MDCalc

Insulin Sensitivity Factor (ISF)

Estimates how much 1 unit of insulin will lower blood glucose to help calculate a corrective insulin dose for hyperglycemia.

This dosing tool is intended to assist with calculation, not to provide comprehensive or definitive drug information. 

  • On this page, you will find: 
    • The 1500 Rule for use with short-acting insulin. 
    • The 1800 and 1700 Rules for use with rapid-acting insulin; the default is the 1800 Rule because it is more commonly used and most recently endorsed by endocrinology society guidelines.
  • This tool is also known as the Insulin Correction Factor (CorrF or CF).

  • To best determine the total daily dose (TDD) of insulin, add up all insulin doses, regardless of type, administered to a patient within a 24-hour window. 
  • For best results, calculate the TDD from a day when glycemic targets were achieved and the patient followed their usual diet and activity patterns. 
  • For otherwise healthy adult patients who are new to insulin, starting TDD may also be approximated based on weight: 
    • 0.5 units/kg/day in type 1 diabetes. 
    • 0.3–0.5 units/kg/day in type 2 diabetes.
units/day

Type of insulin

Result:

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Advice

This tool is intended to assist with calculation and should not be the sole basis for dosing; clinicians should use clinical judgment and a full clinical evaluation to individualized care. 

  • Results provide a baseline estimate that should be further refined based on subsequent glycemic values.
  • Before administering insulin, always consider the clinical context, including the influence of the patient’s: 
    • Diet.
    • Activity.
    • Other medications with glycemic effects (e.g., other diabetes medications, steroids). 
  • Corrective insulin doses are given in addition to prandial and basal insulin doses, when applicable. 
    • Insulin on board should also be considered for patients receiving insulin more frequently than every 4 hours for rapid-acting insulin or every 6 hours for short-acting Regular human insulin. 
    • Although additional insulin doses may occasionally be necessary, caution is warranted when administering correctional insulin at intervals shorter than every 4–6 hours, as insulin stacking may result in unpredictable glycemic effects and increase the risk of hypoglycemia.
Management
  • Results estimate how much a patient’s BG will decrease with administration of 1 unit of short-acting insulin.
  • This result, along with the patient’s target BG, can be used to determine how many units of insulin to give for correction of hyperglycemia according to a custom sliding scale:

Table: Example sliding scale insulin plan created from ISF and the upper limit of normal (ULN) for target BG.* 

Blood glucose (mg/dL)

Insulin (units)

≤ (ULN Target BG)

0

(ULN + 1) - (ULN + ISF)

1

(ULN + ISF + 1) - (ULN + 2 * ISF)

2

(ULN + 2 * ISF + 1) - (ULN + 3 * ISF)

3

(ULN + 3 * ISF + 1) - (ULN + 4 * ISF)

4

(ULN + 4 * ISF + 1) - (ULN + 5 * ISF)

5

*Add rows to this table in the same pattern until BG values are >400 mg/dL, above which point a healthcare professional should be contacted for further evaluation and treatment decisions.

  • Another way to determine a corrective insulin dose is to calculate the difference between the patient’s current BG and their target BG, then divide that difference by the ISF:

    • Corrective insulin dose = (Current BG – Target BG) / ISF

    • This is how an insulin pump might use the ISF and target BG setting to determine how much insulin to deliver. 

    • For example, a patient with current BG 310 mg/dL, target BG 110 mg/dL, and ISF 40 mg/dL/unit should receive a corrective insulin dose of 5 units. 

    • This is separate and often in addition to prandial insulin doses, which are given with meals to counteract future postprandial BG elevations.

  • This tool provides a starting point for insulin corrective dosing. 

    • Hypoglycemia or persistence of hyperglycemia despite these doses should prompt modification of the insulin regimen. 

    • With more patient-specific data, ISF may be found to vary throughout the day, leading to meal-specific sliding scales or time-dependent insulin pump programming. 

    • Consult a diabetes specialist for more information.

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