# Elimination rate constant

The **elimination rate constant** *K* is a value used in pharmacokinetics to describe the rate at which a drug is removed from the system.[1]

It is often abbreviated *K* or *K*_{e}. It is equivalent to the fraction of a substance that is removed per unit time measured at any particular instant and has units of T^{−1}. This can be expressed mathematically with the differential equation

- ,

where is the blood plasma concentration of drug in the system at a given point in time , is an infinitely small change in time, and is the concentration of drug in the system after the infinitely small change in time.

The solution of this differential equation is useful in calculating the concentration after the administration of a single dose of drug via IV bolus injection:

*C*is concentration after time_{t}*t**C*is the initial concentration (_{0}*t*=0)*K*is the**elimination rate constant**

## Derivation

In first-order (linear) kinetics, the plasma concentration of a drug at a given time t after single dose administration via IV bolus injection is given by;

where:

*C*is the initial concentration (at_{0}*t*=0)*t*is the half-life time of the drug, which is the time needed for the plasma drug concentration to drop to its half_{1/2}

Therefore, the amount of drug present in the body at time t is;

where *V _{d}* is the apparent volume of distribution

Then, the amount eliminated from the body after time t is;

Then, the rate of elimination at time t is given by the derivative of this function with respect to t;

And since is fraction of the drug that is removed per unit time measured at any particular instant, then if we divide the rate of elimination by the amount of drug in the body at time t, we get;

## References

- Svensén CH, Brauer KP, Hahn RG, et al. (September 2004). "Elimination rate constant describing clearance of infused fluid from plasma is independent of large infusion volumes of 0.9% saline in sheep".
*Anesthesiology*.**101**(3): 666–674. doi:10.1097/00000542-200409000-00015. PMID 15329591.