Km value indicates:
Purity of Enzyme
Physiological role
The substrate concentration at half maximal velocity
Aﬃnity
The dose rate-Cpss relationship is linear only in case of drugs eliminated by first order kinetics. For drugs (e.g. phenytoin) which follow Michaelis Menten kinetics, elimination changes from first order to zero order kinetics over the therapeutic range. Increase in their dose beyond saturation levels causes an increase in Cpss which is out of proportion to the change in dose rate.
In their case: