Lifespan Pharmacokinetics: Pediatric & Geriatric Dosing

Patient Age

Age 40 yr (adult)

Dose & Clearance Route

Dose 10.0 mg/kg

Live Readouts

In therapeutic range

Peak conc. C₀

14.3 mg/L

Half-life t½

7.8 h

Clearance

0.090 L/h/kg

Vol. distribution

0.70 L/kg

Compare Ages

How it works

The body is modeled as one well-mixed compartment. After a dose is fully absorbed, blood concentration falls exponentially:

C(t) = C0 * e^(-k*t)
C0   = Dose / Vd          (peak concentration)
k    = CL / Vd            (elimination rate constant)
t1/2 = ln(2) / k           (half-life)

Clearance (CL) and volume of distribution (Vd) are not fixed — they change across the lifespan:

Renal clearance is only ~30% of adult level at birth (immature glomerular filtration), reaches the adult plateau by about age 2, then declines roughly 1%/year after age 40 as nephron mass and renal blood flow fall.
Hepatic clearance is also low at birth, but many CYP enzymes then overshoot adult activity in early childhood because a child's liver mass is large relative to body weight — this is why some pediatric doses per kg exceed the adult per-kg dose. It falls back toward the adult level through adolescence, then declines again in old age as hepatic blood flow and liver mass shrink.
Volume of distribution tracks total body water: a neonate is nearly 75% water by weight (high Vd/kg), settles near 60% through adulthood, then rises again in the elderly as lean mass is replaced by fat.

Move the age slider and toggle the clearance route to see why the same mg/kg dose can be sub-therapeutic in one age group and toxic in another — the reason pediatric and geriatric dosing guidelines exist at all. Use "Snapshot" to freeze the current curve and compare it against a different age.

Live concentration curve Therapeutic window Distribution compartment

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