Updated 28 June 2020
The point is that if antibiotic dosing in children is serious business, it is also easy. Be mindful, but remain practical.
Use a reversion to the mean approach that should cover the large number of circumstances for most primary care settings. Because it is primary care and not, say, a university hospital clinic, most patients for all intents and purposes will be previously reasonably well and, so, you can expect that they will be appropriate weight for age. But one look at them will confirm that, if you are in doubt.
A one year old child, for instance, will invariably be close to the 10 kg weight mark. Of course, some infants will weigh more and some less (obvious at a glance of any percentile weight chart) but, generally speaking, 10 kg is the plumb-line from which to work with in one-year-olds.
Children’s antibiotic suspensions (most if not all children’s oral drugs, in fact), come in concentrations for the dosing to conveniently fall within the relative proximity of 5 mL.
The dose should never be an order of magnitude higher or lower than that 5 mL guide.
If calculations suggest 50 mL per dose or, conversely 0.5 mL per dose, for any ORAL formulation, then chances are you have miscalculated. Time to recheck!
According to the American Academy of Pediatrics (AAP), for most acute conditions—such as acute otitis media (AOM)—amoxicillin is dosed at 40 mg/kg of the child’s weight, given in two divided doses. A one-year old child of 10-kg weight will need 40 x 10 = 400 mg amoxicillin each DAY (or 200 mg in each of two doses per day).
In Australia, amoxicillin, which remains first-line treatment for AOM, comes in three differing suspension strengths for convenient childhood dosing: somewhere in the vicinity of 5 mL or a traditional tea-spoon full, including for neonates.
- 100 mg / mL [20 mL bottle with dropper]: neonate
- 125 mg / 5mL [100 mL bottle]: infant / toddler
- 250 mg / 5 mL [100 mL bottle]: pre-school / school-aged
Prescribe a daily dose of 400 mg amoxicillin for a one-year-old (∼10 kg) child with otitis media: i.e. 200 mg amoxicillin (4 mL of amoxicillin 250 mg / 5 mL concentration) for each of two daily doses. That is, 4 mL BID. That’s it.
A weight-for-age one-year-old with otitis media needs 4 mL of amoxicillin (250/5) twice a day. End of story.
Problem: A one year old child has an inflamed, bulging right tympanic membrane and they are off their food with fever and have vomited. There is no history of allergies.
Antibiotic Treatment: Amoxicillin 250/5, 4 mL BD PO x 5 days.
Question: What antibiotic and dose will you give a weight-for-age one-year old vomiting child with otitis media?
Answer: If no allergy to penicillin, 4 mL BD of the 250/5 concentration of amoxicillin.
It’s that easy. How easy? 1 year-old, otitis media, no allergies: 4 mL BD amoxicillin (250/5) for 5 days. Too easy.
Take a look at the (one-year-old) child and confirm that they’re about 10 kg. Ask the parents for confirmation if you prefer. Ask about any allergies. Then dose the 250/5 Amoxicillin at 4 mL twice a day.
How much? 4 mL twice a day
Say what? 4 mL twice a day
If the child is 9.2 kg or 10.6 kg in weight, you know quickly and easily for all intents and purposes that you are safely and effectively treating that child with 4 mL BD of 250/5 amoxicillin.
One-year old, middle ear infection: 4 mL amoxicillin (250/5) twice a day
And if the infection is serious, recurrent, or the child looks significantly ill, approximately double the daily dose by adjusting the individual dosage and frequency to 2.5 mL three times a day. Good work need not equate with pedantry.
Traditional child dosages for amoxicillin, said to be 20 mg/kg, are based on 1970s trial data. But we know that with respect to pharmacokinetics, children are not small adults:
- composition of intestinal fluid and gut permeability varies in childhood
- gastric pH decreases during infancy to reach adult levels (pH ~ 2) at 2 years of age
- volume of distribution changes throughout childhood
- infants have a higher percentage of extracellular water than older children
- infants have lower concentrations of circulating plasma proteins and, consequently, lower drug-protein binding
- Cytochrome P450 enzymatic changes over the first few months of life mean children may even exceed adult levels at about 2 years of age
- hepatic glucoronidation is thought to take at least 3 years to achieve full activity
- GFR does not reach adult levels until about 2 years of age¹
Dosage regimens based entirely on age are often inaccurate and may lead to adverse effects, toxicity or lack of clinical effect.¹
Because of all these variable parameters, childhood age classes are often divided commensurately into:
When dosing in children, refer to a paediatric reference source rather than to references that use extrapolations and approximations from adult data.
Convenient Tip
But at the same time you need to have a simple rule you can use when you do not have a lot of time. For those who are comfortable in dosing, a middle-line approach night take the half-way point between the traditional 20 mg/kg/day and the AAP’s 40 mg/kg/day: i.e. 30 mg/kg/day.
At three times a day dosing, that conveniently equals 10 mg/kg per DOSE and makes for an easy calculation: 30 mg/kg/day means a 10 kg child requires 300 mg of amoxicillin in a day.
Across three divided doses, that equates to 100 mg per dose which, in mg, is conveniently ten times the child’s weight.
A 10 kg child will need 100 mg per DOSE of amoxicillin using a thrice-daily dosing regimen.
100 mg a DOSE is easy: 4 mL of the 125/5.
- 10 kg child: 100 mg, or 4 mL, TID (125/5)
- 12 kg child: 120 mg, or about 5 mL, TID (125/5)
- 15 kg child: 150 mg, or about 3 mL, TID (250/5)
Notice the pattern, weight and dosing: 10 kg —> 100 mg, 12 kg —> 120 mg, 15 kg —> 150 mg.
30 mg/kg per day means 10 mg/kg per dose in three divided doses. That way you can easily multiply the child’s weight by 10 to calculate the per DOSE in mg from which to quickly translate to volume (mL) according to the concentration of suspension you prescribe or dispense.
So when using TID dosing at the 30 mg/kg/day dosage, the calculation becomes straightforward: 10 mg/kg per DOSE — and we all know that multiplication using 10 (especially when busy) is easier.
Child antibiotic dosing, is it child’s play? It is important. It is not a game. But neither need it be complicated and time-consuming or another impost on an already-taxed analytical capacity.
Annotations
- 125/5 just means 25 mg active drug in each 1 mL suspension
- 250/5 just means 50 mg active drug in each 1 mL suspension
Simple.
30 mg/kg/day
Similar formulaic conversion applies to many other antibiotics. Consider:
- erythromycin, for penicillin allergic or atypical, often dosed at the convenient 30 mg/kg/day [avoid in neonates (increased risk for pyloric stenosis) and cardiac arrhythmia (arrhythmogenic)]
- cephalexin, for UTI, also often dosed at 30 mg/kg/day
- metronidazole, for intestinal infestations, you could do worse than dose at 30 mg/kg/day, though that would be getting toward, but not at, the upper limit of dosing
- phenoxymethyl penicillin benzathine (oral penicillin) with a range between 15-50 mg/kg/day can also conveniently be dosed at 30 mg/kg/day (10 mg/kg/dose) at TID intervals
Downloads
CdM-Antibio1-DosageGuidelines-Children-en
Clinical Practice Guidelines _ Antibiotics– RCH
References
O’Hara, Kate. “Paediatric pharmacokinetics and drug doses.” Aust Prescr 2016;39:208–10. Available at http://dx.doi.org/10.18773/austprescr.2016.071. Accessed 28 June 2020.
