstrual cycle
Before you test
Make sure you consider these safety pointers before you do your test:
If your blood glucose level drops below 4 mmol/L at any point in the test, stop the test and treat the low blood sugar as you would normally.
If blood glucose levels reach 15 mmol/L stop the test, perform a ketone test, and act in accordance with your sick day management plan if you have ketones.
How to perform an active insulin time test
Test your blood glucose level 4 hours after your last rapid-acting-insulin dose. If it is more than 3 mmol/L above your target blood glucose level, you can complete an AIT test. If you are using a continuous glucose monitor or Freestyle Libre then scan/view the reading and note it down.
Deliver your correction dose and note down the time. This is the start of the test.
Go about your usual activities, but do not eat or exercise until the end of the test.
If you are finger stick testing, check your blood glucose levels every 30 minutes. If you are using a continuous glucose monitor or Freestyle Libre you can view your blood glucose level in real time and watch the trend arrow.
When your blood glucose level stops dropping, or the trend arrow stabilizes, the insulin is no longer lowering your blood glucose. This is the end of the test and your AIT will be the duration of time from the start, to the end.
Don’t be alarmed if the result from your AIT test is different to the one programed into your pump or bolus calculator, just be sure to discuss it with your diabetes health care team. It is best to repeat the test a few times before making any changes, so take a few test results with you to your next diabetes education appointment.
Active insulin and target blood glucose are vital components of calculating insulin doses. Like carbohydrate ratios and insulin sensitivity factors, they should be reviewed regularly. If you haven't looked at yours in a while, chat to your educator or make an appointment at the Family Centre's clinic by clicking here.