Elimination Rate Constant

What is this formula?

The elimination rate constant quantifies the speed at which a drug is removed from the body. It is a fundamental pharmacokinetic parameter used in first-order elimination models.

A larger value of k indicates faster drug elimination, while a smaller value indicates slower elimination.

When to use it

Use this formula when drug concentrations are measured at two different times during the elimination phase and first-order kinetics can be assumed.

It is commonly used in pharmacokinetic studies, therapeutic drug monitoring, clinical pharmacology, and drug development.

Example

A drug concentration decreases from 20 mg/L to 10 mg/L over 4 hours.

Data:

C1 = 20 mg/L

C2 = 10 mg/L

t2 − t1 = 4 h

Formula:

k = ln(C1/C2)/(t2 − t1)

Substitution:

k = ln(20/10)/4

k = 0.6931/4

k = 0.1733 h⁻¹

Result:

k = 0.173 h⁻¹

Applications

- Determination of drug half-life

- Pharmacokinetic modeling

- Dose regimen design

- Therapeutic drug monitoring

- Drug development studies