When should insulin therapy be initiated in a patient with diabetes?

When to Start Insulin for a Patient with Diabetes

Insulin therapy should be initiated immediately in all patients with type 1 diabetes at diagnosis, and in type 2 diabetes patients when there is severe hyperglycemia (HbA1c ≥9.0% or fasting glucose ≥11.1 mmol/L [≥200 mg/dL]), symptomatic hyperglycemia, evidence of catabolism, ketosis/ketoacidosis, or failure to achieve glycemic goals within 3 months of optimal oral medication therapy. 1, 2

Type 1 Diabetes: Immediate Insulin Initiation

All patients with type 1 diabetes require insulin at diagnosis to sustain life. 1, 3

• Start with multiple daily injections (basal plus prandial insulin) or continuous subcutaneous insulin infusion (CSII) 2, 1
• Initial total daily dose: 0.4-1.0 units/kg/day, with 0.5 units/kg/day as typical starting dose for metabolically stable patients 1
• Approximately one-third of total daily insulin should be basal insulin, with short-acting premeal insulin covering the remainder 4
• Insulin must be started immediately—any delay increases risk of diabetic ketoacidosis 1

Type 2 Diabetes: Specific Thresholds for Insulin Initiation

Immediate Insulin Initiation Required When:

Start insulin without delay if any of the following are present: 1, 2, 3

• HbA1c ≥10% (≥86 mmol/mol) with symptomatic or catabolic features 1, 3
• Random blood glucose ≥250 mg/dL or fasting glucose ≥300-350 mg/dL 1
• HbA1c ≥9.0% or fasting glucose ≥11.1 mmol/L (≥200 mg/dL) with symptomatic hyperglycemia 2, 1
• Ketosis or ketoacidosis present 1, 3
• Evidence of ongoing catabolism (weight loss despite hyperglycemia) 2, 1
• Symptomatic hyperglycemia (polyuria, polydipsia, blurred vision) 2, 1

Early Insulin Initiation (Within 3 Months):

Initiate insulin within 3 months when glycemic goals are not achieved with lifestyle intervention and oral hypoglycemic agents. 2, 1

• This applies when HbA1c remains ≥7.5% (≥58 mmol/mol) despite optimal use of oral medications 3
• Do not delay insulin therapy beyond 3 months of recognized treatment failure 2, 1

Short-Term Intensive Insulin Therapy:

Consider 2 weeks to 3 months of intensive insulin treatment for newly diagnosed type 2 diabetes patients with: 2, 1

• HbA1c >9.0% or fasting glucose ≥11.1 mmol/L with symptomatic hyperglycemia 2
• This approach can restore beta-cell function and may allow subsequent de-escalation to oral agents 2

Hospitalized Patients: Different Thresholds

For hospitalized patients, insulin therapy should be initiated when blood glucose is persistently ≥180 mg/dL (10.0 mmol/L) on two occasions. 2

• Target glucose range: 140-180 mg/dL (7.8-10.0 mmol/L) for most critically ill and non-critically ill patients 2
• More stringent goals of 110-140 mg/dL may be appropriate for selected patients (e.g., post-cardiac surgery) if achievable without significant hypoglycemia 2
• Use basal plus correction insulin for poor oral intake; use basal, nutritional, and correction components for good nutritional intake 1

Initial Insulin Regimen Selection

For Type 2 Diabetes:

Start with basal insulin (long-acting) once daily while continuing metformin: 1, 4, 3

• Initial dose: 10 units or 0.1-0.2 units/kg/day 1, 4
• Administer at the same time each day (any time, but consistent) 4
• Continue metformin to reduce weight gain, lower insulin dose requirements, and decrease hypoglycemia risk 3

Alternative for short-term intensive therapy: 2, 1

• Premixed human insulin or premixed insulin analogs 1-3 times daily 2
• Multiple insulin injections (basal plus prandial) or CSII 2

Intensification Strategy:

If HbA1c remains above target despite optimized basal insulin, add prandial insulin or GLP-1 receptor agonist. 1, 3

• Titrate basal insulin using fasting plasma glucose values 3
• Add rapid-acting insulin at meals when basal insulin alone is insufficient 3, 5

Critical Pitfalls to Avoid

Never delay insulin initiation when clearly indicated—prolonged hyperglycemia increases complication risk. 1

• Therapeutic inertia in starting insulin is a major barrier to optimal diabetes control 6
• Do not abruptly discontinue oral medications when starting insulin due to rebound hyperglycemia risk 3
• Sliding scale insulin alone is strongly discouraged in the inpatient setting 1
• Do not use insulin for diabetic ketoacidosis treatment via subcutaneous route in acute setting—intravenous insulin is required 4

Monitoring During Insulin Initiation

Increase frequency of blood glucose monitoring during any insulin regimen changes. 4, 3

• Use fasting plasma glucose to titrate basal insulin 3
• Use both fasting and postprandial glucose to titrate mealtime insulin 3
• Rotate injection sites within the same region to reduce lipodystrophy risk 4
• Avoid injecting into areas of lipodystrophy as this causes erratic absorption and hyperglycemia 4