Formula sheets, the key method steps for every topic and 6 practice problems, with full solutions at the back.
When should a patient take a once-daily proton pump inhibitor such as omeprazole?
Why is strict aseptic technique and a limited hang time needed for propofol infusions?
A child weighing 36 kg is prescribed 10 mg/kg of a drug. The stock solution contains 40 mg/mL. Draw up the dose: what volume V do you give?
A child weighing 8 kg is prescribed 10 mg/kg per dose of an oral suspension that contains 40 mg in 1 mL. What volume V do you give?
Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 160 mg of gentamicin. The level back-extrapolated to time zero is 6 mg/L and the half-life is 2 h. Find the clearance CL in L/h.
Practice exercise (made-up values, not clinical guidance). A 41-year-old woman weighs 78 kg and has a serum creatinine of 1.8 mg/dL (159 µmol/L). Estimate the creatinine clearance with Cockcroft–Gault: CrCl (mL/min) = (140 − age) × weight (kg)/(72 × SCr in mg/dL), × 0.85 for women. The usual gentamicin regimen is 80 mg every 8 h for a creatinine clearance of 120 mL/min. Keeping the dose, the interval is lengthened in proportion to the fall in clearance. Find the new dosing interval τ. Round to one decimal place.
Answer: A · About 30–60 minutes before the first meal of the day
Proton pump inhibitors block pumps that are active, so they work best when taken before breakfast. Food then activates the pumps when the drug level is high.
Answer: A · Its lipid emulsion lets bacteria grow if it is contaminated
Propofol is formulated in a soybean oil emulsion, which is a good growth medium for bacteria. Contaminated propofol has caused sepsis, so it is handled aseptically and tubing and containers are changed at set intervals.
Answer: V = 9 mL
Drawing up a dose: volume = (prescribed dose/stock strength) × stock volume. Convert to the same units first, then check that the volume fits the syringe and lands on one of its graduations.
On this 10 mL syringe every small line is 0.2 mL.
Answer: V = 2 mL
Dose calculation: the amount to give equals what is prescribed divided by what you have, times the quantity it comes in: amount = (desired dose/stock strength) × stock volume. Always keep the units the same (convert g to mg, mg to mcg first) and check that the answer is sensible.
Answer: CL = 9.242 L/h
Clearance and volume: at steady state the dosing rate equals the elimination rate, so the maintenance rate = CL × Css. A loading dose is LD = Vd × C/(S × F), with S the salt factor (fraction of active drug in the salt) and F the bioavailability (1 for IV). From two levels, k = ln(C₁/C₂)/Δt; for an IV bolus Vd = dose/C₀ and CL = k × Vd. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.
Answer: τ = 19 h
Designing a regimen: with rapid IV doses the level falls from peak to trough during the interval, so τ = ln(Cpeak/Ctrough)/k and the dose that restores the peak is Vd × (Cpeak − Ctrough). The average steady-state level is Css,avg = S·F·dose/(CL·τ). An infusion rises as C = (R/CL)(1 − e−kt). For a drug cleared by the kidneys, the dose is scaled with the creatinine clearance. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.
In practice the interval is rounded to a convenient value such as 12, 24, 36 or 48 h.
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