Example:

A 70 year-old man is on gentamicin for urinary sepsis and is found to have a toxic blood level of 16mg/L

  • The blood level you want is 2mg/L
  • Drug half-life is 8 hours

 

Question: How long will it take for the blood level to fall back to the level you want?

 

Answer
Remember Half-Life is time taken for blood level to reduce by 50%

Therefore:
16mg to 8mg = 8 hours
8mg to 4mg = 8 hours
4mg to 2mg = 8 hours
Total = 3 x 8 hrs = 24 hours

Therapeutic index = a ratio that compares the dose of a drug that causes the therapeutic effect to the dose that causes toxicity

Drugs with a low therapeutic index (or narrow therapeutic window)
 Digoxin
 Gentamicin
 Carbamazepine
 Phenytoin
 Lithium
 theophylline

Examples of drugs where plasma concentration is measured

 Phenytoin: to ensure suppression of seizures
 Gentamicin: to prevent toxicity

So how do you change the plasma concentration?

To change the plasma concentration either:

 Change the dose or

 Change the dosage interval

Dihydrocodeine dose chart

So it’s better to give dihydrocodeine as 30mg every 4 hours rather than 60mg every 8.

Copyright eBook 2019, University of Leeds, Leeds Institute of Medical Education.