Paeds SAQs · clinical-pharmacology-and-therapeutics
Weight-based dosing, body-surface area and dose calculation — formative SAQs
Two MedVellum formative short-answer questions on paediatric weight-based dosing, body-surface-area calculation and dose-calculation safety: the Mosteller formula worked at the bedside, the maximum adult-dose cap, the leading-zero and no-trailing-zero rules, the choice of weight scalar in obesity, and the therapeutic drug monitoring loop for vancomycin and gentamicin. The marks and timing support transparent self-assessment. They are not an official board format or pass standard.
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Target exams
SAQ 1 — An adolescent requiring a body-surface-area dose
Question 1 — 10 formative marks; suggested time 15 minutes [1]
A 14-year-old girl weighing 52 kg with a height of 160 cm is to start a medicine that is dosed at 50 mg per square metre per dose, with a maximum adult single dose of 90 mg per dose. The formulary states the dose is given once daily. The ward nurse asks you to calculate and write the dose. [1]
- State the formula you will use to estimate the body-surface area, and work the calculation for this child. (3 marks)
- What is the calculated dose in milligrams, and what is the maximum adult single dose cap you must apply? (2 marks)
- State the two writing rules that defend against a tenfold decimal error, and explain why each matters. (3 marks)
- Name the final step in the calculation pathway before the drug is given, and one safeguard that should accompany it. (2 marks) [1]
Full-credit answer — SAQ 1
Reveal full-credit answer for SAQ 1
1. Body-surface-area formula and calculation
The bedside formula is the Mosteller formula: body-surface area in square metres equals the square root of the height in centimetres multiplied by the weight in kilograms, divided by 3600. For this child, height 160 cm and weight 52 kg gives 160 times 52 equals 8320, divided by 3600 equals 2.311, and the square root of 2.311 equals 1.52 square metres. [1]
2. Calculated dose and the adult cap
The calculated dose is 1.52 square metres times 50 mg per square metre, which equals 76 mg per dose. The maximum adult single dose stated by the formulary is 90 mg per dose, so the cap does not bind here and the prescribed dose is 76 mg once daily. The cap is checked explicitly, because the principle is that the weight-based or body-surface-area dose never exceeds the adult maximum single or daily dose; in a larger adolescent the cap would bind and the dose would stop at the adult ceiling. [1]
3. The two writing rules
The first rule is the leading zero: write 0.5 mg, never .5 mg, because a bare decimal can be misread as 5 mg and produce a tenfold overdose. The second rule is no trailing zero: write 5 mg, never 5.0 mg, because the trailing zero can be misread as 50 mg and also produce a tenfold overdose. Both rules exist because a misplaced decimal is invisible to the eye and lethal in a small body, and the independent double check is the second defence that catches what the writing rule prevents. [3]
4. Final step and safeguard
The final step before the drug is given is an independent double check by a second clinician, who recalculates the dose, confirms the cap and confirms the units without referring to the original calculation. The accompanying safeguard is to confirm the route, the frequency and the formulation are age-appropriate, and that the concentration of any oral liquid is the one the calculation assumed. [2] [3]
SAQ 2 — An overweight child on vancomycin
Question 2 — 10 formative marks; suggested time 15 minutes [8]
An 11-year-old boy with a body mass index above the ninety-fifth centile, weighing 58 kg, is admitted with a complicated skin infection and started on vancomycin. He has normal renal function. The team is debating whether to use total body weight or ideal body weight for the dose, and how to monitor the response. [8]
- State the therapeutic drug monitoring target for vancomycin in a serious staphylococcal infection, and how the sample is timed. (3 marks)
- Outline the principle for choosing the weight scalar in an obese child for a narrow-therapeutic-index drug, and state the consequence of defaulting to total body weight for every drug. (3 marks)
- Describe the dose-adjust-measure loop you will run over the first days of treatment. (2 marks)
- Name two non-dosing causes of a vancomycin level that is higher or lower than expected despite a correct calculation. (2 marks) [8]
Full-credit answer — SAQ 2
Reveal full-credit answer for SAQ 2
1. Therapeutic drug monitoring target and sampling
The consensus target for vancomycin in a serious methicillin-resistant Staphylococcus aureus 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 once steady state is reached. The dose time is written on the sample tube, because a trough drawn too early overestimates the level and a level without a recorded dose time is uninterpretable. [7] [8]
2. Weight scalar in obesity
The principle is to choose the scalar deliberately rather than by habit: total body weight for drugs that distribute into fat, ideal body weight for water-soluble narrow-therapeutic-index drugs, and adjusted body weight as a middle path for drugs with partial distribution into fat. Defaulting to total body weight for every drug gives water-soluble narrow-therapeutic-index drugs at toxic doses, because the dose is scaled to weight that does not correspond to the volume of distribution or the clearance. The choice is guided by the drug's volume of distribution, its therapeutic index and the available paediatric evidence, and where evidence is limited the level is monitored. [4] [7]
3. The dose-adjust-measure loop
The loop runs as dose, measure the level at the correct time, interpret against the dose time, adjust the dose toward the target, and re-check the calculation at each adjustment, with the weight re-measured as the child's clinical state changes. For vancomycin this means dosing toward an AUC of 400 to 600 mg per litre per hour, drawing the trough before a dose at steady state, and titrating the dose and the interval to the target. [7] [8]
4. Non-dosing causes of an unexpected level
Two non-dosing causes are a sample drawn at the wrong time relative to the dose (the most common, which is why the dose time must be written on the tube), and a change in renal clearance that raises or lowers the level independent of the calculation. A drug interaction that inhibits or induces clearance is a third cause to consider when the arithmetic and the timing are correct. [7]
References
- [1]El Edelbi, R; Lindemalm, S; Eksborg, S Estimation of body surface area in various childhood ages--validation of the Mosteller formula Acta Paediatrica, 2012.PMID 22211780
- [2]Hirata, K M; Kang, A H; Ramirez, G V Pediatric Weight Errors and Resultant Medication Dosing Errors in the Emergency Department Pediatric Emergency Care, 2019.PMID 28976456
- [3]Lesar, T S Tenfold medication dose prescribing errors Annals of Pharmacotherapy, 2002.PMID 12452740
- [4]Procaccini, D; Kim, J M; Lobner, K Medication Errors in Overweight and Obese Pediatric Patients: A Narrative Review Joint Commission Journal on Quality and Patient Safety, 2022.PMID 35045950
- [7]McNeil, J C; Kaplan, S L Vancomycin Therapeutic Drug Monitoring in Children: New Recommendations, Similar Challenges Journal of Pediatric Pharmacology and Therapeutics, 2020.PMID 32839650
- [8]Rybak, M J; Le, J; Lodise, T P Therapeutic Monitoring of Vancomycin for Serious Methicillin-resistant Staphylococcus aureus Infections: A Revised Consensus Guideline and Review by the American Society of Health-system Pharmacists, the Infectious Diseases Society of America, the Pediatric Infectious Diseases Society, and the Society of Infectious Diseases Pharmacists Clinical Infectious Diseases, 2020.PMID 32658968