Blood Glucose Reduction with 1 Unit of Regular Insulin
The insulin sensitivity factor is highly individualized, but 1 unit of regular insulin typically decreases blood glucose by approximately 30-50 mg/dL (1.7-2.8 mmol/L) in insulin-sensitive patients, though this can vary from as little as 15 mg/dL to as much as 100 mg/dL depending on individual insulin sensitivity. 1
Understanding the Insulin Sensitivity Factor
The insulin sensitivity factor (also called correction factor) represents how much blood glucose will decrease with 1 unit of insulin and must be calculated individually for each patient. 1
Common Calculation Methods
The "1500 Rule" for regular insulin: Divide 1500 by the total daily insulin dose to estimate the blood glucose drop (in mg/dL) per 1 unit of insulin 1
- Example: If a patient uses 50 units/day total, 1500 ÷ 50 = 30 mg/dL drop per unit
The "1800 Rule" for rapid-acting analogs: Divide 1800 by total daily dose for rapid-acting insulin analogs 1
Example from insulin pump therapy: A typical ratio might be 1:3, meaning 1 unit of insulin decreases blood glucose by 3 mmol/L (approximately 54 mg/dL) 1
Factors That Modify Insulin Sensitivity
Patient-Specific Variables
Baseline glucose level significantly affects response: Higher baseline glucose concentrations prolong insulin action and may alter the magnitude of glucose reduction 2
- At glucose 276 mg/dL, peak insulin action occurs at 4.7 hours
- At glucose 130 mg/dL, peak action occurs at 4.3 hours 2
Body weight and insulin resistance: Heavier patients and those with greater insulin resistance require more insulin per unit of glucose reduction 1
Time of day variations: Many patients require more insulin per gram of carbohydrate (and thus have different sensitivity factors) in the morning due to counter-regulatory hormones like cortisol and growth hormone 1
Clinical Context: Intravenous Insulin Infusion
Expected Glucose Reduction Rate
In DKA management: Continuous IV regular insulin at 0.1 units/kg/hour typically decreases plasma glucose at 50-75 mg/dL per hour 1, 3
If glucose fails to drop by 50 mg/dL in the first hour: Verify hydration status and double the insulin infusion rate hourly until achieving steady decline of 50-75 mg/dL/hour 1, 3
Critical Monitoring Points
Blood glucose should be checked every 1-2 hours during IV insulin infusion until stable, then every 4 hours 4
When glucose reaches 250-300 mg/dL during DKA treatment: Add dextrose infusion while continuing insulin to prevent hypoglycemia and continue correcting ketosis 4
Common Clinical Pitfalls
Hypoglycemia Risk
In hyperkalaemia treatment: When 10 units of regular insulin is given with 25 g dextrose, the median blood glucose reduction is 24 mg/dL, but 22% of patients develop hypoglycemia (glucose <70 mg/dL) 5
Higher baseline glucose predicts greater reduction: Patients with baseline glucose around 110 mg/dL who received insulin/dextrose for hyperkalaemia dropped to median 52 mg/dL 5
Variable Response Factors
Insulin antibodies do not significantly affect pharmacokinetics, but ambient glucose levels do 2
Subcutaneous absorption varies: Duration of action ranges from 7.7 hours at normal glucose to 9.1 hours during hyperglycemia 2
Pump therapy considerations: The insulin sensitivity factor is preprogrammed and adjusted every 3-6 months based on physiological changes including weight, exercise, and menstruation 1
Practical Application Algorithm
Calculate initial sensitivity factor using 1500 rule (for regular insulin) or 1800 rule (for rapid-acting analogs) 1
Adjust based on observed response: Monitor actual glucose changes after correction doses and refine the factor 1
Account for "insulin on board": Modern pumps calculate remaining active insulin to prevent "stacking" and hypoglycemia 1
Modify for clinical context:
The key principle is that there is no single universal answer—the response must be individualized based on total daily insulin dose, but typical ranges are 30-50 mg/dL per unit for most patients. 1