peak-to-trough ratio peak-to-trough ratio peak-to-trough ratio (ptr), also known as peak-to-trough variation or peak-to-trough fluctuation, is a parameter in pharmacokinetics which is defined as the ratio of cmax (peak) concentration and cmin (trough) concentration over a dosing interval for a given drug. a drug with an elimination half-life of 24 hours taken once per day will have a peak-to-trough ratio of approximately 2. peak-to-trough ratio is proportional to half-life and to dosing interval, with longer half-lives and shorter dosing intervals giving smaller ratios. peak-to-trough ratio in pharmacokinetics is the ratio of peak (cmax) and trough (cmin) levels of a drug over its dosing interval (τ) at steady state.