Insulin Indications in Diabetes Mellitus
Type 1 Diabetes: Insulin is Always Required
Insulin is the primary and essential treatment for all patients with type 1 diabetes from the time of diagnosis 1, 2, 3. There is no alternative—these patients have absolute insulin deficiency and will develop diabetic ketoacidosis without exogenous insulin 4, 1.
Initial Regimen for Type 1 Diabetes
Start with multiple daily injections using a basal-bolus approach 1, 2, 5:
- Total daily dose: 0.4-1.0 units/kg/day (typically 0.5 units/kg/day for metabolically stable patients) 6
- Split approximately 50% as basal insulin (long-acting) once daily 6
- Split approximately 50% as prandial insulin (rapid-acting) divided among three meals 6
- Rapid-acting insulin analogues (aspart, lispro, glulisine) given 0-15 minutes before meals 1, 2
Basal insulin must be combined with short-acting insulin—basal insulin alone is insufficient in type 1 diabetes 4, 1.
Higher Doses Required in Specific Situations
- Immediately following ketoacidosis presentation: higher weight-based dosing needed 6
- During puberty, pregnancy, and acute illness: may exceed 1.0 units/kg/day 6
Type 2 Diabetes: Insulin When Oral Agents Fail or Severe Hyperglycemia
For type 2 diabetes, insulin becomes necessary when oral medications and lifestyle modifications fail to achieve glycemic targets, or when presenting with severe hyperglycemia 7, 1.
Absolute Indications for Immediate Insulin Initiation
Start insulin immediately in these scenarios 7, 8:
Severe hyperglycemia with symptoms:
Advanced chronic kidney disease or liver cirrhosis 9
Relative Indications: When to Add Insulin
Consider starting basal insulin when 7, 8, 1:
- HbA1c ≥7.5-9.0% despite optimal oral medications (metformin plus additional agents) for 3 months 7, 1
- HbA1c ≥9.0%: consider insulin earlier in the treatment algorithm 6
- Fasting glucose ≥200 mg/dL (11.1 mmol/L) 8
- Glucose toxicity preventing oral agents from working effectively 1
- Contraindications to or failure of oral antidiabetic medications 1
Initial Insulin Dosing for Type 2 Diabetes
Starting Dose for Basal Insulin
Begin with basal insulin (insulin glargine, detemir, or NPH) once daily 7, 6:
- Standard starting dose: 10 units once daily OR 0.1-0.2 units/kg/day 7, 6, 4
- For severe hyperglycemia (HbA1c ≥9%): consider 0.3-0.5 units/kg/day as total daily dose 6
- Administer at the same time each day 6, 4
Continue Metformin
Do not stop metformin when starting insulin (unless contraindicated)—it reduces insulin requirements, limits weight gain, and provides complementary glucose-lowering effects 7, 6.
Titration Algorithm
Adjust basal insulin every 3 days based on fasting glucose 7, 6:
- If fasting glucose 140-179 mg/dL: increase by 2 units 7, 6
- If fasting glucose ≥180 mg/dL: increase by 4 units 7, 6
- Target fasting glucose: 80-130 mg/dL 7, 6
- If hypoglycemia occurs: reduce dose by 10-20% immediately 7, 6
When to Add Prandial Insulin
Do not continue escalating basal insulin indefinitely—this leads to "overbasalization" with increased hypoglycemia and poor postprandial control 6.
Critical Threshold: Stop Escalating Basal Insulin When
Add prandial insulin rather than continuing to increase basal insulin when 7, 6:
- Basal insulin exceeds 0.5 units/kg/day 7, 6
- Fasting glucose is controlled (80-130 mg/dL) but HbA1c remains above target after 3-6 months 7, 6
- Significant postprandial glucose excursions persist 7, 6
Starting Prandial Insulin
Begin with rapid-acting insulin before the largest meal 7, 6:
- Starting dose: 4 units OR 10% of current basal dose 7, 6
- Titrate by 1-2 units every 3 days based on postprandial glucose 6
- Add to additional meals as needed based on glucose patterns 6
Special Situations Requiring Insulin
Checkpoint Inhibitor-Associated Diabetes (CIADM)
Insulin is mandatory for checkpoint inhibitor-associated diabetes because these patients develop acute beta-cell destruction similar to type 1 diabetes 7:
- Grade 2 (fasting glucose 160-250 mg/dL): initiate insulin immediately with urgent endocrine consultation 7
- Grade 3-4 (glucose >250 mg/dL or ketoacidosis): admit for inpatient management with basal-bolus insulin 7
- Starting dose: 0.3-0.4 units/kg/day total, split 50% basal and 50% prandial 7
- Long-acting insulin alone is insufficient—these patients require both basal and prandial coverage 7
Hospitalized Patients
For hospitalized patients requiring insulin 6:
- Insulin-naive or low-dose insulin: start 0.3-0.5 units/kg/day total daily dose, with half as basal 6
- High-dose home insulin (≥0.6 units/kg/day): reduce total daily dose by 20% to prevent hypoglycemia 6
- High-risk patients (elderly >65 years, renal failure, poor oral intake): use lower doses of 0.1-0.25 units/kg/day 6
Steroid-Induced Hyperglycemia
For patients on high-dose steroids 7:
- Without diabetes: consider single morning dose of NPH 6
- With diabetes: add 0.1-0.3 units/kg/day glargine to usual regimen 6
- Adjust doses as steroids are tapered to prevent hypoglycemia 7
Critical Pitfalls to Avoid
Do Not Delay Insulin Initiation
Delaying insulin in patients not achieving glycemic goals is harmful 7, 6. Many months of uncontrolled hyperglycemia should be specifically avoided to prevent long-term complications 6.
Do Not Continue Escalating Basal Insulin Beyond 0.5 units/kg/day
Continuing to increase basal insulin beyond 0.5-1.0 units/kg/day without addressing postprandial hyperglycemia leads to suboptimal control and increased hypoglycemia risk 6. This is "overbasalization"—recognize it by 6:
- Bedtime-to-morning glucose differential ≥50 mg/dL
- Basal dose >0.5 units/kg/day
- Frequent hypoglycemia
- High glucose variability
Do Not Stop Metformin When Starting Insulin
Metformin should be continued when adding or intensifying insulin therapy (unless contraindicated) because it reduces total insulin requirements and limits weight gain 7, 6.
Do Not Use Insulin Glargine for Diabetic Ketoacidosis
Insulin glargine is not recommended for treatment of diabetic ketoacidosis—use intravenous regular insulin instead 4.
Monitoring Requirements
Daily fasting blood glucose monitoring is essential during insulin titration 7, 6. Reassess and modify therapy every 3-6 months to avoid therapeutic inertia 6. Patients should be educated on 7, 6: