The Insulin Sensitivity Factor (ISF), often called the correction factor, is a personalized measurement used in diabetes management to determine appropriate insulin dosing. This metric quantifies how much a person’s blood glucose level is expected to drop after administering one unit of rapid-acting insulin. Knowing this factor is fundamental for safely and accurately correcting high blood sugar levels back down to a target range. An accurate ISF allows for individualized dosing, helping prevent dangerous lows (hypoglycemia) or prolonged highs (hyperglycemia).
Understanding the Role of Insulin Sensitivity Factor
The Insulin Sensitivity Factor serves a distinct purpose from the Insulin-to-Carbohydrate Ratio (ICR), which calculates mealtime insulin doses. The ISF is used exclusively for correction dosing—administering insulin to correct existing high blood glucose levels. It is generally added to the mealtime dose only when the pre-meal glucose reading is above the target level.
This factor reflects the body’s responsiveness to injected insulin: a higher ISF number indicates greater insulin sensitivity, while a lower number suggests insulin resistance. For example, if a person’s ISF is 50, one unit of rapid-acting insulin is anticipated to lower the blood glucose concentration by 50 milligrams per deciliter (mg/dL). To calculate the correction dose, the difference between the current and target blood glucose is divided by the ISF number.
Calculating Your Initial ISF (The 1800 Rule)
The most common method for establishing a starting estimate of the Insulin Sensitivity Factor is the “1800 Rule.” This rule provides a provisional calculation based on the total amount of insulin a person uses each day. The formula is 1800 divided by the Total Daily Dose (TDD) of insulin, which yields the estimated blood glucose drop per unit of rapid-acting insulin.
The TDD is the sum of all basal (long-acting) and bolus (short or rapid-acting) insulin units administered over a full 24-hour period. If daily intake varies, doses should be averaged over several days to establish a representative TDD. For instance, if a person uses 40 units of total insulin per day, the calculation is 1800 divided by 40, which equals 45. This means the initial estimated ISF is 45. The 1800 Rule provides a practical starting point, but it represents an educated guess that requires further personalization and supervision from a healthcare professional.
Testing and Validating Your ISF
The initial ISF derived from the 1800 Rule must be validated through controlled observation to ensure accuracy. To properly test the factor, the blood glucose level should be stable and elevated, ideally between 150 and 250 mg/dL. Successful validation requires that there is no active insulin on board (AIOB) from a previous bolus and that no food has been consumed for several hours (typically four to five hours).
Once these conditions are met, a correction dose is calculated using the initial ISF estimate and administered. The blood glucose level should then be monitored every one to two hours for the next four to six hours, which is the typical duration of action for rapid-acting insulin. The goal is for the final reading to land within 20 mg/dL of the target without causing hypoglycemia.
If the resulting drop in glucose is less than expected, the ISF number needs to be decreased. Conversely, if the drop is too large, the ISF number should be increased.
Daily Variables Affecting Sensitivity
Even after validation, the Insulin Sensitivity Factor is not a fixed number and is subject to daily fluctuations caused by various physiological and lifestyle factors. The time of day is a significant variable, as sensitivity is often lower in the evening compared to the morning due to natural circadian rhythms. This contributes to the “dawn effect,” where morning glucose levels may rise without a clear cause.
Physical activity directly influences how the body responds to insulin, with exercise generally improving sensitivity and reducing insulin needs. Conversely, states of stress or illness release hormones like cortisol, which temporarily increase insulin resistance. This means a higher correction dose may be temporarily required.
Additionally, hormonal cycles in women can cause predictable changes in sensitivity throughout the month. Inadequate sleep has also been shown to reduce insulin sensitivity. Recognizing these temporary influences allows individuals to make minor, flexible adjustments to their dosing without permanently altering their validated ISF.