Therapeutic drug monitoring - specific drug concentrations and their interpretation, including digoxin, lithium, antibiotics, and anticonvulsants

The drugs

Digoxin, lithium, aminoglycoside/vancomycin antibiotics, and anticonvulsants (e.g. phenytoin, valproate) each have established target concentration ranges, though the appropriate target and its clinical significance vary by agent - some (aminoglycosides) use peak and trough concentrations for different purposes, while others (lithium, digoxin) rely primarily on trough/steady-state levels
These represent the classic, most frequently monitored drug group in clinical practice, given their combination of narrow therapeutic index and clinically important toxicity if levels are exceeded

Digoxin target range correlates with reduced heart failure hospitalisation at lower levels, with toxicity risk rising disproportionately at higher levels - explaining a shift toward lower target ranges than historically used, since higher "traditional" targets offered little additional benefit while meaningfully increasing toxicity risk
Aminoglycosides use peak levels to confirm adequate bactericidal concentration and trough levels to assess accumulation/toxicity risk - reflecting the drug's concentration-dependent killing mechanism, distinct from time-dependent antibiotics where peak level is less clinically relevant

Sample timing must be correct for valid interpretation - trough levels drawn immediately before the next dose (at steady state, typically after several half-lives of consistent dosing), and for aminoglycosides, peak levels drawn at a defined interval after administration matched to the specific agent's absorption/distribution profile
See for the detailed timing principles and common sampling errors

Dose adjustment guided by the specific target range for each agent, with the understanding that "therapeutic range" does not mean identical target for every patient - some patients require individualised targets based on clinical response and tolerance
Toxicity management specific to the agent (e.g. digoxin-specific antibody fragments for severe digoxin toxicity)

Digoxin's narrowed modern target range (lower than historical targets, given the disproportionate toxicity risk at higher levels without added benefit), aminoglycoside peak-trough dual monitoring rationale, valproate free vs total level interpretation in hypoalbuminaemia
Current evidence supports a lower digoxin target range than was historically used, since higher "traditional" levels provided little additional heart failure benefit while substantially increasing toxicity risk - this shift illustrates how a therapeutic range itself can be revised as evidence evolves, rather than being a fixed, unchanging number

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