last updated 23-10-2021

Drug elimination

How do drugs leave the body?

Typically xenobiotics are removed by the liver via metabolism, or the kidneys via excretion. Both eliminate drugs at different rates and it is important for clinicians to know how quickly it is removed so they know when to re-dose a drug.

To evaluate this, imagine circulating blood through an elimination system that removes the drug from our container representing the volume of distribution. If the concentration before and after the elimination system is measured (CA and CV), the difference in concentration can be determined. The extraction ratio represents the change in concentration which is a fixed percentage and is calculated by dividing the difference by the concentration in the blood that arrives at the clearing organ.

Extraction ratio = (CA - CV) / CA

If the extraction ratio is multiplied by the volume that reaches the clearing organ per time (Q), an expression is produced that indicates the volume that is totally cleared of the drug per time. This measure is known as the clearance.

Clearance (Cl) = Q · (CA - CV) / CA

 See how a single dose of a drug is removed from the body by clicking here and selecting the summary model. First increase the dose rate to reach a concentration of 5. Then increase the clearance rate. The flow rate of water out the cylinder decreases with the decreasing concentration in the cylinder.

Then try to maintain the drug concentration at 6.5 by adjusting the drug flow in and setting the clearance rate at normal.