Therapeutic drug tests

Measurement of therapeutic drug levels is usually onlynecessary for drugs with a ‘narrow therapeutic window’: that is, there is only a small difference between too little (ineffective) and too much (toxic) drug. Having a therapeutic drug blood test too soon after you take your medication can distort the test results.

In general, you should always have the test JUST BEFORE your next dose of medication is due, UNLESS your doctor has specified a particular time for you to have the test.


Please note the specific collection requirements for these drugs:

  • Lithium: blood specimens should not be collected within 12 hours of your last dose.
  • Digoxin: blood specimens should not be collected within 8 hours of your last dose.
  • Cyclosporin: in some circumstances, your doctor may request that you have a blood test 2 hours after your last dose of medication. Unless this has been specifically requested, the test should be performed just before your next dose.

For all therapeutic drug tests

The Collection Centre staff will note the date, time, and amount of your last dose on your blood test request form.

Although it is important that you follow the above rules, if possible, you should avoid missing or delaying doses of essential medication by more than 2 hours.

item number 34177 - Issue date February 2021