How do you calculate the dose of an antibiotic for a child? By weight or age?

Follow the directions in the leaflet and the paediatrician’s advice when calculating an antibiotic dose for a child. You must account for the child’s weight or age when calculating the antibiotic dose, as well as the instructions provided in the patient’s leaflet.

Drug dosages, including antibiotic doses, are different for children and adults. Neonatal patients require extra caution when being treated. Due to underdeveloped enzymatic and hormonal activity, decreased kidney filtration, low body weight, and slower therapeutic agent excretion in children, the dosage of the drug must be decreased. All of these factors increase the likelihood of a drug overdose. Drugs taken in high doses can be toxic to a child’s body.

How to calculate the dose of antibiotic for a child?

The dose of antibiotic for a child is generally based on the child’s weight and simple proportions. 

Example: If you need to give your child an antibiotic suspension and you know that:

• the child weighs 20 kg, 
• a single dose of the drug is 7.5 mg / kg of body weight,
• and in 5 ml of suspension there is 250 mg of the drug,

then, to calculate the dose of antibiotic, follow these simple steps:

• 20×7.5=150 mg – this is the recommended single dose of the drug for a child weighing 20 kg,
• then create the proportion: 5 ml –> 250 mg ; x –> 150 mg, so x = 750:250 = 3 ml – this is the amount of suspension that should be given to the child at one time. 

The dosage of antibiotics for a child cannot be determined using a general formula. The best method of calculating the antibiotic dose is to follow the instructions in the leaflet of the given preparation and the doctor’s advice because not all antibiotics are created equally and may have different safety profiles. Formulas for determining the maximum dosages of medications for children can be used in the case of antibiotics with a broad safety range.

How to calculate the dose of medicine for a child?

The highest single and daily drug doses that a child should take can be determined using formulas. Their use is constrained, though. For medications with a broad range of safety, formulas are employed (broad therapeutic index). Since aminoglycoside antibiotics are extremely powerful antibacterial medications and their doses must be very precise, the formulas do not apply to them.

When calculating doses for children, the following are taken into account:

• child’s age,
• body weight,
• body surface.

The following formulas can be used to determine the maximum antibiotic doses for children (as well as other medications). The maximum single dose for an adult is substituted into the formulas in order to determine the drug’s single maximum dose. The maximum daily dose for an adult is substituted in order to determine the daily dose.

Dosage calculation for children under 1 year of age

Fried formula:

X = child’s age in months X maximum adult dose / 150

Calculation of the dose of the drug for infants

Clark formula:

X = maximum adult dose X child’s age in months / 70

Calculation of the dose of the drug for children from 2 to 12 years of age

Young’s formula:

X = maximum adult dose X child’s age in years / (child’s age in years + 12)

Calculation of the dose of the drug for children over 12 years of age

Cowling formula:

X = child’s age in years X maximum adult dose / 24

Calculating the dose of the drug based on the child’s weight

This formula is not recommended for obese children as the result of the calculation may be higher than the recommended dose for the child. 

X = maximum adult dose X child’s weight in kg / 70

How to prepare an antibiotic suspension?

Antibiotics can come in different forms. In the case of a child, drugs in the form of granules are often prescribed, which must be made into a suspension. Dosing an antibiotic in this form may seem difficult . Here are some tips:

1. Before preparing the medicine, wash your hands with soap and water and let the boiled water cool down (hot water can change the effect of the antibiotic).
2. Shake the powder thoroughly.
3. Check the leaflet how much water you need and what capacity is on the medicine package.
4. Add water to the medicine bottle according to the information in the leaflet. 
5. Shake the medicine bottle well to obtain a homogeneous suspension. There must be no powder residue on the walls. 
6. Check the leaflet on how to store the antibiotic : in the fridge or not. 
7. Check the shelf life of the drug in the leaflet and do not use the antibiotic after this date. 
8. Give your child the recommended dose . Manufacturers often add a syringe, spoon or cup to the drugs, with which you can measure the right amount.