Calculating Correction Factors and Ketone Corrections
The calculations completed for basal insulin needs and bolus insulin needs for carbohydrate coverage represent 100% of the total daily dose (TDD). Despite this, additional calculations are needed to determine how much rapid-acting insulin should be given for the correction of hyperglycemia (the correction factor) and the correction for the presence of ketones (ketone correction). These calculations for “correction insulin” are discussed below.
More rapid-acting insulin is given in addition to the insulin needed for carbohydrate coverage when there is pre-prandial hyperglycemia in an effort to “correct” the post-prandial blood glucose (2-3 hours after injection) to within the normal target blood glucose range. The extra insulin needed for this correction of hyperglycemia is called the correction factor. The importance of this correction factor is demonstrated in Figure 1.
The first step in deriving a patient’s correction factor is determining their insulin sensitivity factor (ISF). ISF is an estimate of insulin sensitivity and represents the theoretical total number of points that a patient’s blood glucose is lowered after administering a single (1) unit of insulin. For example, an ISF of 50 means that 1 unit of insulin will lower a patient’s blood glucose by 50 points (e.g. from 200mg/dL to 150mg/dL).
ISF is calculated using the “1800-rule”:
Example: Calculating ISF from the 1800-Rule:
- Using our 6-year-old patient from prior examples with TDD of 10.2 u/day, what is the patient’s ISF?
- ISF = 1800/TDD
- ISF = 1800/10.2
- ISF = 176
- Thus, 1 unit of insulin will theoretically drop this patient’s blood sugar by 176 points, and thus, 0.5 units of insulin will drop it by half of this (176/2) = ~90 points.
- Using our 14-year-old patient from prior examples with TDD of 44.3 u/day, what is the patient’s ISF?
- ISF = 1800/TDD
- ISF = 1800/44.3
- ISF = 41
- Thus, 1 unit of insulin will theoretically drop this patient’s blood sugar by 41 points.
Once the ISF is determined, this is combined with the desired blood glucose above which to start correcting for hyperglycemia so that blood glucose can return to a normal range. The resulting combination is the correction factor.
Correction Factor = 1 unit for every [ISF] over [target blood glucose]
As a general rule of thumb:
- ISF is generally rounded to the nearest 25 to 50-point increment
- Target blood glucose is adjusted in increments of 50 (starting from 150)
The correction factor is then utilized to generate a correction factor scale, which allows for easy use and visualization of the additional amount of insulin needed to be added to carbohydrate coverage at meals.
Example: Defining the Correction Factor
- From our prior examples, our 6-year-old was found to have an ISF of 176. What is his correction factor and corresponding correction factor scale?
- 1 unit of insulin will drop his BG by 176 points (indicating he is very sensitive to insulin)
- 0.5 units of insulin will drop his BG by ~90 points, which can be rounded to 100 (per our rule of thumb)
- As the normal glycemic range is 70-150mg/dL, starting to correct BG > 150mg/dL with this patient’s ISF would potentially cause hypoglycemia (as 151mg/dL – 100 = 51mg/dL). Therefore, this patient’s target blood glucose should be set to 200, so that the patient receives his first 0.5 unit correction when BG is 201 – 300.
- This way, if he is given 0.5 units at a BG of 201, his lowest predicted BG would be 101mg/dL
- Therefore his correction factor is: 0.5 units for every 100 over 200
- His corresponding correction factor scale would be written as:
- 201 – 300, give + 0.5 unit
- 301 – 400, give +1 unit
- 401 – 500, give +1.5 units
- >500, call house officer
- As the normal glycemic range is 70-150mg/dL, starting to correct BG > 150mg/dL with this patient’s ISF would potentially cause hypoglycemia (as 151mg/dL – 100 = 51mg/dL). Therefore, this patient’s target blood glucose should be set to 200, so that the patient receives his first 0.5 unit correction when BG is 201 – 300.
- From our prior examples, our 14-year-old was found to have an ISF of 41. What is her correction factor and corresponding correction factor scale?
- 1 unit of insulin will drop BG by 41 points (indicating average to mild insulin resistance)
- Can be rounded to 50 points (per our rule of thumb)
- Correcting for BG starting > 150mg/dL would result in correction of most BG back into target range of 70-150mg/dL
- Therefore, her correction factor is: 1 unit for every 50 > 150mg/dL
- Her corresponding correction factor scale is:
- 151 – 200, give + 1 unit
- 201 – 250, give +2 units
- 251 – 300, give +3 units
- 301 – 350, give +4 units
- 351 – 400, give +5 units
- 401 – 450, give +6 units
- 451 – 500, give +7 units
- >500, call house officer
- 1 unit of insulin will drop BG by 41 points (indicating average to mild insulin resistance)
At other institutions, families may be taught a different method for hyperglycemia correction that allows for a more precise determination of the amount of insulin needed for hyperglycemia correction. However, this method requires higher math skills, which may be difficult for many families. This correction factor is calculated as follows:
Hyperglycemia correction dose (in units) = [Current BG – Target BG]/ISF
Additional information about the protocol for the management of hyperglycemia here at the University of Iowa Stead Family Children’s Hospital is discussed further in Unit 4.
The last step in generating a basal/bolus insulin regimen is the calculation of the ketone corrections, which is needed if a patient develops ketonemia or ketonuria.
Management of ketones will vary from one institution to another. Here at the Stead Family Children’s Hospital, a ketone correction consists of giving:
10% of TDD for small or moderate ketones
20% of TDD for large ketones
Example: Calculating Ketone Corrections
- What would the ketone corrections be for our 6-year-old from our prior examples with TDD of 10.2 u/day?
- For small to moderate ketones:
- 10% of TDD
- 0.1 x 10.2 u/day
- =1 unit for small/moderate ketone correction
- For large ketones:
- 20% TDD
- 0.2 x 10.2u/day (or 2 x the small/moderate ketone correction)
- =2 units for large ketone correction
- For small to moderate ketones:
- What would the ketone corrections be for our 14-year-old from our prior examples with a TDD of 44.3u/day?
- Small/Mod ketone correction = 10% TDD = 0.1 x 44.3 = 4.4 units -> rounded to nearst 1/2 unit = 4.5 units
- Large ketone correction = 20% TDD = 0.2 x 44.3 = 8.8 units -> round to 9 units
Our institution’s specific protocol for treating ketones is discussed further in Unit 4: “Ketone Management”.
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