Synopsis
Order and interpret therapeutic drug concentrations using a defined clinical question, valid sampling and the patient’s response, then make a safe monitoring or dose decision.
- A drug concentration is useful only when it answers a clinical question. Establish the medicine, indication, formulation, actual doses, sampling time and organ-function context before changing treatment.
- Distinguish a trough, post-distribution sample, peak and timed random concentration. They describe different points in the concentration–time profile and cannot be compared indiscriminately with the same target.
- Lithium monitoring normally uses a sample 12 hours after the last dose; stable levels are generally checked every 3–6 months, more often with higher risk, alongside renal, thyroid and calcium surveillance.
Reasoning priorities
Ensure the concentration can be interpreted against the intended target.
Document the exact last-dose time, sample time, dose, formulation and duration on the current regimen. For lithium, the usual comparison point is 12 hours after dosing. For digoxin, avoid the early distribution period by sampling no sooner than 6 hours, preferably 8–12 hours, when the clinical situation allows.
Worked reasoning
A stable 44-year-old takes lithium at 08:00 and 20:00. She has no toxicity symptoms and normal unchanged kidney function. A routine sample taken at 10:00 measures 0.95 mmol/L; her documented target applies to a 12-hour post-dose sample.
- Calculate elapsed time from the actual 08:00 dose to the 10:00 sample: 2 hours. Identify that this is not the 12-hour comparison point used by her monitoring plan, so the target cannot be applied as if the sample were a trough.
- Check that the supplied stability is accurate: ask about tremor, coordination, gastrointestinal symptoms, missed doses, illness and new interacting medicines. If toxicity were suspected, the clinical response would be urgent and would not wait for a routine retimed sample.
- For the supplied asymptomatic stable case, avoid changing the dose solely from the 2-hour result. Arrange a correctly timed sample approximately 12 hours after an actual dose, before the next scheduled dose in this twice-daily regimen, with clear phlebotomy instructions.
- Give the final action as repeat valid monitoring rather than automatic dose reduction. Independently verify that an 08:00 administration followed by a sample just before 20:00 gives approximately 12 hours, document the timing and assign the result to the clinician managing her lithium.
A patient taking a monitored medicine develops compatible new adverse effects.