Diabetes Q&A: 70/30; "Rule of Fifths"; Split-Mix; Basal-Bolus-- Which Is Best?

February 1, 2007

During my training, most pediatric endocrinologists were using the "70/30 rule" or the "rule of fifths" to determine the insulin dosage for patients with new-onset diabetes. Now that I am in practice, I have heard that carbohydrate counting is a more accurate method of determining a child's insulin needs.

Q: During my training, most pediatric endocrinologists were using the "70/30 rule" or the "rule of fifths" to determine the insulin dosage for patients with new-onset diabetes. Now that I am in practice, I have heard that carbohydrate counting is a more accurate method of determining a child's insulin needs.

Is carbohydrate counting, in fact, a better approach to determining the insulin dosage for a patient with newly diagnosed diabetes? If so, how is this done for both regular and long-acting insulin? Do age and weight play a role?

A: There is no one "right"method for determining the appropriate initial insulin dosage for children with new-onset type 1 diabetes (T1D). Choosing an insulin regimen often involves a trade-off between accuracy and simplicity.

The split-mix regimen. Until recently, most pediatric patients with T1D were started on a "2-shot split-mix" regimen, which combined short- and long-acting insulins in a single injection given twice daily at a total daily dose of approximately 1 unit/kg. Typically, two thirds of the total daily dose was given at breakfast and one third before dinner. Two thirds of the morning dose was given as long-acting insulin (such as NPH or Lente) and one third as short-acting (such as Regular or one of the rapid-acting analogs). Half of the evening dose was given as long-acting and half as short-acting insulin. For example, for a child who weighs 36 kg, the initial insulin doses would be 8 lispro/aspart plus 16 NPH at breakfast and 6 lispro/aspart plus 6 NPH at dinner.

Many clinicians are now moving away from the 2-shot split-mix regimens because they are usually insufficient to meet intensive glycemic goals without unacceptable swings in blood sugar levels. Three-shot regimens move the dinner NPH dose to bedtime to provide better overnight coverage. Extra "touch-up" doses of short-acting insulin are often needed at lunch or in the midafternoon. The split-mix regimen does require consistency and regularity in meal portions and timing.

The "basal-bolus" method. With the development of once-daily long-acting insulin analogs (glargine and detemir), many clinicians have completely abandoned the split-mix regimen in favor of the basal-bolus method. This more physiologic insulin replacement strategy uses a single dose of a long-acting "basal" insulin to provide background insulin coverage, plus "bolus" doses of rapid-acting insulin analogs for every meal and snack. This method does require that the patient and/or caregiver have some knowledge of carbohydrate counting:

•Infants and toddlers may require 1 unit of rapid-acting insulin for every 20 to 50 g of carbohydrate.

•Preadolescents may require 1 unit per 10 to 20 g.

•Adolescents may require 1 unit per 5 to 10 g.

Alternatively, the insulin-to-carbohydrate ratio (ICR) may be determined by the "450 rule." To determine the ICR, divide 450 by the child's total daily dose of insulin (TDD). For example, for a child with a TDD of 36 units, the ICR would be 450/36 = 12.5, or 1 unit per 12 g of carbohydrate. The basal-bolus strategy allows more freedom in the amounts and timing of meals.

As always, the "best" method for determining insulin dosing should take into account a realistic assessment of the family's and child's abilities and life situation. Many older children and adolescents will tolerate 4 or 5 injections per day for flexibility in meal content and timing. Younger children typically prefer to eat consistently and have fewer shots.

Generally, we still use a 2-shot regimen during the first several months after diagnosis, while patients still have some residual insulin secretion (the "honeymoon period"). After the honeymoon--when blood sugar levels cannot be controlled adequately with 3 shots daily--we make the transition to basal-bolus therapy. This involves either multiple daily injection therapy using glargine or detemir plus aspart or lispro, or continuous subcutaneous insulin therapy (the pump).