Weigh it, cap it, check it — weight-based dosing and dose calculation

Setting the scene

Role: You are the paediatric registrar leading the prescribing for a seven-year-old admitted to a regional ward. The nurse preparing the first intravenous dose has paused because the order looks large for the child's size. Your task is to recalculate the dose from the beginning, apply every safety check, and communicate the corrected plan to the team and the family. [2] [3]

The encounter

A seven-year-old boy weighing 23 kg, with a height of 122 cm, is admitted with a spreading cellulitis. The admitting order written under time pressure was for a weight-based antimicrobial, and the dose on the chart appears roughly double what the nurse expects. The drug is later confirmed as vancomycin. You are asked to confirm or correct the dose before it is given. [2]

Task 1 — Obtain the foundation (2 minutes)

Take a measured weight in kilograms on a calibrated scale and a height in centimetres, and record both once on the chart. Confirm that the weight was not estimated and was not transcribed in pounds, because a pound weight read as kilograms multiplies the dose by about 2.2 and is a classic source of a large-for-size order. [2]

Task 2 — Recalculate from the formulary (4 minutes)

State that vancomycin is dosed by weight in milligrams per kilogram per dose at an interval set by age and renal function. Calculate the dose from the current paediatric formulary figure, then apply the maximum adult-dose cap, because the weight-based dose never exceeds the adult maximum single or daily dose. Work the arithmetic aloud, apply the leading-zero rule (write 0.5 mg, never .5 mg) and the no-trailing-zero rule (write the dose, never with a trailing .0), and round to a dose the available formulation can deliver. [2] [3]

Task 3 — The independent check (2 minutes)

Ask a second clinician to recalculate the dose independently, without referring to your calculation, and to confirm the cap and the units. State that the independent double check is the single strongest defence against a tenfold decimal error, which is invisible to the eye and lethal in a small body. Confirm the route, the frequency and the formulation are age-appropriate before the dose is given. [3]

Task 4 — The monitoring loop (3 minutes)

Plan the therapeutic drug monitoring: the consensus target for vancomycin in a serious staphylococcal infection is an area-under-the-curve over twenty-four hours of 400 to 600 mg per litre per hour, assessed by a Bayesian approach or a trough drawn just before a dose at steady state. Write the dose time on every sample tube, because a trough drawn too early overestimates the level and a level without a recorded dose time is uninterpretable. State the dose-adjust-measure loop: dose, draw the level at the correct time, interpret against the dose time, adjust toward the target, and re-check the calculation at each step. [7] [8]

Task 5 — Communication and the family (2 minutes)

Tell the family in plain language what the medicine is, why the dose was rechecked, and what to watch for. If the child goes home on an oral medicine, write the discharge dose in milligrams and in millilitres of the specific concentration dispensed, provide an oral dosing syringe rather than a spoon, and check back by teach-back, because the home is where the last calculation is performed. Report the original near miss through the local incident system so the calculation weakness is fixed, not just survived. [2]

Examiner prompts

• "The dose on the chart is roughly double what you expect. Name the two most likely causes before you recalculate." (A pound-versus-kilogram unit error, or a missing adult-dose cap on a larger child; a tenfold error would be ten times, not two.)
• "Work the Mosteller body-surface area for this child and say when you would use it." (122 cm and 23 kg: 122 times 23 is 2806, divided by 3600 is 0.779, square root is 0.88 square metres; used for body-surface-area drugs such as chemotherapy, not for vancomycin.)
• "The level comes back higher than target. Walk me through the differential before you change the dose." (A sample drawn too early, a change in renal clearance, a drug interaction, or an incorrect weight scalar in an obese child; check the dose time on the tube first.) [7]