How is the '500 rule' applied to a 40kg child with newly diagnosed type 1 diabetes (T1D) to estimate and adjust the total daily dose of insulin based on blood glucose levels?

The 500 Rule in Pediatric Type 1 Diabetes

For a 40kg child with newly diagnosed type 1 diabetes, start with a total daily dose of 20-40 units (0.5-1.0 units/kg/day), divided 50% basal and 50% prandial insulin, then use the 500 rule to calculate insulin-to-carbohydrate ratios by dividing 500 by the total daily dose—though evidence shows this rule significantly underestimates actual insulin needs, particularly at breakfast. 1, 2

Initial Total Daily Dose Calculation

• Begin with 0.5 units/kg/day (20 units total) if the child is prepubertal, metabolically stable, and did not present with diabetic ketoacidosis. 1
• Use 1.0 units/kg/day (40 units total) if the child presented with DKA, is on steroids, or is pubertal. 1
• Divide this total dose equally: 50% as basal insulin (long-acting) once daily and 50% as prandial insulin (rapid-acting) distributed across three meals. 1, 2

Applying the 500 Rule for Carbohydrate Counting

• Calculate the insulin-to-carbohydrate ratio (ICR) by dividing 500 by the total daily dose. 3
• For a 40kg child on 20 units/day: 500 ÷ 20 = 25 grams of carbohydrate per 1 unit of insulin. 3
• For a 40kg child on 40 units/day: 500 ÷ 40 = 12.5 grams of carbohydrate per 1 unit of insulin. 3

Critical Limitation: The 500 Rule Underestimates Insulin Needs

Research demonstrates that prepubertal children require substantially more bolus insulin than the 500 rule predicts, particularly at breakfast. 3, 4

• Actual ICR values in well-controlled prepubertal children show a median "rule" of 211 for breakfast (meaning 211/TDD, not 500/TDD) and 434 for other meals. 3
• A prospective study of 201 children found that using formulas of 301-309/TDD for morning, 317-331/TDD for afternoon, and 362-376/TDD for evening meals provided more accurate initial estimates than the 500 rule. 4
• Bolus insulin requirements are highest at breakfast due to counter-regulatory hormones like cortisol and growth hormone, requiring more aggressive dosing in the morning. 3, 4

Adjusting Insulin Based on Blood Glucose Patterns

Basal Insulin Adjustment

• Titrate basal insulin based on fasting blood glucose patterns over several days, not single readings. 5
• Increase basal insulin by 1-2 units every 2-3 days if fasting glucose consistently exceeds 130 mg/dL (7.2 mmol/L). 5
• Decrease basal insulin by 10-20% if hypoglycemia occurs or fasting glucose drops below 80 mg/dL (4.4 mmol/L). 5

Prandial Insulin Adjustment

• Adjust ICR if postprandial glucose (2 hours after meals) is consistently out of target range. 5
• If postprandial glucose is consistently >180 mg/dL, decrease the ICR denominator (give more insulin per gram of carbohydrate). 5
• If postprandial hypoglycemia occurs, increase the ICR denominator (give less insulin per gram of carbohydrate). 5

Correction Factor (Insulin Sensitivity Factor)

• Calculate the correction factor using the 100 rule: 100 ÷ total daily dose = mg/dL drop per 1 unit of insulin. 3
• For a child on 20 units/day: 100 ÷ 20 = 5 mg/dL drop per unit. 3
• For a child on 40 units/day: 100 ÷ 40 = 2.5 mg/dL drop per unit. 3
• Adjust the correction factor by 10-20% if correction doses consistently fail to bring glucose into target or cause hypoglycemia. 5

Special Considerations for Pediatric Patients

Honeymoon Phase

• Within weeks of starting insulin, many children enter a "honeymoon phase" where insulin requirements may drop well below 0.5 units/kg/day. 1
• During this phase, children may require only minimal basal insulin with small prandial doses. 1
• Continue monitoring closely and reduce doses as needed to prevent hypoglycemia. 1

Pubertal Children

• Pubertal children may require insulin doses as high as 1.5 units/kg/day (60 units/day for a 40kg child) due to growth hormone and sex hormone effects. 1
• Expect increasing insulin resistance during puberty and adjust doses upward accordingly. 1, 2

Young Children with Erratic Eating

• Consider administering rapid-acting insulin after meals in young children with unpredictable food intake to more accurately match insulin to actual carbohydrate consumption. 2
• In children with predictable eating patterns, pre-meal dosing results in better postprandial glucose control. 2

Monitoring Requirements

• Perform at least four glucose tests daily (before meals and at bedtime) to achieve optimal control. 2
• Include preprandial, postprandial (2 hours after meals), and nocturnal glucose measurements for comprehensive dose adjustments. 2
• Target fasting glucose of 80-130 mg/dL (4.4-7.2 mmol/L) and postprandial glucose <180 mg/dL (10 mmol/L). 5
• Aim for HbA1c <7% (53 mmol/mol) while avoiding severe hypoglycemia. 6

Common Pitfalls to Avoid

• Do not rely solely on the 500 rule without empiric adjustment—it consistently underestimates insulin needs, especially at breakfast. 3, 4
• Do not use the same ICR for all meals—morning insulin requirements are typically 40-50% higher than afternoon or evening meals. 3, 4
• Do not recalculate total daily dose daily—adjust ICR and correction factors based on patterns over 3-7 days, not single readings. 5
• Do not delay dose adjustments when patterns emerge—timely titration every 2-3 days is essential for achieving glycemic targets. 5
• Do not use diluted insulin without proper training—infants and toddlers may require diluted insulin or 0.5-unit increment pens for precise dosing. 1