What are the recommended parameters for starting insulin therapy in an adult patient with diabetes based on hemoglobin A1c (HbA1c) levels?

When to Start Insulin Based on HbA1c

Initiate insulin therapy when HbA1c is ≥10-12%, especially if the patient has symptomatic hyperglycemia, blood glucose ≥300-350 mg/dL, or catabolic features (ketosis, unintentional weight loss); consider starting insulin at HbA1c ≥9% with dual therapy or basal insulin alone depending on symptom severity. 1

HbA1c Thresholds for Insulin Initiation

Immediate Insulin Required (HbA1c ≥10-12%)

• Start basal insulin plus mealtime insulin as the preferred initial regimen when HbA1c is 10-12% or higher, particularly if the patient has symptomatic hyperglycemia, blood glucose 300-350 mg/dL or greater, or catabolic features like ketosis or unintentional weight loss 1
• This represents severe hyperglycemia requiring urgent intervention to prevent metabolic decompensation 1

Consider Insulin at HbA1c ≥9%

• Initiate therapy at this stage when blood glucose levels are 300-350 mg/dL or HbA1c levels are ≥9%, especially if symptomatic 1
• At this threshold, you can choose between dual oral therapy (metformin plus second agent) or proceeding directly to basal insulin, depending on whether the patient has marked symptoms 1, 2
• If markedly symptomatic with hyperglycemia or blood glucose ≥300-350 mg/dL, proceed directly to insulin rather than oral agents 2

Insulin as Add-On Therapy (HbA1c ≥7.5% on Optimized Oral Agents)

• Add basal insulin when HbA1c remains above target despite optimal oral therapy, typically when it rises to ≥7.5% (58 mmol/mol) 2
• Do not delay insulin initiation in patients not achieving glycemic goals with oral medications 1

Starting Dose Parameters

Basal Insulin Initiation

• Start basal insulin at 10 units OR 0.1-0.2 units/kg of body weight 1
• Use basal insulin (NPH, glargine, detemir, or degludec) typically in combination with metformin and perhaps one additional non-insulin agent 1
• For augmentation therapy (adding to oral agents), start at 0.3 units/kg 3

Full Insulin Replacement Therapy

• For replacement therapy (when oral agents are inadequate or contraindicated), start at 0.6-1.0 units/kg total daily dose 3
• Give 50% of total daily dose as basal insulin and 50% as bolus insulin divided before breakfast, lunch, and dinner 3
• When basal insulin plus mealtime insulin is needed (HbA1c ≥10-12%), this represents the preferred initial regimen 1

Titration Strategy After Initiation

• Adjust basal insulin dose based on fasting blood glucose levels using self-monitoring of blood glucose (SMBG) 1
• When basal insulin has been titrated to appropriate fasting glucose but HbA1c remains above target, add prandial coverage with either a GLP-1 receptor agonist or rapid-acting insulin (lispro, aspart, or glulisine) before meals 1
• Timely dose titration after insulin initiation is critical—do not leave patients on inadequate doses 1

Target HbA1c After Starting Insulin

• Set target HbA1c at 7.0% (53 mmol/mol) for most patients on insulin therapy, as insulin is associated with hypoglycemia risk 2, 4
• For patients at high risk of hypoglycemia consequences (impaired awareness, fall risk, operates machinery), target 7-8% or 7.0-8.5% 5, 4
• Avoid targeting HbA1c <6.5%, as this increases mortality risk, hypoglycemia, and weight gain without clinical benefit 2, 5

Continuing Oral Medications with Insulin

• Continue metformin when starting insulin—it reduces all-cause mortality and cardiovascular events in overweight patients, decreases weight gain, lowers insulin dose requirements, and reduces hypoglycemia compared to insulin alone 1, 3, 6
• Withdraw sulfonylureas, DPP-4 inhibitors, and GLP-1 receptor agonists when using complicated insulin regimens beyond basal insulin alone 1
• Thiazolidinediones or SGLT2 inhibitors may be continued to improve glucose control and reduce total daily insulin dose, but use thiazolidinediones cautiously in patients with heart failure risk 1
• Do not abruptly discontinue oral medications when starting insulin due to risk of rebound hyperglycemia 6

Critical Pitfalls to Avoid

• Do not delay insulin initiation when patients are not meeting glycemic goals on oral therapy—this is a common error that prolongs exposure to hyperglycemia 1
• Avoid intramuscular injections, especially with long-acting insulins, as severe hypoglycemia may result 6
• Use the shortest needles (4-mm pen or 6-mm syringe needles) to avoid intramuscular injection and reduce pain 6
• Rotate injection sites properly to prevent lipohypertrophy, which distorts insulin absorption 6
• Monitor for SGLT2 inhibitor-associated ketoacidosis if continuing these agents with insulin 1