Medication safety - prescribing in special populations, relating to age, pregnancy, renal impairment, liver failure, or critical illness

Overview

Prescribing in renal impairment, hepatic impairment, and critical illness each requires specific adjustment beyond standard dosing - renal impairment affecting renally-cleared drugs, hepatic impairment affecting hepatically-metabolised drugs and protein binding, and critical illness altering volume of distribution, organ perfusion, and protein binding simultaneously in ways that are harder to predict from any single laboratory value
Relevant across essentially all hospital and complex chronic disease prescribing, given the high prevalence of organ impairment and critical illness in these populations

Renal impairment reduces clearance of renally-eliminated drugs/metabolites, requiring dose reduction or interval extension, with dosing typically guided by estimated GFR/creatinine clearance formulas that themselves have specific limitations in populations with unusual muscle mass or rapidly changing renal function
Critical illness produces complex, often unpredictable pharmacokinetic changes - altered volume of distribution (fluid shifts, capillary leak), reduced protein binding (hypoalbuminaemia increasing free drug fraction), and variable organ perfusion - meaning standard dosing nomograms validated in stable outpatients may not reliably apply to critically ill patients

Renal and hepatic function should be actively assessed before prescribing any drug with significant renal or hepatic clearance dependence, with dose adjustment per validated references/local protocols rather than empirical estimation
In critical illness, therapeutic drug monitoring (where available for the specific agent) is particularly valuable given the reduced reliability of standard dosing predictions in this population

Dose reduction or interval extension for renally-cleared drugs in renal impairment, avoidance or careful titration of hepatically-metabolised drugs in significant hepatic impairment, augmented renal clearance in some critically ill patients (a less commonly recognised scenario requiring higher, not lower, doses of renally-cleared drugs)

Some critically ill patients (particularly younger patients with sepsis/hyperdynamic circulation) develop augmented renal clearance rather than impairment - a less commonly recognised phenomenon where renally-cleared drugs may actually require higher doses than standard, not the reduced doses that might be assumed by default in a critically unwell patient

Critical illness pharmacokinetics are often less predictable than either renal or hepatic impairment alone, since multiple factors (volume of distribution, protein binding, organ perfusion, and sometimes augmented rather than reduced clearance) interact simultaneously - this is why therapeutic drug monitoring, where available, is particularly valuable in this population rather than relying on standard dosing nomograms alone

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