📘 Insulin sensitivity 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 approximate calculation, use the 1800 rule divided by the total daily insulin dose (for rapid analogs) [1].

If you use 40 units 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 hours and is determined through testing [2]. Like the insulin-to-carb ratio, ISF can vary throughout the day. Most patients have higher sensitivity at night (one unit lowers blood glucose more) and lower in the morning [3].

For correction, calculate: (current blood glucose - target blood glucose) ÷ sensitivity factor = units needed [4]. If you have 250 mg/dl (13.9 mmol/L), your target is 100 mg/dl (5.6 mmol/L) and the 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 insulin on board (IOB) before correction to avoid dose stacking [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 slower later. At night, give only 50% of the calculated correction for a daytime situation (or use a higher ISF, specially for nighttime), but this percentage should be individually adjusted 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 insulin action in the morning due to increased secretion of "counter-regulatory" hormones, such as cortisol, glucagon, growth hormone [7]. You are more sensitive to insulin action 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 [8], while stress or illness decrease it [9].

Test the factor separately for different times of day, morning (6-12), afternoon (12-18), evening (18-24) and night (24-6) [2]. Many patients use pumps precisely to program different insulin sensitivity factors, automatically, throughout the day.

Don't make corrections in the first 1-2 hours after a meal [5]. In this interval the meal insulin still acts strongly and you risk hypoglycemia through insulin stacking. Avoid corrections before exercising [8], when you have consumed alcohol or when blood glucose has a downward arrow on the sensor indicating rapid drop.

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). In case of illness, corrections can be ineffective due to stress hormones [9]. In this case larger and more frequent doses may be needed [10]. Avoid making corrections when you cannot monitor blood glucose in the following hours or when you are alone without possibility of help in case of hypoglycemia.

Correctly calculate insulin on board before a new correction [5]. Most pump calculators do this automatically. If you have an insulin pen, consider that after one hour you still have 75% of insulin still active, after two hours 40%, 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 tested sensitivity factor and adjust it periodically.

Check blood glucose two hours after correction. If you have frequent hypoglycemia after corrections, your factor is too aggressive and should probably be increased by 18 mg/dl (1 mmol/L) [1]. Take into account trend arrows on CGM. If blood glucose is already dropping, wait for it to stabilize. Always keep fast carbs on hand 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 avoid insulin stacking. Rapid analog insulin has peak action at one hour but continues to act for four hours [6]. Too frequent corrections are an important cause of severe hypoglycemia, because the previous dose is still active and overlaps with the new dose [5].

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

Yes, insulin sensitivity changes significantly with age [11]. Young children are very sensitive to insulin, adolescents at puberty become resistant due to sex and growth hormones [12], and young adults return to moderate sensitivity. After 50 years, sensitivity can increase again, requiring adjustments to avoid hypoglycemia.

At puberty, insulin needs can double (the sensitivity factor decreases), due to growth and sex hormones [11]. Girls may have cyclical variations with 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 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 [8], illness [10], major stress or alcohol. Make the calculated correction and measure blood glucose every hour for three hours. Blood glucose should gradually drop to near target, without approaching hypoglycemia.

If blood glucose drops too much (risk of hypoglycemia), 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 10%, with testing of the result before a new change. Test the correction factor (insulin sensitivity factor) on several different days for more certainty in making modification decisions. Test separately for different times of day [2]. Carefully document starting blood glucose, insulin dose, subsequent blood glucose and other factors that can influence its evolution.

During illness, stress hormones (cortisol, adrenaline) increase blood glucose and reduce insulin sensitivity [9]. Your usual correction factor can temporarily even halve. During this period you must expect more aggressive corrections (150% of normal dose) and repeated more often (every 1-2 hours) [10]. Check ketone bodies if 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 blood glucose every 1-2 hours and never stop basal insulin, even if you don't eat [10]. Establish with your doctor a protocol for sick days. If blood glucose remains above 320 mg/dl (20 mmol/L) for more than six hours with repeated corrections, you have high ketone bodies and significant nausea (or vomiting), go to the emergency room because you may have diabetic ketoacidosis.

Correction can fail for several reasons. Check if the insulin has deteriorated (exposure to extreme temperatures, expiration, changed appearance) and that it's not injected in areas with lipodystrophy that absorb unpredictably [13]. Other reasons include a significant infection or inflammation (stress hormones increase) [9] or the pump set is occluded (the cannula has bent).

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