For drugs with first order kinetics the time required to achieve steady state level can be predicted from: (AIIMS Nov 2012)
|A||Volume of distribution|
- For all drugs with first order kinetics, the time required to achieve steady-state levels can be predicted from the half life.
- The steady state is reached in 3-5 half lives unless dose interval is very much longer than t1/2. Some drugs become clinically effective only when these reach steady state. Prominent example is lithium.
- As regarding other options affecting steady state level
Clearance affects the steady state plasma concentration (Css) of the drug. There is no effect on the time required to achieve the steady level.
Clearance is inversely proportional to the steady state concentration (Css).
Volume of distribution
- Volume of distribution is inversely proportional to Css. There is no relation to the time required to achieve the level.
- If the volume of distribution is increased, the Css level falls.
Rate of drug infusion
If rate of drug infusion is increased, the Css increases but the time required to achieve the steady state level remains the same.
The following figure shows the rate of attainment of steady state concentration of drug in plasma and the rate of washout of drug when the drug infusion is stopped.