The absorption rate constant K<sub>a</sub> is a value used in pharmacokinetics to describe the rate at which a drug enters into the system. It is expressed in units of time<sup>âÂÂ1</sup>. The K<sub>a</sub> is related to the absorption half-life (t<sub>1/2a</sub>) per the following equation: K<sub>a</sub> = ln(2) / t<sub>1/2a</sub>.
K<sub>a</sub> values can typically only be found in research articles. This is in contrast to parameters like bioavailability and elimination half-life, which can often be found in drug and pharmacology handbooks.