Basal bolus calculation for insulin
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Here is a comprehensive breakdown of basal-bolus insulin calculation:
Basal-Bolus Insulin Calculation
Step 1: Estimate Total Daily Dose (TDD)
The TDD is the foundation for all calculations. Common starting estimates:
| Patient Type | TDD Estimate |
|---|---|
| General starting point | 0.5 units/kg/day |
| Insulin-naive / elderly / renal impairment | 0.2–0.3 units/kg/day (conservative) |
| Insulin-resistant / obese / high steroid use | 0.6–1.0 units/kg/day |
| Type 1 DM (established) | 0.5–0.7 units/kg/day |
Step 2: Split TDD into Basal and Bolus
According to the Developing a Diabetes Mellitus Comprehensive Care Plan 2022 Update (p. 61):
Basal = 40–50% of TDD Bolus (prandial) = 50–60% of TDD
Example (70 kg patient, starting at 0.5 u/kg/day):
- TDD = 35 units
- Basal = ~17–18 units (long-acting: glargine, detemir, degludec)
- Bolus = ~17–18 units total → divided across meals (e.g., ~6 units per meal)
Step 3: Insulin-to-Carbohydrate Ratio (ICR)
Determines how many grams of carbohydrate 1 unit of rapid-acting insulin covers.
| Insulin Type | Formula |
|---|---|
| Rapid-acting (lispro, aspart, glulisine) | ICR = 500 ÷ TDD |
| Regular insulin | ICR = 450 ÷ TDD |
Example (TDD = 50 units, rapid-acting):
- ICR = 500 ÷ 50 = 1:10 (1 unit per 10 g carbs)
- Default starting point if TDD unknown: 1:15 (1 unit per 15 g carbs)
Meal bolus dose = Carbs eaten (g) ÷ ICR
Step 4: Insulin Sensitivity Factor (ISF) / Correction Factor
Estimates how much 1 unit of insulin lowers blood glucose (in mg/dL).
| Insulin Type | Formula |
|---|---|
| Rapid-acting | ISF = 1800 ÷ TDD |
| Regular insulin | ISF = 1500 ÷ TDD |
Example (TDD = 50 units, rapid-acting):
- ISF = 1800 ÷ 50 = 36 mg/dL per unit
Correction bolus dose = (Current BG − Target BG) ÷ ISF
- Common target BG: 100–120 mg/dL preprandial
Step 5: Total Mealtime Bolus Dose
Total bolus = Meal bolus + Correction bolus
Total bolus = (Carbs ÷ ICR) + [(Current BG − Target BG) ÷ ISF]
Worked Example
- Patient: 70 kg, Type 2 DM, starting insulin
- TDD: 0.5 × 70 = 35 units
- Basal: 17 units glargine at bedtime
- ICR: 500 ÷ 35 ≈ 1:14 (round to 1:15 to start)
- ISF: 1800 ÷ 35 ≈ 51 mg/dL/unit
- Pre-dinner scenario: BG = 210, eating 60 g carbs, target BG = 120
- Meal bolus = 60 ÷ 15 = 4 units
- Correction = (210 − 120) ÷ 51 = 90 ÷ 51 ≈ 1.8 units → round to 2 units
- Total dinner bolus = 6 units
Key Clinical Notes
- These formulas are starting points only — all doses must be titrated empirically based on glucose monitoring patterns (Developing a Diabetes Mellitus Comprehensive Care Plan 2022, p. 61).
- Basal titration: adjust by 1–2 units every 3 days targeting fasting glucose at goal.
- Bolus titration: review 2-hour postprandial glucose; adjust ICR if consistently above or below target.
- Avoid "stacking" — account for insulin on board (IOB) before giving correction doses (especially relevant in pump therapy and CGM-integrated systems).
- In Type 1 DM, carbohydrate counting with individualized ICR is the standard of care.
- Mobile apps (e.g., MySugr, Glucose Buddy) can assist with real-time bolus calculations.
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