"Many drugs are only effective at certain blood levels. This level varies over 24 hours. So when should we measure them?"

Drug blood level monitoring

Some drugs need to be monitored to ensure they are having an optimal effect in the patient. As a results we need to take bloods at a specific time to obtain meaningful information.

So when should you make your appointment to measure certain drug levels?

We aim to measure trough levels of drugs and this logically occurs just before the next dose is due. However when multiple drugs are given and both need monitoring then the best time is just before the next combo dose (as this will be a dual trough level).

Phenobarbitone: just before next dose

Bromide: just before next dose

Phenobarbitone and bromide: bloods just before next bromide/phenobarb combo dose

Gemfibrozil (lopid) and bromide: just before the next combo dose.

Thyroxine (given as a medicine): just before next due dose

The effect of drugs on blood levels

A related question is when should we measure a biochemical blood level eg glucose, in relation to the administration of a drug that controls it?

Insulin: effect on Glucose
We are trying to measure trough blood glucose levels, so blood sampling occurs at 6 hours post insulin injection. Advice: Client feeds am, injects insulin, and no further feeding til bloods done 6 hrs later.

Neomercazole: Effect on T4
We are trying to see how low neomercazole drags the T4 level. Usually therefore measure at maximum effect of neomercazole. Neomercazole is ideally given 8 hourly so max effect is about 4 hours after a dose. Thus blood sampling occurs at 4 hours post neomercazole dosing.