Therapeutic drug monitoring - indications for monitoring plasma concentrations, the importance of understanding the therapeutic range, and how to respond to concentrations outside that range

Which drugs

Therapeutic drug monitoring is reserved for drugs with a narrow therapeutic index (small margin between effective and toxic concentration), significant inter-individual pharmacokinetic variability, and a defined, clinically validated target concentration range correlating with efficacy/toxicity - not applied routinely to drugs with a wide therapeutic margin where monitoring adds cost/burden without meaningful safety benefit
Applies to a relatively small but clinically important group of drugs (digoxin, lithium, several antibiotics including aminoglycosides and vancomycin, and several anticonvulsants) where these criteria are met

The rationale for monitoring rests on the existence of a demonstrated relationship between plasma concentration and clinical effect (both efficacy and toxicity) - for drugs where efficacy/toxicity correlates poorly with plasma level (e.g. many drugs with a wide therapeutic margin, or where clinical effect is more directly observable and titratable), monitoring adds limited value over clinical response-guided titration alone

Understanding why a specific drug requires monitoring (narrow margin, variable pharmacokinetics, defined target range) helps correctly interpret an individual result and avoid inappropriately extending monitoring logic to drugs where it does not apply
Concentrations outside the established range should prompt clinical correlation and consideration of the underlying cause (adherence, altered clearance, interacting drugs) rather than an isolated dose adjustment based on the number alone

Dose titration guided by monitored levels for the specific agents meeting monitoring criteria, with target ranges and appropriate action thresholds specific to each drug (see )
Avoiding unnecessary or inappropriately frequent monitoring for drugs without a validated level-effect relationship, given the cost and burden without proportionate benefit

Narrow therapeutic index as the core monitoring criterion, drugs with wide therapeutic margins not requiring routine monitoring despite occasional clinician assumption otherwise, concentration outside range requiring cause investigation not just dose adjustment

Not every drug benefits from therapeutic drug monitoring - it is reserved specifically for agents with a narrow therapeutic index, significant pharmacokinetic variability, and an established concentration-effect relationship, and applying monitoring logic to drugs without these features (or skipping monitoring for drugs that do have them) both represent recognised prescribing errors

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