Medicines in Clinical Practice
Dose, route and patient factors: renal, hepatic, frailty and pharmacokinetic risk
Applied learning on dose, route and patient factors: renal, hepatic, frailty and pharmacokinetic risk.
Learning objectives
• Recognise patient factors that alter medicine exposure or effect. • Explain why a previously safe dose may become unsafe after acute illness or physiological change. • Use current medicine-specific references rather than memorised universal dose rules. • Consider route, formulation and swallowing ability as part of prescribing. • Identify situations requiring pharmacy or specialist advice.
Overview
Learning objectives
- Recognise patient factors that alter medicine exposure or effect.
- Explain why a previously safe dose may become unsafe after acute illness or physiological change.
- Use current medicine-specific references rather than memorised universal dose rules.
- Consider route, formulation and swallowing ability as part of prescribing.
- Identify situations requiring pharmacy or specialist advice.
The prescription must fit today’s patient
Dose safety is dynamic. Renal function, liver function, hydration, body weight, frailty, acute illness and interacting medicines can change rapidly. A dose tolerated for months may become unsafe during sepsis, dehydration, acute kidney injury or substantial weight loss. Medication review should therefore respond to physiological change rather than assume continuity equals safety.
Renal function
Many medicines or active metabolites depend on renal clearance. The relevant renal estimate and dose-adjustment method can differ by medicine and patient group; therefore this programme does not teach a single numeric threshold as a universal rule. The clinician should check current renal function, trend, acuity and the medicine-specific recommendation in an authoritative source. In unstable kidney function, any estimate may lag behind the clinical state.
Hepatic impairment
Liver disease can alter metabolism, protein binding and bleeding risk, but the relationship between routine liver tests and drug clearance is not simple. Medicine-specific advice, the severity of liver disease, comorbidity and clinical status should inform decisions. When evidence is uncertain, specialist pharmacy or hepatology advice may be appropriate.
Frailty and pharmacodynamic sensitivity
Frailty may increase sensitivity to sedatives, hypotensive agents, anticholinergic burden and other medicines even when standard laboratory values appear acceptable. Falls, delirium, postural symptoms, reduced appetite and functional decline may be medication-related signals. ‘Normal bloods’ do not exclude clinically important medicine harm.
Route and formulation
Route is part of the treatment decision. Swallowing difficulty, enteral tubes, vomiting, malabsorption, inability to use an inhaler correctly or problems with dexterity can make the prescribed formulation ineffective or hazardous. Crushing or opening preparations should not be improvised: modified-release, enteric-coated and hazardous medicines may require specific advice.
Weight, age and changing physiology
Body composition and fluid status can change drug distribution. Low body weight, obesity, oedema and cachexia may all influence dosing for particular medicines. Older age itself should not be used as a simplistic dose rule, but age-related physiological change and multimorbidity increase the need for individualisation.
Sources for this module
When to seek help
Specialist advice is appropriate when the therapeutic window is narrow, evidence is uncertain, renal or hepatic function is rapidly changing, the medicine has complex interactions, or the consequences of error are high. Seeking advice is not an abdication of responsibility; it is part of responsible prescribing when the decision exceeds the prescriber’s expertise.
Sources for this module
- GMC prescribing guidance
- BNF/SmPC as medicine-specific references
- NICE medicines optimisation
- Specialist Pharmacy Service monitoring resources
Educational note: medicine-specific doses, thresholds and rapidly changing regulatory requirements should be checked against current authoritative guidance at the point of care.
