Ka is the absorption rate constant. It is a value that is used to describe the rate at which a drug enters into the systemic circulation once ingested by oral route or any other route except injectables. The “absorption rate constant” is expressed in the unit of time and is closely related to the time at which it enters the systemic circulation for making it bioavailable in the body fluids. Thus, it denotes the rate of absorption of a drug from its place of application using first-order kinetics which is in relation with time.

Ka is determined directly or indirectly as the slope of the linear relationship between the logarithm of the amount of unabsorbed drug and time expressed as “t”. The application of Absorption Rate Constant is mainly for research purposes and used in various pharmacokinetic studies.