Therapeutic drug monitoring - monitoring of renal or hepatic function

Overview

Renal and hepatic function monitoring accompanies therapeutic drug monitoring for many agents given the close relationship between organ clearance function and drug level - declining renal/hepatic function can cause drug accumulation even without a dose change, meaning organ function should be actively monitored alongside, not instead of, the drug level itself
Relevant to any drug with significant renal or hepatic clearance dependence, particularly in populations where organ function can change relatively quickly (acute illness, ageing, concurrent nephrotoxic/hepatotoxic drug exposure)

A drug level that was appropriately therapeutic at a stable dose can become supratherapeutic purely from declining organ clearance function, without any change in the prescribed dose - this explains why organ function monitoring should be paired with, not replaced by, drug level monitoring, since organ function change is often the underlying driver of an unexpected level change

Baseline renal/hepatic function assessment before starting a monitored drug, with periodic reassessment at a frequency matched to the drug's clearance dependence and the patient's risk of changing organ function
A rising drug level should prompt renal/hepatic function reassessment as part of investigating the cause, alongside considering adherence and new interacting medications

Dose or interval adjustment in response to declining organ function, even before a drug level itself becomes available or abnormal, particularly for drugs where level monitoring is less frequent than organ function could plausibly change
Temporary dose reduction or cessation during acute illness/dehydration, given the potential for transient renal function decline to affect clearance of renally-dependent monitored drugs

Lithium and renal function interaction (both lithium's own nephrotoxic potential with chronic use and its dependence on renal clearance for elimination), acute kidney injury precipitating unexpected drug accumulation, sick-day dose adjustment principles
Lithium presents a particularly important bidirectional relationship - it depends on renal function for clearance (declining renal function raises lithium levels) while chronic lithium use can itself cause nephrotoxicity over time, making regular renal function monitoring throughout lithium treatment essential in both directions, not just as a one-off baseline check

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