Insulin prescribing

Normal glucose values (mmol/L)

• Non-diabetic (random) = 3.5-7.8
• Type 1 diabetic = 4-9
• Type 2 diabetic = 4-8.5
• Hyperglycaemia = >11
• Hypoglycaemia = <4

Types of insulin

• Rapid-acting (given at start of meal)
  • Novorapid (Aspart)
  • Humalog (Lispro)
  • Apidra (Glulisine)
• Short-acting (given 30 minutes before a meal)
  • Actrapid
  • Humulin S
• Intermediate-acting (usually given once/twice daily or as part of mix)
  • Humulin I
  • Insulatard
  • Insuman basal
• Long-acting (usually given once daily)
  • Levemir (Detemir)
  • Lantus (Glargine)
  • Tresiba (Degludec) – ultra-long-acting

Mixtures (for twice daily pre-mixed regimens)

Intermediate + short (given 30 minutes before breakfast and dinner)

• Humulin M3 (30% short-acting)
• Insuman comb 15 or 25 or 50 (15/25/50% short-acting respectively)

Intermediate + rapid (given at start of breakfast and dinner)

• NovoMix 30 (30% rapid-acting)
• Humalog Mix 25 or 50 (25/50% rapid-acting respectively)

Regimens 

• Basal bolus regimen (basal long-acting insulin given at night with rapid-acting insulin given before every meal):

• Twice daily pre-mixed regimen (mixed intermediate-acting and short-/rapid-acting insulin given twice daily, before breakfast and before dinner):

• Intermediate insulin once/twice daily may also be used in type 2 diabetes to supplement oral hypoglycaemics and aid glycaemic control in the day (OM), at night/in morning (ON) or both (BD)

Adjusting insulin regimens (in response to glucose levels)

• Review the capillary glucose monitoring chart and work out the pattern of variation in glucose levels in relation to meals (readings are usually taken before meals)
• Use common sense – if a patient is eating less, the insulin dose will need to be reduced; on the other hand, certain conditions may require it to be increased (see box)
• Adjusting a basal bolus regimen:
  • Levels high/low before breakfast (or in night) → increase/decrease bedtime long-acting insulin
  • Levels high/low before lunch or dinner or bed → increase/decrease rapid-acting insulin given with meal before
• Adjusting a twice daily pre-mixed/intermediate regimen:
  • Levels high/low before bed and before breakfast → increase/decrease evening insulin
  • Levels high/low before lunch and before evening meal → increase/decrease morning insulin
• Adjusting a once daily morning regimen:
  • Levels high/low before lunch and evening meal → increase/decrease insulin
• Adjusting a once daily evening regimen:
  • Levels high/low before breakfast → increase/decrease insulin
• Doses are usually adjusted by approximately 10% depending on how abnormal the glucose levels are. Capillary glucose levels must then be closely monitored and dose-adjusted as required (it’s partly trial and error).
• Remember, hypoglycaemia is more dangerous than hyperglycaemia
• If you cannot get on top of it, ask the diabetes team for help (insulin type may need changing)

Changes in insulin requirements

Increased requirement

• DKA/hyperosmolar hyperglycaemic state
• Sepsis
• Illness
• Steroids
• Pancreatitis
• Dehydration

Decreased requirement

• Reduced calorific intake
• Exercise
• Reduced renal function (may reduce drug excretion)
• Alcohol

Insulin pumps

• Insulin pumps continuously infuse a basal rate of rapid-/short-acting insulin subcutaneously
  • The rate can be changed depending on requirements (e.g. during exercise, diet or capillary glucose level)
  • A button is pressed to give an insulin bolus at the start of a meal
• If patients are nil by mouth, the pump should be set to continue basal rate insulin but no boluses should be given. (5% dextrose infusion can be given and their basal rate adjusted accordingly – they do not need a variable rate insulin infusion.)

Variable rate insulin infusion (formerly called ‘sliding scale’)

NB: This is different to a fixed rate insulin infusion, as used in DKA.

• Used for patients with diabetes who are nil by mouth, e.g. perioperatively
• The capillary glucose is checked 1-2 hourly and the rate of insulin infusion is modified according to a predetermined protocol
• Continuous IV fluids containing glucose must be given alongside to maintain patient glucose levels and hydration 
• Starting a variable rate insulin infusion:
  • Most hospitals have a variable rate insulin infusion chart which just needs a signature 
  • Continuous IV fluids also need prescribing as below
  • If the patient is taking long-acting insulin, this should be continued throughout (but short-/rapid-acting insulin must be suspended)
• During a variable rate insulin infusion:
  • Continuous IV fluids, for example:
    • Surgical patients: 5% glucose/0.45% saline/0.15% KCl at 80ml/hour
    • Medical patients: 5% glucose (1L with 20mmol KCl) at 100ml/hour (unless capillary glucose is >15mmol/L, in which case give 0.9% saline until it returns to <15mmol/L)
  • Check plasma Na⁺ and K⁺ daily
  • Re-sign the variable rate insulin infusion chart daily
  • The protocol’s infusion rates can be modified if the patient is particularly insulin-resistant/sensitive
• Stopping a variable rate insulin infusion: confirm patient is eating and drinking and ensure the patient has received their long-acting insulin. (If not, a proportionate dose should be given at least 1 hour before stopping.) Give their usual mixed/rapid-acting insulin with meal and wait 30 minutes before stopping the variable rate insulin infusion. Monitor capillary glucose QDS for at least 24 hours.

Insulin prescribing rules

• Use insulin prescription chart if available (write ‘insulin as per insulin prescription chart’ at relevant times on main chart)
• You must write ‘UNITS’ (do not abbreviate to ‘U’)
• Specify the brand name and indicate the device the patient uses (e.g. disposable pen, vial, pen cartridge)
• Write ‘pre-breakfast/lunch/dinner’ rather than times if the insulin must be taken before meals 
• Ensure you corroborate their prescription if unsure of a dose – never estimate!

Test your knowledge with a scenario!

You review a 40 year old male patient with type 1 diabetes on the ward because the nurses are concerned his capillary blood sugar readings have just said “high” for the last few hours. The patient was admitted with COVID-19 and is currently on dexamethasone day 3. He normally take 5 units of Novorapid with meals, and 30 units of Tresiba at night. The patient has been taking his insulin as normal, but has been vomiting today and is not tolerating meals.