The elimination rate constant K is a value used in pharmacokinetics to describe the rate at which a drug is removed from the system. It is often abbreviated K or Ke. 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.