Insulin prescribing

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

Normal 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

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)

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

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)

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!