steadystatekoncentrationer

Steady-state concentrations, or steadystatekoncentrationer in some languages, refer to the concentration of a substance in a biological fluid that remains constant over time because the rate of input equals the rate of elimination. This concept is central in pharmacokinetics, toxicology and chemical reactor theory. In pharmacology, the term Css is used for the plasma drug concentration at steady state.

When a drug is administered repeatedly, or by continuous infusion, it builds up until input and elimination balance. For a drug given extrinsically with bioavailability F, dose D, dosing interval τ, and systemic clearance Cl, the average steady‑state concentration can be approximated by Css,avg = (F × D) / (Cl × τ). For a continuous infusion at rate R0, Css = R0 / Cl. At steady state, peak and trough concentrations depend on the regimen; fluctuations decrease as clearance increases or as the dosing interval shortens.

The time to reach steady state is governed mainly by the drug’s half-life, t1/2 = 0.693 × Vd / Cl. In most cases, steady state is approached after about 4–5 half-lives, though exact levels depend on absorption and distribution kinetics.

Clinically, achieving an appropriate steady-state concentration is important to balance efficacy and toxicity, especially for drugs with narrow therapeutic windows. Monitoring may be required, and dosing may need adjustment for age, organ function, drug interactions, body weight, or disease states. In addition, the pharmacologically active fraction is the unbound (free) concentration, which may differ from the total Css due to protein binding.

In systems biology and chemical kinetics, the concept extends to reactions and chemostats, where input flux equals consumption or removal, yielding a constant concentration over time.