Overlapping Two Basal Insulins: Not Recommended in Standard Practice
You should not routinely overlap two different basal insulins (glargine and detemir) as this approach is not supported by clinical guidelines and creates unnecessary complexity with increased hypoglycemia risk. 1
Why Overlapping Is Not Standard Practice
The question of "overlapping" two basal insulins suggests either:
- Transitioning from one basal insulin to another
- Using two different basal insulins simultaneously
Neither scenario requires or benefits from overlapping administration. 1, 2
Key Pharmacologic Principles
- Basal insulins are designed to provide 24-hour coverage with relatively flat action profiles to restrain hepatic glucose production between meals and overnight 3
- Glargine provides >24 hours of duration and is dosed once daily 1, 3
- Detemir has <24 hours of duration and often requires twice-daily dosing 3, 4
- These insulins should never be mixed due to glargine's low pH, which would alter the pharmacokinetics of both preparations 2, 5
If Switching Between Basal Insulins
Glargine to Detemir Conversion
When converting from glargine to detemir, expect to need approximately 38% higher total daily dose of detemir (range 8-77% higher) to achieve equivalent glycemic control 4
- Start detemir at 1.4 times the glargine dose, split into twice-daily injections 4
- Make the switch abruptly - give the last dose of glargine, then start detemir the next day at the calculated dose 4
- Monitor fasting glucose daily for the first 2-4 weeks and adjust by 2-4 units every 3 days until fasting glucose reaches 80-130 mg/dL 2, 5, 6
Detemir to Glargine Conversion
When converting from detemir to glargine, reduce the total daily dose by approximately 28% (inverse of the 1.38 ratio) 4
- Calculate total daily detemir dose (if twice-daily, add both doses together) 4
- Divide by 1.4 to get the glargine dose, given once daily 4
- Switch directly - no overlap period needed 6
If Inadequate Basal Coverage
If a patient on once-daily basal insulin has inadequate 24-hour coverage (fasting hyperglycemia with controlled daytime glucose, or vice versa), the solution is NOT to add a second type of basal insulin. Instead:
For Glargine Inadequacy
Consider splitting glargine to twice-daily dosing if once-daily administration fails to provide 24-hour coverage, particularly in type 1 diabetes with high glycemic variability 2
- Divide the total daily dose into two injections (typically 50:50 or 60:40 split) 2
- Administer morning and evening at consistent times 2
For Detemir Inadequacy
Most patients require twice-daily detemir - this is expected, not a failure 3, 4
- If on once-daily detemir with inadequate coverage, split to twice-daily dosing 4
- Increase total daily dose by 10-20% when splitting to maintain glycemic control 4
Critical Threshold: When to Add Prandial Insulin Instead
When basal insulin exceeds 0.5 units/kg/day and approaches 1.0 units/kg/day, adding prandial insulin is more appropriate than manipulating basal insulin regimens 1, 2, 5
Signs of Overbasalization
- Basal dose >0.5 units/kg/day 1, 2
- Bedtime-to-morning glucose differential ≥50 mg/dL 2
- Hypoglycemia (aware or unaware) 1, 2
- High glucose variability 1, 2
Adding Prandial Coverage
Start with 4 units of rapid-acting insulin before the largest meal, or use 10% of the current basal dose 2, 5
- Titrate prandial insulin by 1-2 units every 3 days based on postprandial glucose readings 2
- Continue basal insulin at current dose while adding prandial coverage 2, 5
Common Pitfalls to Avoid
- Never mix glargine with any other insulin due to its low pH diluent - this requires separate injections 5, 6
- Do not assume unit-per-unit equivalence between glargine and detemir - detemir requires substantially higher doses 4
- Avoid continuing to escalate basal insulin beyond 0.5-1.0 units/kg/day without addressing postprandial hyperglycemia with prandial insulin 2, 5
- Do not delay switching insulin types if the current regimen is inadequate - prolonged hyperglycemia increases complication risk 2