Drug dosing and pharmacokinetics
loading and maintenance dosehalf-lifeIV rate calculationsBSA dosingdrip rate
Loading dose, maintenance rate, half-life, surface-area dosing and the two IV rate sums, in the order a prescription is worked out.
Loading Dose from Volume of Distribution
The dose that fills the apparent volume of distribution to a target plasma concentration in one step, before any elimination.
Maintenance Dose Rate from Clearance
The rate of drug administration that exactly replaces what clearance removes, holding a steady-state plasma concentration.
Drug Half-Life from Clearance and Volume of Distribution
Elimination half-life for a one-compartment drug: the time to fall by half, set entirely by the volume of distribution and the clearance.
Body Surface Area Dosing (mg per m²)
The dose for a person from a protocol quoted per square metre of body surface area, the convention used across oncology.
Body Surface Area (Mosteller Formula)
Body surface area in square metres by Mosteller's 1987 square-root simplification of the older exponential fits.
IV Infusion Rate (mL per Hour)
The rate to set on a volumetric infusion pump: the volume to be infused divided by the time it should take.
IV Drip Rate (Drops per Minute)
Drops per minute for a gravity-fed IV set: the volume, multiplied by the tubing's drop factor, divided by the time.
How they fit together
Two independent properties govern a drug, and nearly every dosing question reduces to which one you are asking about. Volume of distribution sets how much is needed to reach a concentration, and clearance sets how fast it disappears. So the loading dose depends on volume of distribution alone and not at all on kidney or liver function, while the maintenance rate depends on clearance alone. A patient in renal failure gets the same loading dose as anyone else and a much smaller maintenance dose, which looks inconsistent until you see that the two formulas share no variable.
Half-life is the one that ties them together, being volume divided by clearance with a log of 2 in front, and it is a derived quantity rather than a property in its own right. It answers the two timing questions that matter at the bedside: steady state arrives after roughly four to five half-lives whatever the dose, and after stopping, the drug is essentially gone on the same schedule. A drug with a long half-life takes just as long to leave as it took to arrive.
Body surface area dosing is the oncology convention, and it needs a BSA, which is why Mosteller travels with it. Mosteller won on arithmetic rather than accuracy: it is a single square root that can be done on a ward without a calculator, and it agrees with the older equations closely enough that nothing clinical turns on the choice. The last two are the arithmetic that reaches the patient. mL per hour is what you type into a pump, and drops per minute is what you count when there is no pump, using the drop factor printed on the tubing. Macrodrip sets run 10, 15 or 20 drops per mL and microdrip is always 60, and using the wrong factor is a threefold error in the delivered rate. Every figure here is a starting point for a clinician, not a substitute for one.