When do you initiate insulin therapy in a patient with diabetes mellitus?

When to Initiate Insulin Therapy in Diabetes

Initiate insulin immediately in patients with type 2 diabetes presenting with severe hyperglycemia (HbA1c ≥9%, fasting glucose ≥11.1 mmol/L [200 mg/dL], or symptomatic hyperglycemia), and start insulin within 3 months when oral agents fail to achieve glycemic targets. 1

Type 1 Diabetes: Immediate Insulin Required

All patients with type 1 diabetes require insulin therapy immediately upon diagnosis to sustain life. 1, 2

• Start with multiple daily injections (3-4 injections per day) or continuous subcutaneous insulin infusion (CSII) 1
• Total daily dose typically 0.4-1.0 units/kg/day, with approximately 50% as basal insulin and 50% as prandial insulin 1, 3, 4
• Use rapid-acting insulin analogs before meals combined with long-acting basal insulin 1, 2

Type 2 Diabetes: Specific Initiation Criteria

Immediate Insulin Initiation (Start Today)

Begin basal-bolus insulin immediately in the following scenarios:

• Newly diagnosed patients with HbA1c >9.0% or fasting glucose ≥11.1 mmol/L (200 mg/dL) with symptomatic hyperglycemia 1
• HbA1c ≥10-12% with symptomatic or catabolic features (weight loss, polyuria, polydipsia) 1, 4
• Blood glucose ≥300-350 mg/dL regardless of HbA1c 1, 3
• Suspected type 1 diabetes or underweight patients with hyperglycemia 4

For these severe presentations, start with 0.3-0.5 units/kg/day total daily dose, split 50% basal and 50% prandial insulin 1, 3, 4

Urgent Insulin Initiation (Within 3 Months)

Start basal insulin within 3 months when oral medications fail to achieve glycemic targets: 1

• HbA1c remains <7.0% after 3 months of maximal tolerated doses of metformin plus additional oral agents 1
• Progressive deterioration despite dual or triple oral therapy 5
• HbA1c ≥7.5% despite optimal oral medications 2

The critical pitfall is delaying insulin initiation—this prolongs hyperglycemic exposure and accelerates complications. Studies show patients continue inadequate oral therapy for an average of 30-36 months despite HbA1c levels of 9-10%, incurring substantial glycemic burden equivalent to 32 months at HbA1c of 9%. 6

Standard Basal Insulin Initiation Protocol

For patients with HbA1c <9% failing oral therapy, start with basal insulin only: 1, 4

• Starting dose: 10 units once daily OR 0.1-0.2 units/kg/day 1, 3, 4, 7
• Administer at the same time each day (any time, but consistent) 3, 7
• Continue metformin unless contraindicated 1, 4
• Consider continuing one additional non-insulin agent 3

Titration algorithm: 1, 3

• Increase by 2 units every 3 days if fasting glucose 140-179 mg/dL
• Increase by 4 units every 3 days if fasting glucose ≥180 mg/dL
• Target fasting glucose: 80-130 mg/dL

When to Add Prandial Insulin

Add rapid-acting insulin before meals when: 1, 3, 4

• Basal insulin optimized (fasting glucose 80-130 mg/dL) but HbA1c remains >7% after 3-6 months 1, 3
• Basal insulin dose exceeds 0.5 units/kg/day—this is the critical threshold to stop escalating basal insulin alone 1, 3, 4
• Significant postprandial hyperglycemia persists despite adequate fasting control 3

Start with 4 units of rapid-acting insulin before the largest meal, or 10% of current basal dose 3, 4

Special Clinical Situations

Hospitalized Patients

• Non-critically ill with glucose 201-300 mg/dL: start 0.2-0.3 units/kg/day total (half basal, half bolus) 3
• Glucose >300 mg/dL: start 0.3-0.5 units/kg/day total (half basal, half bolus) 3
• Reduce home insulin dose by 20% if patient was on ≥0.6 units/kg/day to prevent hypoglycemia 3

Pregnancy, Surgery, Acute Illness

Insulin is indicated during acute illness, surgery, or pregnancy when oral agents are contraindicated or inadequate 2

Youth with Type 2 Diabetes

Start basal insulin when HbA1c >8.5% without acidosis/ketosis, at 0.5 units/kg/day in addition to metformin 4

Common Pitfalls to Avoid

• Delaying insulin when oral agents fail—this is the most harmful error, prolonging hyperglycemic exposure 4, 6
• Continuing to escalate basal insulin beyond 0.5-1.0 units/kg/day without adding prandial coverage—this causes "overbasalization" with increased hypoglycemia and suboptimal control 3, 4
• Abruptly discontinuing oral medications when starting insulin—continue metformin unless contraindicated 4, 2
• Using sliding scale insulin alone without scheduled basal insulin—scheduled basal-bolus regimens are superior 3

Alternative to Insulin: GLP-1 Receptor Agonists

For patients with HbA1c <9% and no severe symptoms, consider adding a GLP-1 receptor agonist before insulin, as these agents provide cardiovascular benefits and avoid weight gain 1, 3, 4