When should insulin therapy be initiated in patients with diabetes?

Indications to Start Insulin Therapy

Insulin therapy should be initiated immediately in type 1 diabetes at diagnosis, and in type 2 diabetes when A1C ≥10% or blood glucose ≥300 mg/dL with symptoms, or when there is evidence of catabolism, ketosis, or failure to achieve glycemic goals within 3 months of optimal oral therapy. 1, 2, 3

Type 1 Diabetes: Immediate Initiation Required

• All patients with type 1 diabetes require insulin at diagnosis to sustain life and prevent ketoacidosis 1, 2, 4
• Start with multiple daily injections (basal plus prandial insulin) or continuous subcutaneous insulin infusion 1, 2
• Initial dosing typically 0.4-1.0 units/kg/day, with 0.5 units/kg/day as a standard starting point for metabolically stable patients 2

Type 2 Diabetes: Clear Thresholds for Initiation

Immediate Insulin Initiation (Do Not Delay)

The following scenarios mandate immediate insulin therapy 1, 2, 3:

• A1C ≥10% (86 mmol/mol) with any symptoms of hyperglycemia 1, 2, 3
• Blood glucose ≥300-350 mg/dL (16.7-19.4 mmol/L) 1, 2, 3
• Presence of catabolic features including unintentional weight loss 1, 2
• Symptomatic hyperglycemia (polyuria, polydipsia, blurred vision) 1, 2, 3
• Ketosis or ketoacidosis at any glucose level 2, 3, 4

Early Insulin Consideration (Within 3 Months)

• A1C ≥9% (75 mmol/mol) after 3 months of optimal oral therapy 1, 3
• Failure to achieve glycemic goals despite maximum tolerated doses of oral agents and lifestyle modification 1, 3, 5
• The American Diabetes Association emphasizes that treatment intensification should not be delayed beyond 3 months of recognizing inadequate control 1

Short-Term Intensive Insulin Therapy

For newly diagnosed type 2 diabetes patients with severe hyperglycemia 1:

• A1C >9.0% or fasting glucose ≥11.1 mmol/L (200 mg/dL) with symptoms warrants 2 weeks to 3 months of intensive insulin therapy 1
• This approach can restore beta-cell function and may allow subsequent transition to oral agents 1

Special Populations

Youth with Type 2 Diabetes

• Initiate insulin when blood glucose ≥250 mg/dL or A1C ≥8.5% 3
• Start immediately if ketosis/ketoacidosis present or if distinction between type 1 and type 2 diabetes is unclear 3
• Begin with basal insulin at 0.5 units/kg/day while continuing metformin 3

Hospitalized Patients

• Insulin is the preferred agent for inpatient hyperglycemia management 2
• Use basal plus correction insulin for poor oral intake, or basal-bolus-correction for good nutritional intake 2
• Avoid sliding scale insulin alone as it is strongly discouraged 2

Initial Insulin Regimen Selection

For Type 2 Diabetes Starting Insulin

Start with basal insulin as the preferred initial approach 1, 2, 3:

• Dose: 10 units or 0.1-0.2 units/kg/day 2, 3
• Continue metformin for ongoing metabolic benefits 1, 3
• Consider adding one additional non-insulin agent 3

For severe hyperglycemia (glucose >300 mg/dL or A1C >10%) 2, 3:

• Consider starting with basal plus mealtime insulin immediately 3
• Alternative: premixed insulin 2-3 times daily 1, 3

Critical Pitfalls to Avoid

• Delaying insulin initiation when clearly indicated leads to prolonged hyperglycemia and increased complication risk 2
• Do not abruptly discontinue oral medications when starting insulin due to risk of rebound hyperglycemia 4
• Avoid sole reliance on sliding scale insulin in any setting 2
• Do not delay treatment intensification beyond 3 months if glycemic goals are not met 1

Medication Management When Starting Insulin

• Continue metformin unless contraindicated, as it reduces mortality and cardiovascular events 1, 3, 5
• Withdraw sulfonylureas to reduce hypoglycemia risk when using complex insulin regimens 3
• Consider continuing SGLT2 inhibitors for cardiovascular and renal benefits, but monitor for ketoacidosis risk 3
• GLP-1 receptor agonists are preferred over insulin when possible in patients with established cardiovascular disease 1

When Insulin is NOT First-Line

The 2022 American Diabetes Association guidelines emphasize that for patients with or at high risk for atherosclerotic cardiovascular disease, heart failure, or chronic kidney disease, GLP-1 receptor agonists or SGLT2 inhibitors should be initiated first, with or without metformin, regardless of A1C 1. However, this does not apply when the severe hyperglycemia thresholds above are met.