Pick the relation: C = C₀·e−kt, LD = Vd·C/(S·F), maintenance rate = CL·Css/(S·F), τ = ln(Cpeak/Ctrough)/k.
For kidney function, estimate CrCl with Cockcroft–Gault first, then scale the dose or the interval.
Check the result: levels fall by half every half-life, and steady state takes about five half-lives.
Pharmacy calculations
Percentage strength
% w/v = g per 100 mL
Amount in a solution
g = % × mL / 100
Ratio strength
1:X = 1 g in X mL; % = 100 / X
Reconstitution
final volume = diluent + powder volume
Concentration
mg/mL = drug in vial / final volume
Alligation
parts of H = T − L, parts of L = H − T
Water or base
0%
Pediatric & critical-care dosing
Daily dose
mg/day = mg/kg/day × weight (kg)
Doses per day
number = 24 h / dosing interval (h)
Pounds to kilograms
kg = lb / 2.2
Infusion concentration
units/mL = units in bag / bag volume
Pump rate
mL/h = dose per hour / concentration
Titrated drip
mL/h = mcg/kg/min × kg × 60 / (mcg/mL)
Dose from pump rate
mcg/kg/min = mL/h × (mcg/mL) / (60 × kg)
Units
1 mg = 1000 mcg
2Practice problems
1.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 210 mg of tobramycin. The level back-extrapolated to time zero is 9 mg/L and the half-life is 2 h. Find the volume of distribution Vd.
D = 210 mgC₀ = 9 mg/Lt½ = 2 h
Vd = L
2.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A theophylline infusion is started without a loading dose. The half-life in this patient is 9 h. How long t does it take to reach 75% of the steady-state level?
t½ = 9 h75% Css
t = h
3.Pharmacy calculationsMedium
Practice exercise (made-up values, not clinical guidance). A vial contains ampicillin 1000 mg as a powder. The practice label says to add 3.5 mL of sterile water; the powder adds 0.5 mL to the volume. What is the concentration C in mg/mL after reconstitution?
1000 mg3.5 mL+0.5 mL
C = mg/mL
4.Pharmacy calculationsHard
A vial holds 1000 mg of powder. You add 9.2 mL of diluent, and the powder adds 0.8 mL of volume. The prescription is 240 mg. Draw up the dose: what volume V do you give?
1000 mg9.2 mL + 0.8 mL240 mg
V = mL
5.Pediatric & critical-care dosingMedium
Practice exercise (made-up values, not clinical guidance). A practice protocol calls for a heparin bolus of 70 units/kg for a patient who weighs 70 kg. The vial contains 5 000 units/mL. What volume V do you draw up? Round to two decimal places.
70 units/kg70 kg5 000 units/mL
V = mL
6.Pediatric & critical-care dosingHard
Practice exercise (made-up values, not clinical guidance). A bag contains dopamine 400 mg in 250 mL. The order is 8 mcg/kg/min for a patient who weighs 62 kg. At what rate R do you set the pump, in mL/h? Round to one decimal place.
400 mg/250 mL8 mcg/kg/min62 kg
R = mL/h
7.PharmacologyMedium
A patient uses two different eye drops at the same time of day. What should the nurse advise?
APut both drops in at exactly the same moment
BMix the drops in one bottle first
CWait at least 5 minutes between the two drops
DUse only one of them each day
8.PharmacologyHard
Why should a dose never be written as "5.0 mg" on a medication order?
AIt means the dose must be given five times
BIt is too precise for most drugs
CThe decimal point may be missed, and the dose read as 50 mg
DThe pharmacy cannot supply doses that contain a decimal point
3Solutions
1.Clinical pharmacokinetics
Answer: Vd = 23.33 L
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.
Volume of distribution
Vd = 210/9 = 23.33 L
Answer
Vd = 23.33 L
2.Clinical pharmacokinetics
Answer: t = 18 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.
Rate constant
k = 0.693/9 = 0.07702 h⁻¹
Time to the given fraction of steady state
t = −ln(1 − 0.75)/0.07702 = 1.386/0.07702 = 18 h
Answer
t = 18 h
That is 2 half-lives.
3.Pharmacy calculations
Answer: C = 250 mg/mL
Reconstitution: the powder takes up space, so the final volume is the diluent plus the powder volume. Concentration = drug in the vial/final volume, and the volume to give = dose/concentration. On a multi-strength label, use the concentration that belongs to the amount of diluent you actually added. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.
Final volume
3.5 + 0.5 = 4 mL
Concentration
C = 1000/4 = 250 mg/mL
Answer
C = 250 mg/mL
4.Pharmacy calculations
Answer: V = 2.4 mL
Strength to volume: turn the strength into mg per mL first. A w/v percentage is grams per 100 mL, so x% = 10x mg/mL. A ratio strength 1:n is 1 g in n mL. After reconstitution the concentration is the powder's dose over the final volume (diluent + powder volume). Then volume = dose/concentration.
Final volume and concentration
9.2 + 0.8 = 10 mL
C = 1000/10 = 100 mg/mL
Volume
V = 240/100 = 2.4 mL
Answer
V = 2.4 mL
5.Pediatric & critical-care dosing
Answer: V = 0.98 mL
Heparin and insulin infusions: concentration = units in the bag/bag volume (units/mL). Pump rate (mL/h) = units per hour/concentration, and units per hour = mL/h × concentration. For a weight-based order, units/h = units/kg/h × weight first. A bolus volume = units needed/vial strength. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.
Units needed
70·70 = 4 900 units
Volume
V = 4900/5000 = 0.98 mL
Answer
V = 0.98 mL
6.Pediatric & critical-care dosing
Answer: R = 18.6 mL/h
Titrated drips: first find the concentration in mcg/mL (mg × 1000/mL). For a weight-based order, mcg/min = mcg/kg/min × weight. Then mL/h = mcg/min × 60/concentration. Working backwards, mcg/kg/min = mL/h × concentration/(60 × weight). The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.
Concentration
400 mg = 400 000 mcg
400 000/250 = 1600 mcg/mL
Dose per minute and per hour
8·62 = 496 mcg/min
496·60 = 29760 mcg/h
Pump rate
R = 29760/1600 = 18.6 mL/h
Answer
R = 18.6 mL/h
7.Pharmacology
Answer: C · Wait at least 5 minutes between the two drops
Why this is the answer
Waiting several minutes allows the first drop to be absorbed before the next is applied. Eye ointments are usually applied last.
Answer
C · Wait at least 5 minutes between the two drops
8.Pharmacology
Answer: C · The decimal point may be missed, and the dose read as 50 mg
Why this is the answer
Trailing zeros (5.0) and missing leading zeros (.5) are well-known causes of tenfold errors. Safe practice is to write 5 mg and 0.5 mg.
Answer
C · The decimal point may be missed, and the dose read as 50 mg