Question

What is the relationship between dose, clearance, bioavailability and steady state plasma concentration?

Answer 1

The given terms like dose, clearance, bioavailability and steady state plasma concentration are related to Pharmacokinetics.

Pharmacokinetics is a study related to the kinetics of the absorption, distribution and elimination of drugs.

Let's understand the terms using an example,

Suppose you have a filled water bottle in the fridge, and you drink half of the water.

Someone looks that the bottle is half empty so fills it up again.

After some time again you drink up half water. And again the empty part is filled up.

Here, the water levels remains steady even after you consume half of it as it is filled up again.

Now, comparing this activity with pharmacokinetics,

• The quantity of water filled is the dose
• The amount of water consumed is the Clearance

The steady state concentration (C) is maintained when the Rate of drug input (i.e Dose) is equal to the rate of elimination (i.e clearance) of the drug.

Steady state = Rate of drug input i.e / rate of drug elimination i.e Clearance

Now incase of drug dose given to a body system, some amount or fraction of drug absorbed into our body.This is called bioavailability.

Bioavailability affects the steady state concentration.

Steady state concentration also depends on the dose intervals.

Thus the relationship between steady State concentration can be given as-

C = F x Dose / CL x t

Where,

• C- steady state plasma concentration
• F- fraction absorbed (Bioavailability)
• CL- Clearance
• t - dose interval

The steady state concentration is lower when lower dose is given for longer intervals. Whereas the steady state concentration value is higher if higher dose is given for shorter intervals.