How do you calculate insulin dosage?

How to Calculate Insulin Dosage

Insulin dosing is calculated using three core components: basal insulin (covering background needs), bolus/prandial insulin (covering meals based on carbohydrate intake), and correction insulin (addressing elevated blood glucose), with specific formulas derived from total daily dose (TDD) or body weight.

Initial Insulin Dosing

Type 2 Diabetes - Starting Basal Insulin

• Start with 10 units once daily OR 0.1-0.2 units/kg body weight per day for insulin-naive patients, administered at the same time each day 1, 2, 3
• For severe hyperglycemia (blood glucose ≥300 mg/dL or A1C ≥10%), consider higher starting doses of 0.3-0.5 units/kg/day as total daily dose, split between basal and prandial insulin 1, 2, 3
• Continue metformin unless contraindicated when initiating insulin therapy 1, 2

Type 1 Diabetes - Basal-Bolus Regimen

• Total daily dose = 0.5 units/kg/day for metabolically stable patients 2, 3
• Divide as 50% basal insulin (given once daily) and 50% prandial insulin (divided among three meals) 1, 2, 3
• Higher doses (0.5-1.0 units/kg/day) are needed during puberty, pregnancy, and acute illness 2, 3
• Patients in honeymoon phase may require only 0.2-0.6 units/kg/day 2

Titration Algorithms

Basal Insulin Adjustment

• Increase by 2 units every 3 days if fasting glucose is 140-179 mg/dL 1, 2
• Increase by 4 units every 3 days if fasting glucose is ≥180 mg/dL 1, 2
• Target fasting plasma glucose: 80-130 mg/dL 1, 2
• If hypoglycemia occurs without clear cause, reduce dose by 10-20% immediately 1, 2, 3

Adding Prandial Insulin

• When basal insulin is optimized but A1C remains above goal after 3-6 months, add prandial insulin 1, 2
• Start with 4 units before the largest meal OR 10% of current basal dose 1, 2, 3
• Increase by 1-2 units or 10-15% every 3 days based on postprandial glucose readings 1, 2

Insulin Pump Therapy Calculations

Basal Rate Programming

• Total basal dose = 0.48 × TDD (approximately 30-50% of total daily dose) 1, 4
• Most type 2 diabetes patients require only one basal rate; only 20% need two rates 5
• Basal rates can be adjusted with temporary rates for exercise or alternate rates for different activity patterns 1

Carbohydrate-to-Insulin Ratio (CIR)

• CIR = 300/TDD for breakfast 6
• CIR = 400/TDD for lunch and dinner 6
• Alternative formula: CIR = 365/TDD (averaged across all meals) 5
• The ratio defines grams of carbohydrate covered by 1 unit of insulin 1
• Example: with ratio 1:10, every 10g carbohydrate requires 1 unit insulin 1

Insulin Sensitivity Factor (Correction Factor)

• Correction Factor = 1500/TDD (how much 1 unit lowers glucose in mg/dL) 4, 7
• Alternative formula: CF = 1960/TDD 4
• Example: with ratio 1:3,1 unit decreases blood glucose by 3 mmol/L 1
• Used to correct pre-meal hyperglycemia above target 1

Integrated Dosing Relationship

• The relationship between dosing factors: 100/TBD = ICR = CF/4.5 7
• This allows cross-checking calculations for internal consistency 7

Critical Threshold: Recognizing Overbasalization

When basal insulin exceeds 0.5 units/kg/day, stop escalating and add prandial insulin instead 1, 2

Warning Signs of Overbasalization

• Basal insulin dose >0.5 units/kg/day approaching 1.0 units/kg/day 1, 2
• Bedtime-to-morning glucose differential ≥50 mg/dL 1, 2
• Hypoglycemia episodes (aware or unaware) 1
• High glucose variability 1
• Fasting glucose controlled but postprandial hyperglycemia persists 1, 2

Special Clinical Situations

Hospitalized Patients

• Non-critically ill: 0.3-0.5 units/kg/day total, with 50% as basal and 50% as prandial insulin 2, 3
• For patients on high-dose home insulin (≥0.6 units/kg/day), reduce by 20% upon admission to prevent hypoglycemia 2, 3
• High-risk patients (elderly >65 years, renal failure, poor oral intake): use lower doses of 0.1-0.25 units/kg/day 2, 3

Transitioning from IV to Subcutaneous Insulin

• Total subcutaneous dose = 1/2 of IV insulin infused over 24 hours 1
• Give half as basal insulin once in evening 1
• Divide remaining half by 3 for ultra-rapid analogue before each meal 1

Tube Feeding Patients

• 1 unit insulin per 10-15 grams of carbohydrate in the feeding formula 3
• Basal insulin needs are typically 30-50% of total daily requirement 2
• Start with 5 units NPH/detemir every 12 hours OR 10 units glargine every 24 hours 2

Common Pitfalls to Avoid

• Never continue escalating basal insulin beyond 0.5-1.0 units/kg/day without addressing postprandial hyperglycemia—this leads to overbasalization, suboptimal control, and increased hypoglycemia risk 1, 2
• Do not delay insulin initiation in patients not achieving glycemic goals with oral medications 1, 2
• Avoid waiting longer than 3 days between basal adjustments in stable patients—this unnecessarily prolongs time to target 2
• Never mix or dilute insulin glargine with other insulins due to its low pH 2
• Do not use premixed insulin in hospital settings due to unacceptably high hypoglycemia rates 2
• Always reduce home insulin by 20% when admitting patients on high doses (≥0.6 units/kg/day) 2, 3
• Recognize that basal insulin addresses fasting/between-meal glucose only—postprandial hyperglycemia requires prandial insulin, not more basal insulin 2

Monitoring Requirements

• Daily fasting blood glucose monitoring is essential during titration 1, 2
• Assess adequacy of insulin dose at every clinical visit 1, 2
• Reassess and modify therapy every 3-6 months to avoid therapeutic inertia 1
• Monitor for hypoglycemia triggers: reduced oral intake, inappropriate insulin timing, reduced dextrose infusion, interrupted tube feedings 3