The following pharmacological definition has been taken from the Pharmacology and Experimental Therapeutics Department Glossary at Boston University School of Medicine.
The concentration of a drug or chemical in a body fluid – usually plasma – at the time a “steady state” has been achieved, and rates of drug administration and drug elimination are equal. Css is a value approached as a limit and is achieved, theoretically, following the last of an infinite number of equal doses given at equal intervals. The maximum value under such conditions (Css,max) is given by Css,max = C0/(1 -f), for a drug eliminated by first-order kinetics from a single compartment system. The ratio Css,max/C0 indicates the extent to which drug accumulates under the conditions of a particular dose regimen of, theoretically, an infinitely long duration; the corresponding ratio 1/(1 – f) is sometimes called the Accumulation Ratio, R. Css is also the limit achieved, theoretically, at the “end” of an infusion of infinite duration, at a constant rate.
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