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How Illness Can Affect Your Blood Glucose Levels
When you are unwell with an infection or virus, your blood glucose levels can rise. This happens because your body becomes less responsive to the insulin you take, even if you are eating less than usual. It is important to check your blood glucose more often when unwell and to continue taking your insulin. You may need to adjust your insulin doses during illness - your CF diabetes team can support you with this.
High blood glucose levels can make you feel very thirsty and cause you to pass urine more frequently, which may lead to dehydration. They can also make infections more severe or last longer, so keeping your glucose levels well controlled during illness is especially important.
Occasionally, some people may experience lower blood glucose levels if they are eating a lot less. In these situations, a reduction in insulin doses may be needed. Regular blood glucose monitoring is essential to guide safe adjustments to your diabetes treatment.
Steroid courses
Steroids are sometimes prescribed during illness. These can make your body less sensitive to insulin and can cause your liver to release more glucose into the blood.
You are likely to require an increased dose of insulin if you start a steroid course, or you may require an additional insulin alongside your usual regimen. Please contact the diabetes or CF teams if your blood glucose levels become difficult to control.
Important self-care advice during illness with CF Diabetes
- Ensure you get plenty of rest
- Prevent dehydration by drinking 2.5 -3.5 litres of sugar-free fluids throughout each 24-hour period. Aim for at least 100 ml per hour of fluid, including overnight.
- Continue to take insulin regularly
- Eat as normal if possible. It is important to include regular carbohydrates, insulin and fluids to avoid dehydration and prevent the development of diabetic ketoacidosis (see page 4). If you are unable to eat, you can consider the following ideas for light carbohydrate-containing snacks regularly throughout the day (each contain approximately 10 g of carbohydrate):
- 100ml fruit juice
- 200ml milk
- ½ tin of soup (200ml)
- 1 large scoop of vanilla ice cream
- 1 small pot of yogurt (150g)
- 2 Rich Tea or malted milk biscuits
- 1 thin slice of bread
- 1 portion of fruit
- Speak with your CF care team or GP if you think you are unwell and may need antibiotics.
- Monitor blood glucose levels every 4-6 hours, including during the night.
- If your blood glucose level is above 11 mmol/l, you should test your ketone levels.
Diabetic ketoacidosis (DKA)
If blood glucose levels are very high (>11 mmol/l), your body may be unable to use glucose for energy as it should. The body instead breaks down fat for energy, which results in the production of chemicals called ketones. If ketones accumulate in the blood, this can quickly lead to a dangerous condition called diabetic ketoacidosis (DKA).
The symptoms of DKA include:
- High blood glucose levels
- Increased thirst
- Passing more urine than usual
- Feeling and/or being sick
- Abdominal pain
- Drowsiness
- Fast and deep breathing
- Sweet or metallic taste in the mouth
- Pear-drop smelling breath (acetone)
The early signs of DKA can often be treated using extra insulin and fluids if recognised quickly. If it is not identified early, DKA can be life-threatening and needs hospital treatment.
DKA mostly affects people with type 1 diabetes and is usually very rare in CF diabetes, however given the serious nature of the condition, it should always be considered and assessed during periods of sickness and poor glucose control.
Testing for ketones
Type 1 diabetes and CF: use blood ketone testing
If you are testing blood ketones, the results can be interpreted as follows:
|
Blood ketone level (mmol/l) |
Interpretation |
See relevant table for management advice: |
|
Less than 0.6 |
Normal reading |
A |
|
0.6-1.5 |
You may be at risk of developing DKA; repeat test again in 2 hours |
A |
|
1.6-2.9 |
At risk of DKA; contact your diabetes team as soon as possible |
B |
|
3 or above |
Very high risk of DKA; get emergency help as soon as possible |
C |
CF diabetes on multiple daily insulin injections: use urine testing ketone sticks
If you are using urine ketone sticks, the results can be interpreted as follows:
|
Urine ketone result |
Interpretation |
See relevant table for management advice: |
|
Negative / trace |
Normal reading |
A |
|
+/++ |
At risk of DKA; contact your diabetes team as soon as possible |
B |
|
+++/++++ |
Very high risk of DKA; get emergency help as soon as possible |
C |
Table A:
Managing your insulin dose when your blood ketones are less than 1.5 mmol or urine ketones are negative / trace
- Closely follow ‘important self-care advice’ listed on page 3
- Aim to take your usual insulin dose, however if your blood glucose is above 11 mol/l, take additional insulin as below:
|
Blood glucose level (mmol/l) |
Insulin dose |
|
11-17 |
Add 2 extra units to each dose |
|
17-22 |
Add 4 extra units to each dose |
|
More than 22 |
Add 6 extra units to each dose |
Please call the diabetes team if your blood glucose levels still remain elevated despite adjusting your insulin doses as above.
If you start vomiting, are unable to keep fluids down, or are unable to control your blood glucose or ketone levels, you must seek urgent medical advice. Do not stop taking your insulin, even if you are unable to eat.
Table B:
Managing your insulin dose when your blood ketones are 1.5 - 3 mmol or urine ketones are +/++
- Closely follow ‘important self-care advice’ listed on page 3
- Additional insulin will be required.
- Work out your total daily insulin dose (TDD) by adding the total number of units of long-acting and rapid-acting insulins that you take on a typical day, for example:
- If you usually have 8 units of long-acting insulin each day, as well as rapid-acting insulin at approximately 4 units for breakfast and lunch and 7 units for your evening meal, your TDD would be 23 units (8 + 4 + 4 + 7 = 23)
- Take additional rapid-acting insulin every 2 hours (including overnight) as follows
|
Total daily insulin dose (TDD) |
Give an additional 10% of your TDD as rapid-acting insulin every 2 hours |
|
Up to 14 units |
1 unit |
|
15 to 24 units |
2 units |
|
25 to 34 units |
3 units |
|
35 to 44 units |
4 units |
|
45 to 54 units |
5 units |
Please call the diabetes team if your TDD is more than 54 units, or if you are unsure how to adjust your dose
- Test blood glucose levels and blood/urine ketones 2 hourly, including during the night
- If your blood glucose level remains above 11 mmol/l and ketones remain present, please repeat the process
- As your illness resolves, adjust your insulin dose back to normal
If you start vomiting, are unable to keep fluids down, or are unable to control your blood glucose or ketone levels, you must seek urgent medical advice. Do not stop taking your insulin, even if you are unable to eat.
Table C:
Managing your insulin dose when your blood ketones are more than 3 mmol / urine ketones are +++/++++
- Closely follow ‘important self-care advice’ listed on page 3
- Additional insulin will be required.
- Work out your total daily insulin dose (TDD) by adding the total number of units of long- and rapid-acting insulins that you take on a typical day, for example:
- If you usually have 8 units of long-acting insulin each day, as well as rapid-acting insulin at approximately 4 units for breakfast and lunch and 7 units for your evening meal, your TDD would be 23 units (8 + 4 + 4 + 7 = 23)
- Take additional rapid-acting insulin every 2 hours (including overnight) as follows:
|
Total daily insulin dose (TDD) |
Give an additional 20% of your TDD as rapid-acting insulin every 2 hours |
|
Up to 14 units |
2 units |
|
15 to 24 units |
4 units |
|
25 to 34 units |
6 units |
|
35 to 44 units |
8 units |
|
45 to 54 units |
10 units |
Please call the diabetes team if your TDD is more than 54 units, or if you are unsure how to adjust your dose
- Test blood glucose levels and blood/urine ketones 2 hourly, including during the night
- If your blood glucose level remains above 11 mmol/l and ketones remain present, please repeat the process
- As your illness resolves, adjust your insulin dose back to normal
If you start vomiting, are unable to keep fluids down, or are unable to control your blood glucose or ketone levels, you must seek urgent medical advice. Do not stop taking your insulin, even if you are unable to eat.
Seek urgent medical attention if any of the following occur:
- You start vomiting or are unable to keep fluids down
- You become drowsy or confused
- You become breathless
- You have acute abdominal pain
- Your ketones levels do not improve despite increased insulin doses
- You are unable to manage your blood glucose levels despite increased insulin doses
- Your condition worsens
Contact Us:
If you require further advice, the following contacts may be useful:
Diabetes Specialist Nurses, Frimley Park Hospital:
Telephone: 0300 613 4701
Cystic Fibrosis Unit, Frimley Park Hospital:
Telephone: 0300 613 4597 - 0300 613 6665
Contact us
If you have any queries relating to this information, please contact the Dietetics service.
About this information
Service:
Dietetics
Reference:
DT/013
Approval date:
1 April 2026
Review date:
1 April 2029
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Important note
This page provides general information only. It is developed by clinical staff and is reviewed regularly every 3 years for accuracy. For personal advice about your health, or if you have any concerns, please speak to your doctor.