What should the carbohydrate-to-insulin (carb) ratio be for someone taking 40 mg of prednisone and 19 units of NPH (Neutral Protamine Hagedorn) insulin?

Carbohydrate Ratio for Steroid-Induced Hyperglycemia

For a patient on 40 mg prednisone taking 19 units of NPH insulin, start with a carbohydrate ratio of 1:10 (1 unit of rapid-acting insulin per 10 grams of carbohydrate), with the understanding that prandial insulin requirements will likely need to increase by 40-60% above this baseline due to the high-dose glucocorticoid effect. 1

Understanding the Insulin Requirements

The 40 mg prednisone dose qualifies as a high-dose glucocorticoid, which creates substantial insulin resistance particularly during daytime hours. 1 The American Diabetes Association guidelines specifically state that for higher doses of glucocorticoids, increasing doses of prandial and correction insulin by 40-60% or more are often needed in addition to basal insulin. 1

Standard Starting Point

• Initial carbohydrate ratio: 1 unit of rapid-acting insulin per 10-15 grams of carbohydrate 1
• For this patient on high-dose steroids, use the more aggressive end: 1:10 ratio 2
• This translates to 1 unit of rapid-acting insulin for every 10 grams of carbohydrate consumed 1

Expected Adjustments for High-Dose Steroids

Because prednisone causes disproportionate hyperglycemia during the day (peaking 4-6 hours after morning administration), you will likely need to: 1

• Increase lunch and dinner prandial insulin by 40-60% above the 1:10 ratio 1
• This means the effective ratio may become 1:6 to 1:7 for afternoon and evening meals 2
• Breakfast may require less aggressive adjustment since steroid effect hasn't peaked yet 3

Correction Insulin Scale

Initial correction scale: 1 unit of rapid-acting insulin for every 40-50 mg/dL above target (150 mg/dL) 2

• More aggressive correction will be needed in the afternoon and evening when steroid effect peaks 2
• Consider using 1 unit per 30-40 mg/dL above target for post-lunch and post-dinner corrections 2

Monitoring Protocol

Blood glucose monitoring every 2-4 hours initially, with special attention to afternoon and evening values when steroid effect peaks: 2

• Target blood glucose range: 80-180 mg/dL 2
• Expect blood glucose to normalize overnight regardless of treatment 1, 2
• Fasting glucose alone is misleading and will underestimate daytime insulin needs 2

Critical Timing Considerations

The NPH insulin should be administered in the morning to coincide with prednisone administration, as both peak at 4-6 hours: 1, 2

• NPH peaks 4-6 hours after administration 1
• Prednisone reaches peak plasma levels 4-6 hours after ingestion 1
• This synchronization is essential for optimal glycemic control 2

Daily Adjustment Algorithm

1. If afternoon/evening glucose consistently >180 mg/dL: Increase prandial insulin ratio from 1:10 to 1:8, then to 1:6 if needed 2
2. If hypoglycemia occurs: Reduce the corresponding meal's insulin dose by 10-20% 2, 4
3. If fasting glucose <100 mg/dL: This predicts next-day hypoglycemia risk; consider reducing evening prandial insulin 1

Common Pitfalls to Avoid

• Do not rely on fasting glucose alone to guide insulin dosing in steroid-induced hyperglycemia, as this leads to under-treatment of daytime hyperglycemia and potential nocturnal hypoglycemia 2
• Avoid using only long-acting insulin without adding NPH, as this provides inadequate coverage of daytime hyperglycemia 2
• Be prepared for rapid changes: Insulin requirements typically decrease dramatically within days of steroid dose reduction, requiring prompt 10-20% dose reductions to avoid hypoglycemia 2, 4
• Do not use the same carb ratio for all meals: Lunch and dinner typically require more aggressive ratios (1:6-1:7) compared to breakfast (1:10) 2