Lessons · Medicine foundations · the maintenance rate
The maintenance rate: replace exactly what is cleared
At steady state the rate going in equals the rate coming out, so the maintenance rate is the clearance multiplied by the target concentration.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
Half of pharmacokinetics is knowing which of two numbers a question is about: the volume of distribution fills a space once, and the clearance empties it continuously. Loading uses one, maintaining uses the other, and mixing them up is the single most common error on the topic.
How to think about it
Read the unit of what you are given. Litres per hour is a clearance, a rate. Litres per kilogram is a volume of distribution, a space. Multiply the clearance, in litres per hour, by the target concentration, in milligrams per litre, and the litres cancel to leave milligrams per hour.
Worked example
Maintenance rate = clearance × target concentrationRate in equals rate out.
Clearance given as 3 L/h and a target concentration of 15 mg/LNote the hour in the clearance; it carries through to the answer.
3 × 15 = 45 mg/hLitres per hour times milligrams per litre. Milligrams an hour.
Over a whole day: 45 × 24 = 1080 mgThe same rate, expressed over twenty-four hours.
Your turn
The clearance is 5 L/h and the target concentration is 8 mg/L. Write the line that gives the maintenance rate.
5 × 8 = mg/h
Solve one, graded on the server
The trap
Using the volume of distribution here because it is the number you saw first. A volume fills the space once; it says nothing about how fast the space is being emptied, and only the clearance carries an hour in its unit.