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Clinic Chat: Nailing your basal

Lots of members of the Family Centre community are focusing on their diabetes management with new motivation during the COVID-19 crisis, and making sure basal or long-acting insulin doses are correct is a great place to start. This Clinic Chat covers basal testing, which is especially important right now if you’re less active than usual and need to revisit your long-acting insulin dose or basal settings.

Basal and long-acting insulin

Basal and long-acting insulin help the body to metabolise the continuous trickle of glucose that is released by the liver into the bloodstream around-the-clock. An accurate rate or dose should keep blood glucose levels steady overnight and between meals. Basal insulin can be delivered by injections, once or twice daily, or via an insulin pump with different basal rates programmed throughout the day.

Basal testing

You can't build a house without a strong foundation. In the same way, you cannot expect stable blood glucose levels without an accurate basal dose. Only after your basal is set correctly, should you look to your carbohydrate and correction ratios.

Basal testing means committing to a time period where you remove some of the things that have a big impact on your blood glucose levels (such as food, meal insulin, and stress) in order to zero in on two things: the glucose that is released from your liver and your basal insulin only. Basal testing requires you to fast for a period of time.


Fasting means abstaining from food. You can drink water on a fast – if you want some flavour, try a splash of lemon juice, mint leaves or a small amount of diet cordial – and black tea or coffee only if caffeine doesn’t normally impact your blood glucose levels.

Preparing for a basal test

There are a few things to consider in preparation for your basal test:

  • Remember, fasting is required for basal testing, so choose a day where this will work for your lifestyle.

  • Try doing your basal testing on a non-exercise day first. After you have some initial data, you can build exercise in and start to adjust your basal rate or dose in relation to this variable.

  • Choose a time where you can minimize stress, because stress may increase the amount of glucose released from the liver and change your results.

  • You’ll need to commit to finger-prick testing very regularly or wear a continuous or flash glucose monitor (CGM/FGM)

Basal testing safety

If you plan to fast for a period of time to test your basal, it’s important to enlist the advice and support of your diabetes educator to help you do it safely and effectively.

Immediately before you start your basal test, it’s important to test your blood glucose or check your CGM/FGM. If your blood glucose level is above 4 mmol/L and below 15 mmol/L (or your own upper cut point at which you would correct) you can safely begin. If not, treat accordingly and try again another time.

It is important to know that basal tests don’t always go to plan. Your basal test can continue unless blood glucose rises above 15 mmol/L (of your own upper cut point at which you would correct) or drops below 4 mmol/L. If either occurs, you will need to call off the test and treat with insulin or glucose as needed.

How to do a basal test

The key premise of basal testing is fasting for a specific block of time. If you want to see the full picture of how your basal insulin dose or rates are working over the course of a whole day, a 24-hour fast is the best way to see this. However, you may only be interested in what happens during a particular part of the day such as overnight, or fasting for long periods of time might be challenging (especially if you’re a hungry and growing kid!), so you may wish to focus on specific 6- or 12-hour time blocks. This may make interpreting results more challenging, as your system might not be entirely clear of meal insulin and food for much of the test, so it’s best to enlist the support of your educator in approaching shorter basal tests and interpreting the results.

Choose your time period – let’s work with a 24 hour fast as an example, starting with collecting data overnight. Have dinner at your regular time, but avoid large protein serves or a meal that is high in both fat and carbohydrates, as these factors may elevate your blood glucose level well into the night. Think about a meal that will not have a huge impact on your glucose level and that will clear quickly – a chicken salad, with lots of non-starchy vegetables, enough protein to make you feel full, and a serve of healthy fats that isn’t over the top, like a handful of nuts or some avocado. Have the rapid-acting insulin dose you would normally take for a meal like this.

If you’re on a pump, leave your basal rate as it is normally set and don’t touch it for the 24 hour period while the test is happening. If you are on injections, take your long-acting insulin at the usual time(s), whether you are on a single injection or two injections a day.

If you are on a CGM or FGM, your device will collect the data you need, so you can sleep through the night. If you are on fingerpricks, test just before you go to bed, at midnight, 3AM, when you wake up and then every hour during the day. Try and jot a few notes down about how you slept and what happened during the test day – all this is useful to figuring out if these factors were influencing your levels later on.

If you’re in-range when you wake up, great! Continue with your fast until dinner time, making sure you’re well hydrated and following any advice your diabetes educator has given you about executing your basal test. If you’re out of range (below 4mmol/L or above 15mmol/L or your own upper cut point), you’ll need to abort the test – but don’t worry, the data you collected overnight is useful, and will give you and your educator some pointers about adjusting overnight insulin straight away. After you’ve adjusted your overnight to wake up in range, you can continue with the rest of the test to see if your day dose is correct.

Your results

If your blood glucose moves up or down by 2mmol/L or more during the 24 hour test, and that move cannot be attributed to a factor other than basal insulin and liver glucose (such as a stressful event, or some unanticipated exercise), then you may need to consider adjusting the basal insulin that is onboard during that time period. Pumpers can change basal rates for different times easily, whereas people on injections might need to adjust the dose they took hours earlier to make a change later that day. If you feel confident to adjust your insulin yourself, do so conservatively and make sure you test frequently. If not, take your data to your educator and work together to find your sweet spot.

Want to know more about basal testing, or try a test yourself? Contact the Family Centre's Diabetes Educator, Amy Rush, at - she's here to help!

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