How do I calculate insulin dosing for a patient?

Insulin Dosing Calculation

Insulin dosing is calculated using weight-based formulas that differ fundamentally between Type 1 and Type 2 diabetes, with Type 1 requiring 0.5 units/kg/day split 50% basal/50% prandial, while Type 2 typically starts at 10 units or 0.1-0.2 units/kg/day of basal insulin alone. 1

Type 1 Diabetes: Basal-Bolus Regimen Required

Type 1 diabetes patients require both basal and prandial insulin from the outset—dosing based solely on premeal glucose levels without accounting for basal requirements or caloric intake increases the risk of both hypoglycemia and hyperglycemia. 2

Initial Total Daily Dose Calculation

• Start with 0.5 units/kg/day as the total daily insulin dose for metabolically stable Type 1 patients 1
• Divide this dose: 50% as basal insulin (given once daily as glargine/degludec or twice daily as NPH/detemir) and 50% as prandial insulin split among three meals 1
• Total daily requirements typically range from 0.4-1.0 units/kg/day, with higher doses needed during puberty, pregnancy, or acute illness 1
• Patients in the honeymoon phase may require lower doses of 0.2-0.6 units/kg/day 1

Example Calculation for Type 1

For a 70 kg patient with Type 1 diabetes:

• Total daily dose = 70 kg × 0.5 = 35 units/day
• Basal insulin (glargine) = 35 × 0.5 = 17.5 units once daily
• Prandial insulin (rapid-acting) = 35 × 0.5 = 17.5 units total, divided as approximately 6 units before breakfast, 6 units before lunch, 5.5 units before dinner 1

Advanced Pump Calculations for Type 1

• Basal insulin comprises 40-60% of total daily dose in pump therapy (not the traditional 50%) 1
• Carbohydrate-to-insulin ratio (CIR) = 300/TDD for breakfast, or 400/TDD for lunch and dinner 3, 4
• Insulin sensitivity factor (ISF) = 1500/TDD or 1700/TDD depending on individual response 1, 4
• For the 70 kg patient above with TDD of 35 units: CIR = 300/35 = 8.6 grams carb per unit at breakfast and 400/35 = 11.4 grams per unit at other meals; ISF = 1500/35 = 43 mg/dL drop per unit 3, 4

Type 2 Diabetes: Start with Basal Insulin Only

For Type 2 diabetes, begin with basal insulin alone at 10 units once daily or 0.1-0.2 units/kg/day, continuing metformin unless contraindicated. 1

Initial Dosing Algorithm

• Standard starting dose: 10 units once daily of long-acting insulin (glargine, degludec, detemir) at the same time each day 1
• Weight-based alternative: 0.1-0.2 units/kg/day for insulin-naive patients 1
• Severe hyperglycemia (A1C ≥9%, glucose ≥300-350 mg/dL): Consider higher starting doses of 0.3-0.5 units/kg/day as total daily dose, requiring immediate basal-bolus therapy 1

Titration Protocol

• Increase by 2 units every 3 days if fasting glucose is 140-179 mg/dL 1
• Increase by 4 units every 3 days if fasting glucose is ≥180 mg/dL 1
• Target fasting glucose: 80-130 mg/dL 1
• If hypoglycemia occurs: Reduce dose by 10-20% immediately 1

Critical Threshold: When to Add Prandial Insulin

When basal insulin exceeds 0.5 units/kg/day and approaches 1.0 units/kg/day, add prandial insulin rather than continuing to escalate basal insulin alone—this prevents "overbasalization." 1

• Clinical signs of overbasalization include: basal dose >0.5 units/kg/day, bedtime-to-morning glucose differential ≥50 mg/dL, hypoglycemia, and high glucose variability 1
• Start prandial insulin with 4 units of rapid-acting insulin before the largest meal, or use 10% of current basal dose 1
• Titrate prandial insulin by 1-2 units or 10-15% every 3 days based on postprandial glucose readings 1

Special Populations Requiring Dose Adjustments

Hospitalized Patients

• Insulin-naive or low-dose patients: Start with 0.3-0.5 units/kg/day total daily dose, giving half as basal insulin 1
• High-dose home insulin (≥0.6 units/kg/day): Reduce total daily dose by 20% upon admission to prevent hypoglycemia 1
• High-risk patients (elderly >65 years, renal failure, poor oral intake): Use lower doses of 0.1-0.25 units/kg/day 1

Critical Care Setting

• Continuous intravenous insulin infusion is the most effective method for achieving glycemic goals in critical care 2
• Use validated written or computerized protocols that allow predefined adjustments based on glycemic fluctuations 2

Transitioning from IV to Subcutaneous Insulin

• Calculate total subcutaneous dose based on insulin infusion rate during prior 6-8 hours when stable glycemic goals were achieved 2
• Give subcutaneous basal insulin 2 hours before discontinuing IV infusion to minimize rebound hyperglycemia 2
• Consider adding low-dose basal insulin analog (0.15-0.3 units/kg) in addition to IV insulin to reduce duration of infusion and prevent rebound hyperglycemia 2

NPH Insulin Dosing (When Long-Acting Analogs Unavailable)

Split-Dose Regimen

• Divide total NPH dose: 2/3 before breakfast and 1/3 before dinner 5
• When converting from bedtime-only NPH to split regimen: reduce total dose to 80% of current bedtime dose, then divide according to 2/3:1/3 ratio 5
• Add 4 units of short/rapid-acting insulin to each injection, or 10% of the reduced NPH dose, for prandial coverage 5

Initial Once-Daily NPH

• Start with 10 units at bedtime or 0.1-0.2 units/kg/day 5
• Titrate by increasing 2 units every 3 days to reach fasting glucose goal 5

Common Pitfalls to Avoid

Medication Errors

• Insulin is one of the most common medications causing adverse events in hospitalized patients, with errors occurring at prescriber, pharmacy, and nursing levels 2
• Errors include incorrect insulin type, incorrect timing, missed doses, and improper dosing 2

Hypoglycemia Risk Factors

• Kidney failure is a major risk factor for hypoglycemia in hospitals, possibly due to decreased insulin clearance 2
• Improper prescribing of other glucose-lowering medications and inappropriate management of first hypoglycemia episode are common preventable causes 2
• All hypoglycemic episodes should be documented and treatment plans reviewed when blood glucose <70 mg/dL 2

Dosing Errors

• Do not dose insulin based solely on premeal glucose levels in Type 1 diabetes—this ignores basal requirements and caloric intake 2
• Do not continue escalating basal insulin beyond 0.5-1.0 units/kg/day without addressing postprandial hyperglycemia—this leads to overbasalization with increased hypoglycemia risk 1
• Do not delay insulin initiation in Type 2 patients not achieving glycemic goals with oral medications 1
• Do not use premixed insulin in hospital settings due to unacceptably high rates of iatrogenic hypoglycemia 1

Monitoring Requirements

• Daily fasting blood glucose monitoring is essential during titration 1
• Assess adequacy of insulin dose at every clinical visit, looking for signs of overbasalization 1
• Reassess and modify therapy every 3-6 months to avoid therapeutic inertia 1