What is the insulin correction factor (insulin sensitivity factor) and how is it calculated for an adult with type 1 diabetes using the 1800 rule, and how do I determine the correction dose?

Insulin Correction Factor: Definition, Calculation, and Clinical Application

What Is the Insulin Correction Factor?

The insulin correction factor (also called insulin sensitivity factor or ISF) tells you how much one unit of rapid-acting insulin will lower your blood glucose above a target level—typically expressed as mg/dL per unit of insulin. 1

For example, if your correction factor is 30, then 1 unit of rapid-acting insulin will lower your blood glucose by approximately 30 mg/dL. 1

Calculating the Correction Factor Using the 1800 Rule

For adults with type 1 diabetes using ultra-long-acting basal insulin analogs (glargine 300 U/mL or degludec), the 1800 rule provides the most accurate estimate: divide 1800 by your total daily insulin dose (TDD). 2

The Formula

• Correction Factor (ISF) = 1800 ÷ Total Daily Insulin Dose 2
• Alternative formulas include 1500 ÷ TDD or 1700 ÷ TDD, but the 1800 rule has been validated specifically for patients using ultra-long-acting basal analogs 1, 2

Example Calculation

If your total daily insulin dose is 60 units:

• ISF = 1800 ÷ 60 = 30 mg/dL per unit 2
• This means 1 unit of rapid-acting insulin will lower your blood glucose by 30 mg/dL 1

Determining the Correction Dose

To calculate how much correction insulin you need, subtract your target glucose from your current glucose, then divide by your correction factor. 1, 3

The Correction Dose Formula

Correction Dose = (Current Glucose − Target Glucose) ÷ Correction Factor 1, 3

Practical Example

• Current glucose: 250 mg/dL
• Target glucose: 120 mg/dL
• Correction factor: 30 mg/dL per unit

Correction Dose = (250 − 120) ÷ 30 = 4.3 units (round to 4 units) 1, 3

Critical Consideration: Diurnal Variation

The correction factor varies throughout the day, with morning correction doses requiring significantly more insulin than afternoon or evening doses due to counter-regulatory hormones like cortisol and growth hormone. 2, 3

• The 1800 rule accurately predicts morning correction factor requirements 2
• For afternoon and evening, the actual correction factor is significantly higher (meaning you need less insulin per mg/dL) than the 1800 rule predicts 2
• You should calculate separate correction factors for morning, afternoon, and evening to optimize glucose control 2, 3

Time-Specific Adjustment

A practical approach: use the 1800 rule for morning corrections, but recognize you may need 20-30% less insulin for the same glucose elevation in the afternoon or evening. 2

Relationship Between Correction Factor and Carbohydrate Ratio

Your correction factor is mathematically related to your insulin-to-carbohydrate ratio (ICR), and both can be estimated from your total daily dose. 4, 5

The relationship can be expressed as:

• Correction Factor = 4.5 × ICR 4
• Or: 100 ÷ TBD = ICR = CF ÷ 4.5 (where TBD is total basal dose) 4

Linear regression analysis shows that ISF (in mg/dL) = 5.14 × ICR for the same time of day, with 95% accuracy. 2

When to Recalculate Your Correction Factor

Recalculate your correction factor every 3-6 months, or whenever there are significant changes in weight, activity level, or overall insulin requirements. 1

Specific Triggers for Reassessment

• Correction doses consistently fail to bring glucose into target range 1
• Post-correction glucose levels (checked 2-4 hours after correction) are consistently too high or too low 1
• Changes in total daily insulin dose of more than 10-15% 1
• Illness, steroid use, or major changes in physical activity 1

Avoiding Insulin Stacking

Never give another correction dose within 3-4 hours of the previous rapid-acting insulin dose, as the first dose is still active ("insulin-on-board"). 1, 3

Modern insulin pumps and bolus calculators automatically account for insulin-on-board by subtracting remaining active insulin from the calculated correction dose. 1

Common Pitfalls

• Using a single correction factor for the entire day ignores diurnal variation in insulin sensitivity and leads to morning hyperglycemia or afternoon/evening hypoglycemia 2, 3
• Adjusting basal insulin when the problem is inadequate correction dosing—if correction doses fail to normalize glucose, adjust the correction factor, not the basal dose 1
• Daily recalculation of total daily dose for correction purposes—TDD should be recalculated periodically (every few weeks to months), not daily 1
• Failing to account for insulin-on-board leads to insulin stacking and hypoglycemia 1, 3

Integration with Meal Insulin

Your total mealtime insulin dose combines carbohydrate coverage plus correction insulin minus any insulin-on-board. 3

Total Bolus = (Carbs ÷ ICR) + [(Current Glucose − Target) ÷ CF] − Insulin-on-Board 3

Hospital and Special Settings

In hospitalized patients, correction insulin should supplement scheduled basal and prandial insulin, never serve as monotherapy. 6, 1

For non-critically ill hospitalized adults:

• Add 2 units of rapid-acting insulin when pre-meal glucose >250 mg/dL 6
• Add 4 units when pre-meal glucose >350 mg/dL 6
• Target glucose range: 140-180 mg/dL 6