Step-by-Step Calculation
To determine the volume for a single dose, the calculation must be performed in a specific sequence to ensure patient safety. First, the total daily dose is calculated based on the child's weight. Next, this total is divided by the number of doses per day to find the amount per single administration. Finally, the ordered mass is converted to a volume using the concentration of the available suspension.
1. Calculate the Total Daily Dose
The prescription is for
45 mg/kg/day. The child weighs
12 kg.
Total daily dose =
45 mg ×
12 =
540 mg/day.
2. Calculate the Single Dose
The total daily dose is to be divided into two equal doses.
Single dose =
540 mg ÷
2 =
270 mg per dose.
3. Calculate the Volume to Administer
The available suspension has a concentration of
200 mg/5 mL. This means that every
5 mL of liquid contains
200 mg of amoxicillin. To find the volume for the
270 mg dose, a standard ratio and proportion or dimensional analysis method is used:
200 mg /
5 mL =
270 mg / X mL
X = (
270 mg ×
5 mL) ÷
200 mg
X =
1350 ÷
200
X =
6.75 mL.
Clinical Reasoning and Safety Analysis
A critical step in pediatric medication administration is the verification of the calculated dose against established safety parameters. Weight-based dosing is the standard of care in pediatrics because it accounts for developmental differences in drug metabolism and clearance, a practice necessitated by the frequent lack of commercially available, ready-to-administer formulations for children
[4]. The calculated single dose of
270 mg (or
22.5 mg/kg/dose) must be cross-checked against recommended dosing guidelines for amoxicillin, which typically range from
20 to 50 mg/kg/day divided into two or three doses for common infections. The prescribed regimen falls within this standard therapeutic range.
However, the volume calculation yields
6.75 mL per dose, which is not one of the provided options. This discrepancy signals that the listed options correspond to a different interpretation of the prescribed order. If the prescriber's order of "
45 mg/kg/day divided into two equal doses" is misinterpreted to mean that
45 mg/kg is the amount for each of the two doses, the calculation changes drastically and unsafely. Let's examine this error pathway: a single dose would be calculated as
45 mg ×
12 kg =
540 mg. The volume for this erroneous dose would be (
540 mg ×
5 mL) ÷
200 mg =
13.5 mL. This volume is not among the options either.
The correct answer provided,
4.8 mL, is derived from a different, hypothetical prescription or a calculation error. For
4.8 mL to be the correct single dose volume, the single dose mass would be (
4.8 mL ÷
5 mL) ×
200 mg =
192 mg. For this child,
192 mg represents a dosage of
16 mg/kg/dose, or
32 mg/kg/day. This does not match the stated prescription of
45 mg/kg/day. This highlights a fundamental principle in pediatric pharmacotherapy: the extreme vigilance required when calculating and preparing extemporaneous doses. The use of a concentrated suspension (
200 mg/5mL) requires a precise volume measurement, as small errors in drawing up the liquid can lead to significant overdosing or underdosing
[4]. In the context of antibiotic stewardship, ensuring the exact, weight-correct dose is not only a safety issue but also a measure to combat antimicrobial resistance by avoiding sub-therapeutic dosing [1, 2]. The process demands that the nurse independently verifies the mg/kg dose against the child's weight and the clinical indication before administration, a barrier identified as crucial in preventing improper dosing .
References (research sources)