Steady State Css
The plateau reached during multiple-dose therapy when drug input equals elimination — average plasma concentration no longer rises between doses, and the drug's full clinical effect is established.
Definition
During repeated drug dosing at a fixed dose and interval, drug accumulates with each dose until a plateau is reached at which the amount eliminated during one interval exactly equals the dose given. At this point — steady state — average plasma concentration remains constant from cycle to cycle, although concentration still fluctuates between Cmax,ss (peak) and Cmin,ss (trough) within each interval.
The concept is fundamental to all multiple-dose pharmacotherapy. TDM samples should ideally be collected at steady state for the result to be interpretable against published therapeutic ranges. Background on the one-compartment model is in the PK basics guide.
Concentration rises and plateaus near Css after 4–5 half-lives of regular dosing
Time to Reach Steady State
Time to steady state depends only on half-life — not on dose size, dosing interval, or route of administration (for linear kinetics):
Clinical implications: A drug with t½ = 2 h (ibuprofen) reaches steady state in ~10 h. A drug with t½ = 20 h (fluoxetine parent) takes ~4 days. Amiodarone (t½ = 40–55 days) may take months to reach true steady state — which is why it requires a loading protocol. See half-life glossary entry for more.
Steady-State Formulas
τ = dosing interval. Doubling dose doubles Css; halving CL doubles Css.
Accumulation factor R = 1/(1−e−ke·τ). Large when τ << t½.
TDM trough drawn just before next dose; should be at steady state (4–5 × t½).
Achieves target immediately but does NOT shorten time to true steady state.
Clinical Significance
TDM sampling at steady state
Trough levels for drugs such as vancomycin, tacrolimus, and phenytoin should be collected at steady state so that the measured concentration reflects stable long-term exposure. Sampling too early (before steady state) gives a false low result. The 2020 ASHP/IDSA vancomycin guidelines specify that AUC/MIC estimation should be performed after the patient has been on stable vancomycin dosing. See also TDM guide.
Loading doses
For drugs with long half-lives where waiting 4–5 × t½ for steady state is clinically unacceptable, a loading dose is used. Examples:
Dose changes and new steady state
After any dose change (increase, decrease, or addition of an interacting drug), a new steady state is reached in another 4–5 half-lives. This explains why dose adjustments for chronic medications should not be made too frequently — the full effect of a dose change on plasma concentration won't be visible until the new plateau is reached. For fluoxetine (t½ ~50 h for parent; active metabolite norfluoxetine t½ ~7 days), this means waiting 2–4 weeks after a dose change before reassessing.
FAQ
What is steady state in pharmacokinetics?
Steady state is when drug input equals elimination during multiple-dose therapy, so average plasma concentration stays constant between doses. It is reached after approximately 4–5 half-lives.
How long does it take to reach steady state?
4–5 half-lives, regardless of dose size or interval. After 1 t½ = 50% SS; after 3.32 t½ = 90% SS; after 5 t½ = ~97% SS.
How is steady-state concentration calculated?
Css,avg = (F × Dose/τ) / CL. Css is proportional to dose and inversely proportional to clearance. Halving clearance (e.g., in renal failure) doubles Css at the same dose — the basis for dose reduction requirements.
Does a loading dose affect the time to steady state?
No. A loading dose achieves target concentrations immediately but true steady state (balanced input/output for the maintenance dose) is still only reached after 4–5 maintenance-dose half-lives.
Why must TDM samples be collected at steady state?
Therapeutic ranges are validated against steady-state concentrations. A sample collected before steady state underestimates the eventual plateau, leading to unnecessary dose increases. Always confirm the patient has been on stable dosing for at least 4–5 half-lives before interpreting TDM results.