Should I adjust short-acting or long-acting insulin first?

Insulin Adjustment Strategy: Long-Acting First, Then Short-Acting

When adjusting insulin therapy, you should first optimize the basal (long-acting) insulin dose to achieve target fasting glucose levels before making adjustments to prandial (short-acting) insulin. 1

Rationale for Basal-First Approach

The most recent guidelines from the American Diabetes Association (2025) recommend a stepwise approach to insulin adjustment:

1. Start with basal insulin optimization:

   • Set a fasting plasma glucose (FPG) goal
   • Titrate basal insulin dose until FPG goal is reached without hypoglycemia
   • Typical titration: increase by 2 units every 3 days until target is reached
   • For hypoglycemia: determine cause and lower dose by 10-20% if no clear reason 1

2. Only after basal optimization, adjust prandial insulin:

   • Add or adjust prandial insulin when A1C remains above target despite optimized basal insulin
   • Usually start with one dose at the largest meal or meal with greatest postprandial excursion 1

Clinical Algorithm for Insulin Adjustment

Step 1: Assess Basal Insulin Adequacy

• Check fasting glucose values (target typically 90-150 mg/dL)
• Look for signs of overbasalization:
  • Elevated bedtime-to-morning glucose differential
  • Hypoglycemia (aware or unaware)
  • High glucose variability 1

Step 2: Adjust Basal Insulin

• If 50% of fasting glucose values are above target: increase basal dose by 2 units
• If >2 fasting glucose values/week are <80 mg/dL: decrease dose by 2 units 1

Step 3: Once Basal is Optimized, Address Prandial Control

• Initial prandial adjustment: 4 units per dose or 10% of basal dose
• Titrate by 1-2 units or 10-15% twice weekly based on postprandial values
• For hypoglycemia: lower corresponding dose by 10-20% 1

Special Considerations

Mixing Insulins

When using multiple insulin types:

• Rapid-acting insulin can be mixed with NPH, lente, and ultralente
• When mixing rapid-acting with intermediate/long-acting insulin, inject within 15 minutes before meals
• Insulin glargine should not be mixed with other insulins due to its low pH 1, 2

Older Adults

For older adults, simplification may be needed:

• Consider using 70% of total daily dose as basal only in the morning
• May discontinue prandial insulin if ≤10 units/dose and add non-insulin agent 1

Avoiding Common Pitfalls

1. Avoid simultaneous adjustments: Changing both basal and bolus insulin simultaneously makes it difficult to determine which adjustment caused any resulting changes in glycemic control.

2. Beware of stacking: Adjusting short-acting insulin without proper consideration of the basal dose can lead to insulin stacking and increased hypoglycemia risk.

3. Don't ignore patterns: Look for patterns in glucose readings before making adjustments. Random adjustments without pattern recognition can lead to erratic control.

4. Consider insulin pharmacokinetics: Remember that rapid-acting insulins have onset of 15-30 minutes and duration of 3-5 hours, while long-acting insulins have minimal peak and duration of 20-24+ hours 3.

By following this systematic approach of optimizing basal insulin first followed by prandial insulin adjustments, you can achieve better glycemic control with lower risk of hypoglycemia.