Diabetes: Testing Your Correction Factor

Important information: 

Your correction factor, or insulin sensitivity factor (ISF), is how much 1 unit of bolus insulin (or fast-acting insulin) is expected to lower your blood sugar level. Your correction factor tells you how much fast-acting insulin to give to correct high blood sugar.

  • If your correction factor is right for your body, the correction bolus will bring your blood sugar to 20 points above or 20 points below your target blood sugar range. Your correction factor will change over time.

  • Test your correction factor if your sugars have been running too high or too low within 3 hours of administering a correction bolus.   

Instructions for testing your correction factor:

  1. Check and record your blood sugar level and the time you start the test

  • Start the test at least 4 hours after the last meal or correction bolus was given.

  • Make sure the last meal eaten was a low-fat, low-protein meal. Extra protein and fat in a meal increases your blood sugar for 4 to 5 hours after you eat.

  • Start the test when blood sugar is above 200.

  1. Give your correction bolus. Use your current correction factor.

  2. Avoid exercise and carbohydrates for the next 3 hours. They affect blood sugar.

  3. Re-check your blood sugar level 3 hours after the correction bolus.

  4. Record your blood sugar levels, the times taken, and bolus insulin amount to share with your diabetes care team. You can use your blood glucose meter to do a fingerstick or use the sensor glucose reading from your continuous glucose monitor. 

Calculating your correction bolus:

Follow these steps to figure out how much insulin to take when your blood sugar is high:

  1. Subtract your target blood sugar from your current blood sugar.

  2. Divide that number by your correction factor. This gives you your total correction bolus.

  3. Round your bolus insulin:

  • If you use whole units only: Round the total down to the nearest whole number.

  • If you use half units: Divide the total by 2, then round down to the nearest half-unit number.

Example:

Current blood sugar = 210

Target blood sugar = 120

Correction factor = 50

  • 210 minus 120 = 90

  • 90 divided by 50 = 1.8 units

  • If using whole units: take 1 unit of insulin

  • If using half units: 1.8 divided 2 = 0.9 → round down to 0.5 units of insulin

Understanding the results

  • If your blood sugar is within 20 points above or below your target blood sugar, your correction bolus worked and does not need to be changed.

  • If your blood sugar is lower, your correction bolus was too much. Decrease your correction factor. For example, 1 unit for every 50 mg/dL becomes 1 unit for every 60 mg/dL

  • If your blood sugar is higher, your correction bolus was not enough. Increase your correction factor. For example, 1 unit for every 50 mg/dL becomes 1 unit for every 40 mg/dL. 

Tips for safety

If your blood sugar is less than 70 mg/dL, treat with 15 grams of carbohydrate and stop the test. 

If your blood sugar is above 300 mg/dL, stop the test, check ketones and follow the ketone protocol. When ketones are cleared, you can try the test again. 

 

Reviewed July 2025 by Faith Daily, RN, BSN, CDCES, CPT