Half Life — Measured
All calculations should be independently verified prior to clinical use. These calculators are intended to supplement, not replace, clinical judgment.
Calculate the elimination rate constant (kelim) and half-life (t½) of a drug from two plasma concentrations separated by a known time interval.
The elimination rate constant is derived from the natural log of the ratio of the two concentrations divided by the elapsed time between them:
Half-life follows directly:
C1 and C2 need not be the true peak and trough; any two concentrations drawn at known times will suffice, provided C1 > C2. The longer the time interval between levels, the more accurate the estimate.
Level timing matters. This calculation assumes both levels are drawn after the same dose (dose → C1 → C2), during the elimination phase. If levels are drawn around a dose (C2 → dose → C1), the calculation is only valid at steady state. Steady state is reached after 4–5 half-lives have elapsed since the last dosing change, or can be confirmed by two consecutive identical trough levels.
Time interval ≠ dosing interval. The time interval entered here is simply the elapsed time between the two blood draws, not the dosing frequency.
kelim can be used as an input for the Time to Target Level and Steady-state Trough Level calculators.
References
- Bauer LA. Applied Clinical Pharmacokinetics. 3rd ed. McGraw-Hill Education; 2014.
- Winter ME. Basic Clinical Pharmacokinetics. 5th ed. Lippincott Williams & Wilkins; 2009.