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Clearance, Half-Life & Steady State

The three numbers that decide dosing. See how total clearance, half-life, and the time to steady state connect — and why half-life tells you when to dose but clearance tells you how much.

Clearance: volume cleaned per minute

Clearance is the body's single most important elimination number. It is not an amount of drug — it is a volume of plasma fully cleared of drug per unit time (mL/min or L/h). Total body clearance is just the sum of every route added together: renal + hepatic + any others. Think of clearance as the size of the drain: a wider drain empties the tub faster, no matter how much water is in it.

In first-order kinetics — true for most drugs at normal doses — the rate of elimination rises and falls with concentration, but clearance itself stays constant. The amount removed per minute = clearance × current concentration. This is why clearance is so useful: it links the dose you give to the steady concentration you reach.

Half-life: time, not amount

The elimination half-life (t½) is the time for the plasma concentration to fall by half. It is not a fundamental property — it is a result of two more basic numbers: clearance (CL) and the volume of distribution (Vd). The relationship is one of the most important formulas in all of pharmacokinetics.

t½ = 0.693 x Vd / CL          (0.693 = ln 2)

Example: Vd = 50 L,  CL = 5 L/h
   t½ = 0.693 x 50 / 5 = 6.93 h  (~7 hours)

Note what this means:
 - bigger Vd (drug hides in tissues) -> LONGER half-life
 - bigger CL (faster drain)          -> SHORTER half-life
Half-life is a CONSEQUENCE of CL and Vd, not a cause.
Half-life depends on both how big the drug's distribution space is and how fast the body clears it.

Steady state: the plateau

When you dose a drug repeatedly, concentration climbs until the amount going in each interval equals the amount cleared — that balance is steady state. The beautiful, dose-independent rule: it takes about 4–5 half-lives to reach steady state, regardless of how big the dose is. This is time to steady state, and it explains drug accumulation during the climb.

  1. After 1 half-life of dosing you are ~50% of the way to the plateau; after 2, ~75%; after 3, ~88%; after 4, ~94%; after 5, ~97%.
  2. The plateau height is set by clearance and the maintenance dose rate: higher dose rate or lower clearance → higher steady-state level.
  3. The time to reach it depends only on half-life — not on dose. Want to reach the plateau instantly? Give a loading dose (next idea).

So the division of labour is clean: clearance and dose rate set how high the plateau is; half-life sets how long it takes to get there. Set the maintenance dose from clearance, and choose the dosing interval from half-life. This is the heart of rational dosing.