Therapeutic Drug Monitoring
Also written as: TDM, Drug level testing
Blood tests that check whether a medication's level sits where your prescriber intends.
What it measures
The concentration of a specific medication in the blood, drawn at a defined time relative to the dose. It exists for drugs where too little fails and too much causes harm, and where the same dose produces very different blood levels in different people.
Why a doctor orders it
It is standard for medications with a narrow window between effective and harmful - lithium, digoxin, some seizure medications, certain antibiotics, and transplant immunosuppressants among them. Levels are typically checked when the drug is started, after dose changes, after an interacting medication is added or stopped, and periodically once things are stable.
What is generally associated with a higher result
A level above what the prescriber is aiming for is broadly associated with recent dose changes, interacting medications, changes in kidney or liver function that slow the drug's clearance, and samples drawn too soon after a dose. What any given number means depends entirely on the drug, the timing of the draw, and the target the prescriber has set - there is no universal high or low, and the target range is individual.
What is generally associated with a lower result
A level below the intended range is broadly associated with missed or mistimed doses, interacting medications that speed the drug's clearance, and differences in absorption. Draw timing matters just as much here as it does for high results.
Everyday things that move this number
Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:
- Timing of the draw relative to the last dose - most levels are meant to be drawn at a trough, just before the next dose
- Missed doses in the days before the test
- Newly started or stopped medications and supplements that interact
- Hydration and kidney function, for drugs the kidneys clear
- Whether the dose has been stable long enough to reach a steady level
Questions to take to your appointment
- Was this drawn at the right time relative to my last dose?
- What level are you aiming for in my case, and why that target?
- Should I take my usual dose before the blood draw on test day, or wait until after?
- Do any of my other medications or supplements interact with this one?
Build a printable questionnaire with your own results →
Where to read more
MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .
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