When is insulin therapy indicated for patients with diabetes?

Insulin Therapy Indications in Diabetes Management

Type 1 Diabetes: Immediate and Universal Indication

Insulin therapy must be initiated immediately at diagnosis in all patients with type 1 diabetes—this is non-negotiable and represents the cornerstone of treatment. 1, 2

• Multiple daily injections of prandial and basal insulin, or continuous subcutaneous insulin infusion, should be started at diagnosis 1
• Initial dosing typically starts at 0.5 units/kg/day for metabolically stable patients, with half as basal insulin and half as prandial insulin 1
• Rapid-acting insulin analogs are preferred over regular insulin to reduce hypoglycemia risk 1

Type 2 Diabetes: Specific Clinical Triggers

Immediate Insulin Initiation Required:

Start insulin immediately when any of the following severe metabolic derangements are present 1, 2:

• Ketosis or diabetic ketoacidosis 1, 2
• Blood glucose ≥300-350 mg/dL with symptomatic hyperglycemia or catabolic features (unintentional weight loss) 1, 2
• HbA1c ≥10-12% with symptoms or catabolism—in these cases, basal plus mealtime insulin is the preferred initial regimen 1
• Random blood glucose ≥250 mg/dL with HbA1c ≥8.5% and symptoms 2

Consider Early Insulin Initiation:

Insulin should be strongly considered when 1, 2:

• HbA1c ≥9% despite optimal oral therapy—this represents a reasonable threshold to add basal insulin 1
• Evidence of ongoing catabolism (weight loss despite hyperglycemia) 1, 2
• Failure to achieve glycemic goals despite optimal non-insulin therapy—do not delay insulin when targets are not met 1

Insulin as Part of Stepwise Therapy:

For patients with HbA1c ≥7.5% (≥58 mmol/mol) who have not achieved goals with oral agents, insulin can be added alone or in combination 3. However, current guidelines prioritize GLP-1 receptor agonists over insulin in patients with established atherosclerotic cardiovascular disease or high cardiovascular risk 1.

Practical Initiation Approach

For Type 2 Diabetes Starting Insulin:

• Begin with basal insulin at 10 units or 0.1-0.2 units/kg/day 1, 4
• Continue metformin for ongoing glycemic and metabolic benefits 1
• Administer once daily at the same time each day 4
• Titrate every 3-4 days based on fasting blood glucose until targets are reached 5

When Basal Insulin Alone Is Insufficient:

If fasting glucose is controlled but HbA1c remains above target, add prandial coverage 1:

• Add rapid-acting insulin analogs before meals (lispro, aspart, or glulisine) 1
• Alternatively, add a GLP-1 receptor agonist to reduce postprandial excursions 1

Critical Pitfalls to Avoid

Do not delay insulin therapy when clearly indicated—prolonged hyperglycemia increases complication risk and may worsen beta-cell function 1, 2. The American Diabetes Association explicitly states that insulin therapy should not be delayed in patients not achieving glycemic goals 1.

Avoid "sliding scale" insulin as monotherapy in any setting—this approach is strongly discouraged and ineffective 2.

Do not abruptly discontinue oral medications when starting insulin—this risks rebound hyperglycemia; continue metformin and consider continuing other agents based on the insulin regimen complexity 1, 3.

Watch for overbasalization—clinical signals include basal doses >0.5 units/kg/day, high bedtime-to-morning glucose differentials, hypoglycemia, or high glycemic variability; this should prompt addition of prandial insulin or GLP-1 receptor agonist rather than further basal titration 1.