How to find dosage of insulin In a patient
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insulin dosing initiation type 2 diabetes
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Insulin Dosage - How to Find the Right Dose
Insulin dosing is individualized based on the patient's diabetes type, body weight, insulin sensitivity, blood glucose levels, diet, and clinical setting. There is no single fixed dose. The approach involves calculating a Total Daily Dose (TDD), then splitting it into basal and prandial (bolus) components, with a correction factor on top.
Step 1: Calculate the Total Daily Dose (TDD)
Type 1 Diabetes
| Clinical scenario | Starting TDD |
|---|---|
| Newly diagnosed (conservative) | 0.4 units/kg/day |
| Average non-obese patient | 0.5-0.8 units/kg/day |
| Full physiological replacement | 0.3-1.0 units/kg/day |
| Obese, insulin-resistant, adolescent, late pregnancy | Higher doses required |
Example: A 70 kg Type 1 patient - conservative start = 0.4 × 70 = 28 units/day
- Washington Manual of Medical Therapeutics and Goldman-Cecil Medicine
Type 2 Diabetes - Basal Insulin Initiation
When starting insulin in T2DM (when oral agents are no longer sufficient), the approach is:
- Start: 10 units/day (flat dose) OR 0.1-0.2 units/kg/day of long-acting basal insulin once daily
- Titrate by 2 units every 3 days (or per fasting glucose target) until fasting BG is consistently below target (~130 mg/dL)
Step 2: Split the TDD into Basal and Bolus
Once TDD is established, distribute as follows:
| Component | Proportion of TDD | Timing |
|---|---|---|
| Basal insulin | 40-50% of TDD | Once daily (glargine/degludec) or twice daily (NPH/detemir) |
| Prandial (bolus) insulin | 50-60% of TDD | Split equally across 3 meals (~1/3 each) |
Example: TDD = 30 units → Basal = 15 units glargine at night + Prandial = 5 units rapid-acting before each of 3 meals
- Goldman-Cecil Medicine, Washington Manual
Step 3: Calculate Prandial (Meal) Dose
Two methods:
A. Carbohydrate Counting (ICR - Insulin:Carb Ratio)
- Starting rule: 1 unit per 10-15 g of carbohydrate consumed
- Example: A meal with 60g carbs → 60/15 = 4 units rapid-acting insulin
B. Weight-Based Prandial Dose
-
0.1 unit/kg per meal
-
Example: 100 kg patient → 10 units rapid-acting before each meal
-
Check 2-hour postprandial glucose: if rise is 0-50 mg/dL above pre-meal, the dose was correct
-
Textbook of Family Medicine 9e, Goldman-Cecil Medicine
Step 4: Add a Correction Factor (Supplemental Dose)
The correction factor (CF) is used when the pre-meal glucose is above target. It tells you how many units of insulin will drop the glucose by a set amount.
"1800 Rule" (for rapid-acting insulin):
CF = 1800 ÷ TDD
This gives the mg/dL drop expected per 1 unit of rapid-acting insulin.
Examples from Harrison's Principles:
| Patient type | Correction factor |
|---|---|
| Thin/insulin-sensitive | 1 unit lowers BG by 50 mg/dL |
| Obese/insulin-resistant | 1 unit lowers BG by ~25 mg/dL (i.e., 2 units per 50 mg/dL over target) |
Correction dose = (Current BG - Target BG) ÷ CF
Important: Always account for insulin-on-board (active insulin from the previous injection) before giving a correction dose. Rapid-acting insulin stays active for up to 6 hours. Giving a correction before the previous dose is absorbed causes "insulin stacking" - a major cause of hypoglycemia.
- Harrison's Principles of Internal Medicine 22E, Textbook of Family Medicine 9e
Step 5: Titrate the Dose
Adjust the regimen based on:
- Fasting glucose - reflects adequacy of basal dose → adjust basal by 5-10% daily until fasting BG < 130 mg/dL
- 2-hour postprandial glucose - reflects prandial dose → target rise of 0-50 mg/dL above pre-meal
- A1C every 3 months
Insulin Types Quick Reference (Goldman-Cecil)
| Type | Onset | Peak | Duration |
|---|---|---|---|
| Rapid-acting (lispro, aspart, glulisine) | 15-30 min | 30-90 min | 3-4 hrs |
| Regular (short-acting) | ~30 min | 2-4 hrs | 5-8 hrs |
| NPH (intermediate) | 2-4 hrs | 6-10 hrs | 16-20 hrs |
| Glargine/detemir (long-acting) | 1.5-2 hrs | Peakless | 16-20 hrs |
| Degludec (ultra-long) | 1.5-2 hrs | Peakless | >24 hrs |
Special Situations
- ICU / critically ill: Use IV insulin infusion (regular insulin); half-life is minutes, allowing tight titration. Start long-acting insulin 2-4 hours before stopping infusion.
- Hospital (non-ICU): Basal + prandial + correction SC regimen. "Sliding scale alone" (reactive only) is inadequate.
- Pregnancy (Type 1): Bolus insulin needs increase from ~50% to ~75% of TDD by 36 weeks' gestation.
- Adolescents / Obesity / Steroids: Significantly higher TDD may be needed (up to 1.5-2 units/kg/day in some cases).
Key Practical Rules Summary
- TDD = 0.4-0.5 units/kg/day to start (conservative); adjust upward
- 50% basal / 50% bolus split
- Prandial = 1 unit per 10-15g carbs, or 0.1 unit/kg/meal
- Correction = (Current BG - Target) ÷ CF; CF = 1800 ÷ TDD
- Always check insulin-on-board to avoid stacking and hypoglycemia
- Titrate using fasting BG (for basal) and 2-hour post-meal BG (for bolus)
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