Insulin sensitivity factor (correction factor)

The insulin sensitivity factor (ISF) tells you by how many mg/dl your blood glucose drops for each unit of rapid insulin [1]. For example, a factor of 54 means that one unit of insulin lowers blood glucose by 54 mg/dl (3 mmol/L). For an initial, approximate value, use the 1800 rule divided by the total daily insulin dose (for rapid analogs) [1].

If you use 40 U of insulin in total per day, your factor would be approximately 1800÷40=45 mg/dl (2.5 mmol/L). This calculation provides only a starting point. The actual factor differs by hour and is determined through testing [2]. Like the insulin-to-carbohydrate ratio, the ISF can vary throughout the day. Most patients have higher sensitivity at night (one unit lowers blood glucose more) and lower sensitivity in the morning [3].

For a correction, calculate: (current blood glucose - target blood glucose) ÷ sensitivity factor = units needed [4]. If you are at 250 mg/dl (13.9 mmol/L), your target is 100 mg/dl (5.6 mmol/L) and the sensitivity (correction) factor is 50, you calculate: (250-100)÷50=3 units. Round to the nearest half unit for a pen or 0.1 for an insulin pump.

Always check the active insulin (IOB = "insulin on board") before a correction to avoid overlapping doses [5]. Most rapid analog insulins act for four hours [6]. If you made a correction two hours ago, you still have approximately 40% of that dose active, because it is consumed faster initially and more slowly later. At night, give only 50% of the correction calculated for a daytime situation (or use a higher ISF, specific for night). This nocturnal percentage or ISF should be adjusted individually, together with your doctor, based on your history of nocturnal hypoglycemia [3]. Don't make aggressive corrections before exercise or when you intend to drink alcohol.

Insulin sensitivity varies throughout the day [3]. You are more resistant to the action of insulin in the morning because of the increased secretion of "counter-regulatory" hormones, such as cortisol, glucagon, and growth hormone [7]. You are more sensitive to the action of insulin in the afternoon and at night. The sensitivity factor can be 30 in the morning and 60 at night for the same person [2]. Physical activity increases insulin sensitivity for 24 hours, while stress or illness decrease it [8] [9].

Test the factor separately for different times of day: morning (6-10), afternoon (12-17), evening (18-21) and night (23-5) [2]. Many patients use insulin pumps precisely so they can program different insulin sensitivity factors throughout the day.

Don't make corrections in the first 1-2 hours after a meal [5]. During this interval the meal insulin is still acting strongly and you risk hypoglycemia from insulin overaccumulation. Avoid corrections before exercising, when you have drunk alcohol, or when your blood glucose shows a rapid downward arrow on the sensor [8].

At night, be very cautious with correction doses [3]. Use only half of the calculated dose, and only for blood glucose above 180 mg/dl (10 mmol/L). With experience you can gradually lower this threshold. During illness, corrections can be ineffective because of stress hormones [9]. In that case, larger and more frequent doses may be needed [10]. Avoid making corrections when you cannot monitor your blood glucose over the following hours, or when you are alone, without any possibility of help in the event of severe hypoglycemia.

Correctly calculate the active insulin before a new correction [5]. Most pump calculators do this automatically. If you use an insulin pen, assume that after one hour you still have 75% of the rapid analog insulin active, after two hours 40%, and after three hours 20% [6]. Don't make corrections for blood glucose below 150 mg/dl (8.3 mmol/L), especially if you still have active insulin on board. Use the sensitivity factor tested for that time slot and adjust it periodically.

Check your blood glucose two hours after a correction. If you have frequent hypoglycemia after corrections, your factor is too aggressive and should probably be increased (for example, by 10%) [1]. Take the trend arrows on your CGM into account. If your blood glucose is already dropping, wait for it to stabilize. Always keep fast-acting carbohydrates within reach when making corrections, especially at night.

Wait two hours between corrections for rapid analog insulin [5]. Two hours are needed to estimate the full effect of a dose and to avoid insulin overaccumulation. Rapid analog insulin peaks at one hour but continues to act for four hours [6]. Corrections that are too frequent are an important cause of severe hypoglycemia, because the previous dose is still active and overlaps with the new dose [5].

The only exception is blood glucose above 300 mg/dl (16.7 mmol/L), especially with positive ketones in the urine [10]. In that case you can make corrections hourly, with careful monitoring of how the blood glucose evolves. If you have an insulin pump, its bolus calculator automatically accounts for the active insulin, if you use it [4]. Monitor and use the glucose trend displayed by your glucose sensor to estimate correction doses more accurately.

Yes, insulin sensitivity changes significantly with age [11]. Young children are very sensitive to insulin, adolescents at puberty become resistant because of sex and growth hormones, and young adults return to moderate sensitivity. After the age of 50, sensitivity can increase again, requiring adjustments to avoid hypoglycemia.

At puberty, insulin needs can even double (the sensitivity factor drops), because of growth and sex hormones [11]. Girls may have cyclical variations linked to menstruation. Adjust the factor every few months during the growth period and whenever you notice that corrections no longer work well. Parents of children with diabetes must always be prepared for such changes.

For the most accurate testing, start with a stable blood glucose above 180 mg/dl (10 mmol/L), at least four hours after the last meal bolus [6]. Don't test on days with exercise, illness, major stress, or alcohol [8] [10]. Make the calculated correction and measure your blood glucose every hour for three hours. The blood glucose should drop gradually to near target, without any tendency toward hypoglycemia.

If the blood glucose drops too much (a risk of hypoglycemia appears), the factor is too small and should be increased. If it doesn't drop enough, the factor is too large and should be decreased [1]. Changes could be of 10%, testing the result before any new change. Test the correction factor (insulin sensitivity factor) on several different days for greater certainty when making adjustment decisions. Test separately for different times of day [2]. Carefully document the starting blood glucose, the insulin dose, the subsequent blood glucose values, and other factors that can influence how it evolves.

During illness, stress hormones (cortisol, adrenaline) raise blood glucose and reduce insulin sensitivity [9]. Your usual correction factor can temporarily halve. During this period you should expect more aggressive corrections (150% of the normal dose), repeated more often (every 1-2 hours) [10]. Check for ketone bodies if your blood glucose persists above 250 mg/dl (13.9 mmol/L) for more than three hours, or sooner if you have symptoms. If they are present, insulin needs increase even more [10].

Hydrate as well as possible, monitor your blood glucose every 1-2 hours, and never stop your basal insulin, even if you are not eating [10]. Establish a sick-day protocol with your doctor. If your blood glucose stays above 320 mg/dl (20 mmol/L) for more than six hours despite repeated corrections, you have high ketone levels and significant nausea (or vomiting), go to the emergency room, because you may already have quite advanced diabetic ketoacidosis.

A correction can fail for several reasons. Check whether the insulin has deteriorated (exposure to extreme temperatures, expiration, changed appearance) and that it is not injected into areas with lipodystrophy, where absorption is unpredictable [13]. Other reasons include a significant infection or inflammation (which raise stress hormones) or an occluded pump set (the cannula has bent) [9].

First check the insulin (appearance, expiration date), the injection site (rotation, lipodystrophy), and the technique (depth, leaks) [13]. Consider physiological factors such as acute stress, the start of menstruation, medications (corticosteroids), or an exaggerated dawn phenomenon [7] [9]. If everything seems normal but corrections fail consistently, temporarily increase the factor and consult your doctor.