When should insulin therapy be initiated in diabetic patients?

When to Start Insulin Therapy in Diabetic Patients

Initiate insulin immediately in newly diagnosed type 2 diabetic patients presenting with A1C ≥9% with symptoms of hyperglycemia, blood glucose ≥300 mg/dL, or evidence of catabolism such as unexpected weight loss. 1

Immediate Insulin Initiation (Type 2 Diabetes)

Start insulin therapy without delay in the following clinical scenarios:

• Severe hyperglycemia at presentation: A1C ≥9% with symptomatic hyperglycemia (polyuria, polydipsia, weight loss) 1
• Marked glucose elevation: Blood glucose levels ≥300 mg/dL 1
• Catabolic features: Unexpected weight loss indicating severe insulin deficiency 1
• Ketosis or ketoacidosis: Requires immediate subcutaneous or intravenous insulin to rapidly correct hyperglycemia and metabolic derangement 1

For these patients, basal insulin should be initiated while metformin is simultaneously started and titrated. 2, 1

Insulin After Oral Agent Failure (Type 2 Diabetes)

Add insulin when noninsulin therapy at maximal tolerated doses fails to achieve or maintain A1C target over 3-6 months. 2

The specific thresholds are:

• A1C ≥7.5%: Consider insulin alone or in combination with oral agents when glycemic goals are not met 3
• A1C ≥10%: Insulin is essential for treatment when diet, physical activity, and other antihyperglycemic agents have been optimally used 3

This recommendation comes from the American Diabetes Association and represents the standard approach to progressive therapy intensification. 2, 3

Type 1 Diabetes

Insulin is the primary treatment and must be initiated immediately at diagnosis in all patients with type 1 diabetes. 3

• Multiple daily injections should be started at the time of diagnosis 3
• This typically consists of short-acting or rapid-acting insulin analogue given 0-15 minutes before meals together with one or more daily injections of intermediate or long-acting insulin 3
• Basal insulin alone is insufficient—prandial insulin coverage is mandatory 4

Initial Insulin Regimen Selection

Start with basal insulin as the preferred initial approach in type 2 diabetes:

• Starting dose: 0.2 units/kg or up to 10 units once daily for insulin-naive type 2 diabetic patients 4
• Type 1 diabetes: Approximately one-third of total daily insulin requirements should be basal insulin, with short-acting premeal insulin providing the remainder 4
• Basal insulin should be used with metformin and perhaps one additional noninsulin agent 2, 1

The American Diabetes Association specifically recommends basal insulin (NPH, glargine, detemir, or degludec) as the initial regimen. 2

Progression to More Complex Regimens

When basal insulin titrated to appropriate fasting glucose levels fails to achieve A1C target, add prandial coverage:

• Add a GLP-1 receptor agonist or prandial insulin (1-3 injections of rapid-acting insulin before meals) 2, 1
• Consider twice-daily premixed insulin analogues, though their pharmacodynamic profiles make them suboptimal for covering postprandial glucose excursions 2

Titration Strategy

Use an evidence-based titration algorithm with clear targets:

• Set fasting plasma glucose target (typically 80-130 mg/dL) 1
• Increase insulin dose by 2 units every 3 days to reach fasting glucose target without hypoglycemia 1
• If hypoglycemia occurs, determine the cause; if no clear reason is identified, lower the dose by 10-20% 1

Special Clinical Situations

Youth with marked hyperglycemia: Initiate basal insulin while starting and titrating metformin 1

NPO patients requiring insulin: Use basal insulin at 0.1-0.2 units/kg/day with correction doses of rapid-acting insulin every 4-6 hours 1

Pregnancy, advanced chronic kidney disease, liver cirrhosis, or post-transplant diabetes: Insulin should be strongly considered in these populations 5

Common Pitfalls to Avoid

• Therapeutic inertia: Do not delay insulin initiation when indicated—prolonged hyperglycemia worsens outcomes and increases complication risk 1
• Sliding scale insulin alone: Using only correction doses without basal insulin leads to poor glycemic control and should be avoided 1
• Abrupt discontinuation of oral agents: Do not stop oral medications abruptly when starting insulin due to risk of rebound hyperglycemia 3
• Continuing sulfonylureas with complex insulin regimens: When progressing beyond basal insulin, discontinue sulfonylureas, DPP-4 inhibitors, and GLP-1 receptor agonists, but continue metformin 2, 1

Monitoring Requirements

• A1C testing: Every 3 months to assess glycemic control 1
• Blood glucose monitoring: Fasting plasma glucose values should be used to titrate basal insulin; both fasting and postprandial values should guide prandial insulin adjustments 3
• Increased monitoring frequency: During any changes to insulin regimen to detect hypoglycemia or hyperglycemia early 4