Nineteen traps, one for each topic, each with the worked example that shows it biting.
Most calculation marks are lost to setup rather than to arithmetic: the right formula applied to the wrong number, or the right number reported in the wrong unit. The same handful of setups catch students every year, which is why they are worth learning by name. Each entry below names the trap for one topic and shows a worked example of the calculation it hides in.
Anchors to memorize: 1:1,000 = 0.1% (epinephrine), 1:10,000 = 0.01%.
Express 1:2,500 as a percentage. 1:2,500 means 1 g in 2,500 mL, so (1 ÷ 2,500) × 100 = 0.04% w/v.
Practice percentage & ratio strength →The drug amount is unchanged by dilution; only concentration and volume change. Remember to QS to the final volume when the question asks for it.
How many mL of a 20% stock are needed to make 120 mL of a 2% solution? V₁ = (2% × 120) ÷ 20% = 12 mL (then QS to 120 mL with diluent).
Practice dilution →Don't confuse the two: medial = known quantities → find strength; alternate = known strengths → find quantities.
Make 250 mL of 25% dextrose from 70% and 10% stock. Parts of 70% = 25 − 10 = 15; parts of 10% = 70 − 25 = 45; total = 60. Volume of 70% = (15 ÷ 60) × 250 = 62.5 mL (and 187.5 mL of the 10%).
Practice alligation →Valence trap: monovalent ions (Na⁺, K⁺) → mEq = mmol; divalent ions (Ca²⁺, Mg²⁺) → 2 mEq per mmol.
How many mEq are in 1 g of KCl (MW 74.5, valence 1)? mmol = 1,000 ÷ 74.5 = 13.4; mEq = 13.4 × 1 = 13.4 mEq. Osmolarity of 0.9% NaCl = 9 g/L ÷ 58.5 × 2 × 1,000 = 308 mOsm/L.
Practice milliequivalent (meq), millimole & milliosmole →Always round drops to a whole number, and round pump rates exactly as the question instructs.
Infuse 1,000 mL over 8 hours with a set that gives 15 drops/mL. (1,000 × 15) ÷ (8 × 60) = 15,000 ÷ 480 = 31.25 → 31 drops/min.
Practice iv infusion & drip rate →Rounding trap: rounding BSA to one decimal too early can shift the dose by several mg, so carry full precision to the end.
A patient is 170 cm and 70 kg. BSA = √(170 × 70 ÷ 3,600) = √3.306 = 1.82 m². A 175 mg/m² order → 175 × 1.82 = 318.5 mg.
Practice body surface area (bsa) & chemo dosing →Read the ask: total vs nonprotein kcal changes whether you include the amino acid calories.
Calories from 1,000 mL of dextrose 20%? Grams = 1,000 × 20% = 200 g; 200 × 3.4 = 680 kcal.
Practice tpn & nutrition calculation →The exam gives you the E-value. The skill is the 4-step setup, not memorizing E-values.
Make 30 mL of 1% pilocarpine HCl (E = 0.24) isotonic. (1) drug = 0.3 g; (2) 0.3 × 0.24 = 0.072 g; (3) 0.009 × 30 = 0.27 g; (4) 0.27 − 0.072 = 0.198 g.
Practice isotonicity & e-value →Weight rule first, formula second. Using actual weight where AdjBW is required is the classic error.
70-yr-old male, 5'10" (IBW = 73 kg), weighs 90 kg, SCr 1.2. Since 90 < 125% of 73, use IBW. CrCl = (140 − 70) × 73 ÷ (72 × 1.2) = 59.1 mL/min.
Practice creatinine clearance (cockcroft-gault) →Loading dose depends on Vd; maintenance depends on clearance. Exams like to swap the two.
Two levels: 40 mg/L at hour 0, 10 mg/L at hour 8. k = ln(40 ÷ 10) ÷ 8 = 1.386 ÷ 8 = 0.173 hr⁻¹, so t½ = 0.693 ÷ 0.173 ≈ 4 hr.
Practice pharmacokinetics →Always dose-normalize both AUCs. Comparing raw AUCs from different doses is the built-in trap.
IV 100 mg gives AUC 50; oral 200 mg gives AUC 60. First term: 60 ÷ 200 = 0.30; second term: 100 ÷ 50 = 2; F = 0.30 × 2 = 0.60 = 60%.
Practice bioavailability (f) →Stability caps the day supply of an open container, and the 20% reduction only applies going NPH→glargine or Toujeo→Lantus/Basaglar, not the reverse.
Lantus 10 mL vial at 25 units/day: 1,000 ÷ 25 = 40 days, but an open glargine vial is stable 28 days → 28-day supply. Switching NPH 30 units BID to glargine: 60 × 0.8 = 48 units daily.
Practice insulin day supply & switching →90 MME/day is the common safety-review threshold; watch route differences. The same drug by a different route has a different number.
Oxycodone 10 mg three times daily = 30 mg/day. Table: 20 mg oxycodone ≈ 30 mg morphine, so MME = 30 × (30 ÷ 20) = 45 MME/day.
Practice opioid conversion & mme →Round down. You can't bill for a partial day. Inhalers count total actuations (puffs), not mL.
A 5 mL eye drop bottle (20 drops/mL = 100 drops), 1 drop in each eye twice daily = 4 drops/day. 100 ÷ 4 = 25 days.
Practice days supply calculation →mg/kg/DAY vs mg/kg/DOSE is the trap that catches most people here. Read which one the order specifies.
A 66 lb child on amoxicillin 45 mg/kg/day divided twice daily. Kg = 66 ÷ 2.2 = 30 kg; daily = 45 × 30 = 1,350 mg; per dose = 1,350 ÷ 2 = 675 mg.
Practice weight-based (mg/kg) dosing →Add the displacement volume to the diluent when finding concentration. Leave it out and you overestimate the strength.
A 1 g vial reconstituted to 100 mg/mL needs a final volume of 10 mL. If the label says to add 9.5 mL diluent, the powder volume = 10 − 9.5 = 0.5 mL.
Practice reconstitution & powder volume →Numerically, SG equals grams per mL because it's referenced to water (1 g/mL).
Glycerin has a specific gravity of 1.25. The weight of 240 mL = 240 × 1.25 = 300 g.
Practice specific gravity →Only correct calcium when albumin is low. For phenytoin with CrCl < 10, the 0.2 factor becomes 0.1.
Measured calcium 7.5 mg/dL, albumin 2.5 g/dL. Corrected = 7.5 + 0.8 × (4.0 − 2.5) = 7.5 + 1.2 = 8.7 mg/dL.
Practice clinical lab calculation →Never round these to the nearest whole number. 2.19 vials is 3 vials, not 2, and 2.25 bottles of antibiotic is 3, or the patient runs out mid-course. The leftover in the last container is waste, not an error.
An 82 kg patient needs vancomycin 20 mg/kg = 1,640 mg, stocked as 750 mg vials. 1,640 ÷ 750 ≈ 2.19, so you open 3 vials.
Practice dispensing quantity →The full formula sheet, every formula on this site with a worked example.