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Medicine 3% exam weight

Worked Numeric Example

Part of the Saudi GP Board study roadmap. Medicine topic medici-009 of Medicine.

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Worked Numeric Example

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Body Surface Area (BSA) standardizes drug dosing across body sizes. The Mosteller formula is the bedside formula of choice:

BSA (m²) = √[(Height_cm × Weight_kg) / 3600]

For burns, the Rule of Nines gives rapid adult TBSA percentages (head 9%, each arm 9%, each leg 18%, anterior trunk 18%, posterior trunk 18%, perineum 1%). For fluid resuscitation, the Parkland formula delivers 4 mL × kg × %TBSA of Lactated Ringer over 24 hours — half in the first 8 hours.

Creatinine clearance (Cockcroft-Gault) adjusts renally-cleared drug doses:

CrCl = [(140 − Age) × Weight_kg] / (72 × SCr), multiplied by 0.85 in females.

High-yield pointers: memorize Mosteller (not DuBois), use Lund-Browder (not Rule of Nines) in children, give half the Parkland volume in the first 8 hours, and always apply the 0.85 female factor.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Body Surface Area Formulas

BSA standardizes doses for chemotherapy, biologics, and pediatric drugs where fixed adult doses would be unsafe. Three formulas coexist, but the Saudi GP Board expects the Mosteller formula for rapid calculation MCQs because it requires only height, weight, and a square root.

FormulaEquationWhen Used
Mosteller√[(H × W) / 3600]Bedside, exam default
DuBois0.007184 × H^0.725 × W^0.425Historical/research
Haycock0.024265 × H^0.3964 × W^0.5378Pediatric reference

Variables: H = height in cm, W = weight in kg; result in m². Drug dose = mg/m² × BSA.

Burn Assessment and Resuscitation

The Rule of Nines partitions the adult body into 9% regions for rapid TBSA estimation. In children, proportions differ (head ≈ 18% at birth), so the Lund-Browder chart is mandatory. The Parkland formula then drives crystalloid resuscitation:

Volume (mL) = 4 × Weight_kg × %TBSA of Lactated Ringer over 24 hours; half in first 8 h, remainder over next 16 h, measured from the burn time.

Renal-Based Dose Adjustment

When eGFR is unreliable (extremes of body size, pregnancy, amputees), Cockcroft-Gault estimates CrCl from age, weight, and serum creatinine. Always multiply by 0.85 in females, or renally cleared drugs are underdosed.

Typical exam pattern: a 60 kg patient with 30% burns → Parkland = 7,200 mL/24 h, with 3,600 mL in the first 8 hours.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Worked Numeric Example

A 70 kg, 170 cm adult man with 40% TBSA burns needs resuscitation.

  • BSA (Mosteller): √[(170 × 70) / 3600] = √3.306 ≈ 1.82 m²
  • Parkland volume: 4 × 70 × 40 = 11,200 mL over 24 h
  • First 8 hours: 5,600 mL Lactated Ringer
  • Next 16 hours: 5,600 mL
  • CrCl (SCr 1.2 mg/dL, age 50): [(140 − 50) × 70] / (72 × 1.2) = 72.9 mL/min → no dose adjustment needed.

Common Pitfalls

  • Swapping height and weight in Mosteller produces an incorrect BSA — verify H is in cm and W in kg.
  • Rule of Nines in children overestimates TBSA because the head is proportionally larger; use Lund-Browder until age 14.
  • Giving the entire Parkland volume in the first 8 hours causes pulmonary edema — the formula splits the total.
  • Forgetting the 0.85 female factor in Cockcroft-Gault underdoses drugs like enoxaparin and aminoglycosides.
  • Obese patients: use adjusted body weight for chemotherapy dosing, or actual body weight for Parkland — never ideal body weight alone.
  • Confusing mg/m² (dose intensity) with m² (BSA) when computing total dose.

Adjacent Topics and Strategy

Topic 9 connects to oncology dosing (mg/m²), pediatric prescribing (mg/kg), and renal pharmacology (dose-interval extension). Saudi GP Board MCQs usually present a short vignette with two numeric inputs and four formulas to choose between — recognizing which formula fits the stem (burn → Parkland; renal → Cockcroft-Gault) is half the answer.

Practice prompts:

  1. A 25 kg child with 20% burns — compute the 8-hour Parkland volume using the pediatric 2 mL × kg × %TBSA variant often preferred under age 30 kg.
  2. A 65-year-old woman, 60 kg, SCr 1.4 mg/dL — calculate CrCl and decide whether to dose-adjust amikacin.

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