3.2 Dosing by Weight/BSA, Days Supply & Pediatric/Geriatric Dose Adjustments

3.2 Dosing by Weight/BSA, Days Supply & Pediatric/Geriatric Dose Adjustments

Posology—the science of dosage regimens—requires clinicians to calculate patient-specific doses and verify day-supply limits across specialized delivery systems. In pediatric, geriatric, and oncology medicine, fixed adult dosing fails because organ clearance, metabolic capacity, and volume of distribution change dramatically across body size and life stages. Pharmacists must navigate weight-based dosing, body surface area (BSA) determinations, and stringent supply limits for ophthalmics, insulin devices, and respiratory inhalers.

---

Pediatric and Weight-Based Posology Principles

Pediatric doses are conventionally prescribed according to body mass, expressed either as milligrams per kilogram per day ($\text{mg/kg/day}$) or milligrams per kilogram per single dose ($\text{mg/kg/dose}$):

• Daily Divided Posology ($\text{mg/kg/day}$): The total calculated milligram quantity must be divided into equal fractional doses matching the dosing frequency (e.g., BID = divided into 2 doses every 12 hours; TID = divided into 3 doses every 8 hours).
• Per-Dose Posology ($\text{mg/kg/dose}$): The calculated milligram quantity is administered at each individual dosing event without mathematical division.

Clinical Benchmark: Canadian Paediatric Society Guidelines for Acute Otitis Media

In Canadian pediatric practice, the Canadian Paediatric Society (CPS) recommends high-dose amoxicillin as first-line therapy when a child with acute otitis media (AOM) needs an antibiotic, because it covers penicillin-intermediate Streptococcus pneumoniae:

• Recommended Dosage: $75\text{--}90\text{ mg/kg/day}$ divided into two doses every 12 hours (BID); three-times-daily dosing is an alternative.
• Duration: 10 days for children younger than 2 years and for those with a perforated tympanic membrane or recurrent AOM; 5 days for children 2 years and older with uncomplicated AOM.
• Pharmacodynamic Target: The high-dose strategy achieves middle ear fluid concentrations exceeding the minimum inhibitory concentration (MIC) of intermediate-resistant pneumococci for greater than $40\%$ to $50\%$ of the 12-hour dosing interval ($T > \text{MIC}$).

The Mandatory Adult Maximum Cap Rule

Step-by-Step Worked Pediatric Calculation

Clinical Case: A 3-year-old child weighing $15\text{ kg}$ has AOM with ear discharge from a perforated tympanic membrane, so the 10-day CPS course applies. The child is prescribed amoxicillin suspension at $90\text{ mg/kg/day}$ divided BID for 10 days. The pharmacy stocks amoxicillin $400\text{ mg}/5\text{ mL}$ ($80\text{ mg/mL}$) in $100\text{ mL}$ bottles.

1. Calculate Total Daily Dose: $\text{Daily Dose} = 15\text{ kg} \times 90\text{ mg/kg/day} = 1,350\text{ mg/day}$

2. Calculate Dose per Administration (BID): $\text{Single Dose} = \frac{1,350\text{ mg/day}}{2\text{ doses/day}} = 675\text{ mg BID}$ (Check cap: $1,350\text{ mg/day} < 4,000\text{ mg/day}$ ceiling. Verified safe).

3. Calculate Volume per Dose: $\text{Volume per Dose} = \frac{675\text{ mg}}{80\text{ mg/mL}} = 8.4375\text{ mL} \approx 8.4\text{ mL BID}$

4. Calculate Total Regimen Volume & Dispense Package: $\text{Total Volume} = 8.4375\text{ mL/dose} \times 2\text{ doses/day} \times 10\text{ days} = 168.75\text{ mL}$ Because amoxicillin is supplied in $100\text{ mL}$ bottles, dispensing a single $100\text{ mL}$ bottle would provide only 5.9 days of therapy. The pharmacist must dispense two $100\text{ mL}$ bottles ($200\text{ mL}$ total) with a calibrated oral syringe, counselling the parent to discard the remainder after 14 days.

---

Body Surface Area (BSA) and Narrow-Therapeutic-Index Regimens

Body surface area (BSA) correlates more strongly with glomerular filtration rate, cardiac output, and hepatic metabolic capacity than total body weight. Consequently, BSA ($m^2$) is the clinical standard for antineoplastic agents (cytotoxic chemotherapy), monoclonal antibodies, and pediatric burn/intensive care regimens.

The Mosteller Formula

The Mosteller formula is the most widely used BSA method in clinical practice because it is simple and agrees closely with the DuBois and DuBois reference formula. The PEBC formula sheet does not list a BSA formula, so an exam item that needs BSA gives either the formula or the BSA value in the stem:

$\text{BSA (m}^2\text{)} = \sqrt{\frac{\text{Height (cm)} \times \text{Weight (kg)}}{3600}}$

When height is measured in inches and weight in pounds:

$\text{BSA (m}^2\text{)} = \sqrt{\frac{\text{Height (inches)} \times \text{Weight (lbs)}}{3131}}$

Step-by-Step Worked BSA Calculation

Clinical Case: An adult patient with lung carcinoma is $168\text{ cm}$ tall and weighs $65\text{ kg}$. The oncologist prescribes cisplatin at $75\text{ mg/m}^2$ IV every 21 days.

1. Calculate BSA via Mosteller: $\text{Product} = 168 \times 65 = 10,920$ $\text{Quotient} = \frac{10,920}{3600} = 3.0333$ $\text{BSA} = \sqrt{3.0333} \approx 1.7416\text{ m}^2 \approx 1.74\text{ m}^2$

2. Calculate Prescribed Chemotherapy Dose: $\text{Dose} = 1.7416\text{ m}^2 \times 75\text{ mg/m}^2 = 130.62\text{ mg} \approx 131\text{ mg}$

---

Precision Days Supply Calculations for Specialized Delivery Systems

Determining the exact days supply of a prescription is an essential dispensing competency. Underestimating days supply leads to premature refills and public drug plan audit clawbacks (e.g., Ontario Drug Benefit, Fair PharmaCare in British Columbia), while overestimating days supply can leave patients without medication.

Ophthalmic Preparations

Ophthalmic dropper tips are manufactured to deliver standardized droplet volumes, but physical surface tension varies with drug viscosity:

• Standard Exam Calibration: $1\text{ mL} = 20\text{ drops}$ for aqueous solutions ($0.05\text{ mL}$ per drop).
• Viscous Suspensions & Emulsions: $1\text{ mL} = 15\text{ drops}$ (e.g., cyclosporine ophthalmic emulsions, gellified drops).

$\text{Total Droplets} = \text{Bottle Volume (mL)} \times \text{Drop Factor (drops/mL)}$ $\text{Mathematical Days Supply} = \frac{\text{Total Droplets}}{\text{Drops Required per Day}}$

Ophthalmic Example: A preserved glaucoma solution labelled "discard 28 days after opening" is supplied in a $10\text{ mL}$ bottle. The prescription reads: Instill 1 drop into both eyes (OU) twice daily.

• Total drops: $10\text{ mL} \times 20\text{ drops/mL} = 200\text{ drops}$.
• Daily usage: 1 drop × 2 eyes × 2 times = 4 drops/day.
• Mathematical exhaustion: $50\text{ days}$.
• In-use discard limit: $28\text{ days}$.
• Clinical Days Supply: $28\text{ days}$ (the remaining drops are discarded on day 28).

By contrast, latanoprost $0.005\%$ in a $2.5\text{ mL}$ bottle at 1 drop OD daily gives $50$ drops = $50$ days, capped by its 6-week (42-day) in-use limit.

Insulin Pens and Vials

Most commercial insulin formulations are supplied at standard U-100 concentration ($100\text{ units/mL}$):

• Prefilled Pen Cartridges: $3\text{ mL} = 300\text{ units}$.
• Multi-Dose Vials: $10\text{ mL} = 1,000\text{ units}$.

Mandatory Safety Priming ("Air Shot")

Most pen instructions for use tell patients to dial and expel a $2\text{ unit}$ safety test before every injection (Toujeo SoloStar uses 3 units). This clears air from the needle assembly and confirms needle patency. Dosing calculations must account for this priming waste:

$\text{Daily Insulin Usage} = \text{Prescribed Units/Day} + (\text{Number of Injections/Day} \times 2\text{ units})$

In-Use Room-Temperature Stability Limits

Once an insulin pen or vial is removed from refrigeration and placed into active room-temperature use ($15^\circ\text{C}$ to $30^\circ\text{C}$), it degrades over time:

The days supply for an individual insulin pen is: $\min\left(\lfloor\frac{\text{Total Cartridge Units}}{\text{Daily Insulin Usage}}\rfloor, \text{In-Use Stability Days}\right)$.

Metered-Dose Inhalers (MDIs) and Dry Powder Inhalers (DPIs)

Inhaler days supply is governed by total labelled actuations divided by daily inhalation frequency:

• Salbutamol HFA (Ventolin): $200\text{ actuations}$ per canister. If prescribed 2 puffs QID scheduled: $\frac{200}{8\text{ puffs/day}} = 25\text{ days supply}$.
• Fluticasone propionate (Flovent HFA): $120\text{ actuations}$. If prescribed 2 puffs BID: $\frac{120}{4\text{ puffs/day}} = 30\text{ days supply}$.
• Priming Discard: When initializing a new canister or when an MDI has not been used for $>7\text{--}14\text{ days}$, 2 to 4 priming sprays must be sprayed into the air and deducted from available actuations.

---

Pharmacokinetic Considerations in Pediatric and Geriatric Dosing

Pediatric Age-Related Physiology

• Total Body Water: Neonates consist of $75\%\text{--}80\%$ total body water (compared to $55\%\text{--}60\%$ in adults). Hydrophilic medications (e.g., aminoglycosides, beta-lactams) require higher initial $\text{mg/kg}$ loading doses to achieve therapeutic peak concentrations.
• Hepatic Glucuronidation: Phase II conjugation enzymes are severely immature in infants $<2\text{ months}$, leading to toxic accumulation of chloramphenicol (Gray Baby Syndrome) or bilirubin displacement.
• Glomerular Filtration Rate: At birth, GFR is approximately $20\text{--}40\text{ mL/min/1.73 m}^2$, maturing to adult levels around 1 year of age.

Geriatric Age-Related Physiology

• Renal Function Decline: Nephron mass and glomerular filtration decrease by roughly $1\text{ mL/min}$ per year after age 40. Normal serum creatinine values in elderly patients often conceal severe renal impairment due to age-related sarcopenia (reduced muscle mass).
• Altered Body Composition: Increased percentage of body adipose tissue and decreased total body water increase the volume of distribution ($V_d$) and elimination half-life of lipophilic drugs (e.g., diazepam, flurazepam), increasing delirium and fall risks.
• Deprescribing & Beers Criteria: Geriatric posology mandates "starting low and going slow," actively tapering inappropriate anticholinergic, sedative-hypnotic, and NSAID therapies.