What are the clinical indications for adding insulin therapy in a patient with type 2 diabetes, including severe hyperglycemia (fasting or random glucose ≥300 mg/dL with classic symptoms or ketosis/diabetic ketoacidosis), HbA1c ≥10 % (or ≥9 % with symptomatic hyperglycemia), failure of dual or triple oral therapy or glucagon‑like peptide‑1 (GLP‑1) receptor agonist/sodium‑glucose co‑transporter‑2 (SGLT‑2) inhibitor regimen after ≥3 months, pregnancy or planning pregnancy, contraindications or intolerance to oral agents such as estimated glomerular filtration rate (eGFR) <30 mL/min/1.73 m², acute illness or peri‑operative periods, and persistent glucotoxicity (>250 mg/dL)?

Indications for Adding Insulin in Type 2 Diabetes

Insulin therapy should be initiated immediately in adults with type 2 diabetes when fasting glucose ≥300 mg/dL with classic symptoms (polyuria, polydipsia, weight loss) or any evidence of ketosis/diabetic ketoacidosis, regardless of HbA1c level. 1

Absolute Indications for Immediate Insulin Initiation

• Severe hyperglycemia with metabolic decompensation: Start basal-bolus insulin at 0.3–0.5 units/kg/day (split 50% basal, 50% prandial) when fasting glucose ≥300 mg/dL or random glucose ≥300–350 mg/dL with classic symptoms (polyuria, polydipsia, unintentional weight loss), or when any degree of ketosis/ketoacidosis is present. 1, 2, 3, 4

• Markedly elevated HbA1c with symptoms: Initiate insulin immediately when HbA1c ≥10% (86 mmol/mol) with symptomatic hyperglycemia or catabolic features (weight loss, muscle wasting), because oral agents typically lower HbA1c by only 0.9–1.1% and cannot achieve adequate control at this severity. 1, 2, 3, 5

• HbA1c ≥9% (75 mmol/mol) with inadequate response: Begin basal insulin at 10 units daily (or 0.1–0.2 units/kg) when HbA1c remains ≥9% despite 3 months of optimized oral therapy (metformin plus at least one additional agent), because monotherapy is unlikely to achieve adequate control. 1, 2, 3

Relative Indications Based on Treatment Failure

• Dual or triple oral therapy failure: Add basal insulin when HbA1c remains >7% after ≥3 months of maximally tolerated doses of metformin plus two additional oral agents (e.g., SGLT2 inhibitor, DPP-4 inhibitor, sulfonylurea, or thiazolidinedione). 1, 2, 6

• GLP-1 receptor agonist inadequacy: Initiate basal insulin when HbA1c stays >7% after ≥3–6 months of GLP-1 RA therapy combined with metformin, or when basal insulin dose approaches 0.5–1.0 units/kg/day without achieving targets (indicating need for prandial coverage). 2, 3, 7, 8

• Persistent glucotoxicity: Start insulin when fasting glucose consistently exceeds 250 mg/dL despite oral agents, because prolonged severe hyperglycemia (>250 mg/dL for months) specifically increases complication risk and should be avoided. 2, 3

Special Clinical Situations Requiring Insulin

• Pregnancy or planning pregnancy: Discontinue all oral agents except metformin (which may be continued in some cases) and transition to insulin, as most oral antidiabetic medications are contraindicated during pregnancy. 5, 6

• Severe renal impairment: Initiate insulin when eGFR <30 mL/min/1.73 m² because metformin is contraindicated, and most other oral agents require dose reduction or discontinuation at this level of renal function. 1, 2

• Acute illness or peri-operative periods: Start insulin during hospitalization for acute infection, myocardial infarction, stroke, or major surgery, because stress hyperglycemia and counter-regulatory hormones markedly increase insulin requirements (often by 40–60%). 2, 3, 4

• Contraindications to oral agents: Begin insulin when oral medications are contraindicated due to hepatic impairment, heart failure (thiazolidinediones), recurrent urinary tract infections (SGLT2 inhibitors), or gastrointestinal intolerance. 1, 5, 9, 6

Practical Implementation Algorithm

Step 1 – Assess severity:

• If fasting glucose ≥300 mg/dL with symptoms OR ketones present → immediate basal-bolus insulin (0.3–0.5 units/kg/day). 1, 2, 3
• If HbA1c ≥10% with symptoms → immediate basal-bolus insulin. 1, 2, 3
• If HbA1c 9.0–9.9% → start basal insulin 10 units daily plus continue metformin. 1, 2

Step 2 – Evaluate treatment history:

• If on dual/triple oral therapy for ≥3 months with HbA1c >7% → add basal insulin 10 units daily. 1, 2
• If on GLP-1 RA plus metformin for ≥3 months with HbA1c >7% → add basal insulin 10 units daily. 2, 7, 8

Step 3 – Check for special circumstances:

• Pregnancy/planning → transition to insulin immediately. 5, 6
• eGFR <30 mL/min/1.73 m² → start insulin, discontinue metformin. 1, 2
• Acute illness/surgery → initiate insulin during hospitalization. 2, 3, 4

Step 4 – Titrate systematically:

• Increase basal insulin by 2 units every 3 days if fasting glucose 140–179 mg/dL. 3, 4
• Increase basal insulin by 4 units every 3 days if fasting glucose ≥180 mg/dL. 3, 4
• Target fasting glucose 80–130 mg/dL. 3, 4
• When basal insulin reaches 0.5 units/kg/day without achieving HbA1c <7%, add prandial insulin (4 units before largest meal) rather than further basal escalation. 2, 3, 4

Critical Pitfalls to Avoid

• Never delay insulin initiation when fasting glucose consistently exceeds 250 mg/dL or HbA1c ≥9%, because prolonged severe hyperglycemia increases complication risk and should specifically be avoided. 2, 3

• Do not discontinue metformin when adding insulin unless contraindicated (eGFR <30 mL/min/1.73 m²), because metformin reduces total insulin requirements by 20–30% and provides superior glycemic control compared with insulin alone. 2, 3, 4, 7

• Avoid relying solely on oral agents when HbA1c ≥10%, because most oral agents reduce HbA1c by <1% and are insufficient at this baseline severity. 2, 3

• Do not use sliding-scale insulin as monotherapy in place of scheduled basal-bolus therapy, as this reactive approach is condemned by major diabetes guidelines and causes dangerous glucose fluctuations. 3, 4