How do doctors calculate medication doses for children?

The growth and development of children is a continuous and gradual process. In this process, as they age, their body structure and physiological functions are in a continuous development period. Their pharmacological characteristics are significantly different from those of adults, so the conversion of children's medication dosage is more particular. It is good that there are drug specifications suitable for children. The manufacturer will mark the dosage for children on the instructions. So, if there is no indication in the instructions, how do doctors and pharmacists calculate the dosage for children?

There are many methods for calculating dosage for children, including age conversion method, weight conversion method, body surface area conversion method, etc. Among them, the method of calculating dosage based on age is rarely used now because there are many factors that affect the accuracy of dosage, so it will not be discussed in this article.

Calculated based on child's weight

If the dosage per kilogram of a child's body weight is known, simply multiply it by the weight to get the daily or single dose. For example, for oral cefaclor, the dosage is marked as 20-40 mg per kilogram of body weight per day, given in 3 divided doses. If the child weighs 15 kg, then (20-40) × 15 = 300-600 mg, taken in 3 divided doses, i.e. 100-200 mg each time.

If you do not know the dosage per kilogram of child body weight, you can calculate it according to the following formula: Child dosage = adult dosage × child weight / 70 (kg)

If the result is not an integer, it can be slightly adjusted to facilitate medication. When calculating the dosage for older children by weight, the lower limit of the dosage should be used to avoid excessive dosage. Conversely, the upper limit of the dosage can be used for infants and young children to prevent low dosage.

Calculated based on body surface area

In recent years, foreign countries have recommended that drugs be calculated based on children's body surface area, which is suitable for both children and adults and is more scientific.

For those weighing less than 30 kg, the formula for calculating the body surface area is: child's body surface area (square meters) = weight (kg) × 0.035 + 0.1.

Note that this formula is not applicable to children weighing more than 30 kg. For children over 10 years old, for every 5 kg increase in weight, the body surface area increases by 0.1 square meters. For example, 30 kg = 1.15 square meters, 35 kg = 1.25 square meters, and 50 kg = 1.55 square meters. For those weighing more than 50 kg, for every 10 kg increase in weight, the body surface area increases by 0.1 square meters.

If the dose per square meter of body surface area is known, then:

Child dose = drug dose per square meter of body surface area × child body surface area (square meters)

If the dose per square meter of body surface area is unknown, it can be calculated as follows:

Child dose = adult dose × child body surface area (per square meter) / 1.73 (per square meter)

Calculation of dosage of Chinese medicine for children

The calculation of dosage of Chinese medicine for children is different from that of Western medicine. Generally, it is divided into four types according to age. For children under 1 year old, use 1/4 of the adult dosage; for children aged 3 to 4 years old, use 1/3 of the adult dosage; for children aged 4 to 7 years old, use 1/2 of the adult dosage; for children aged 7 to 15 years old, use 2/3 of the adult dosage; for children aged 15 years and above, use the adult dosage.

When using any of the above methods to calculate the dosage, it is necessary to combine it with the specific conditions of the child to obtain a more accurate drug dosage. For example, newborns or infants have poor renal function, so the general drug dosage should be smaller; children with severe illness should use a larger dosage than children with mild illness; drugs that must pass through the blood-brain barrier to work, such as sulfonamides or penicillins for the treatment of purulent meningitis, should also have a larger dosage.

Author: Liu Yanping Chen Ye

Shanghai Pudong New Area People's Hospital