Describe the role of the kidney in drug excretion and the factors affecting this (80% marks). Briefly outline how you would alter the dosing of a drug with high renal excretion in a patient with renal impairment (20% marks).
The preponderance of marks was allocated to a discussion of renal drug excretion and factors altering this function. Detail was expected including a definition of clearance, and the balance between filtration / secretion / reabsorption in the tubules. Specific mention of GFR, molecular weight of filterable compounds, protein binding, and charge effects determining filtration at the glomerular level was
anticipated. Tubular transport mechanisms for secretion and reabsorption in the proximal and distal tubule should have been included in the discussion.Candidates needed to cover factors which alter GFR, competition for transport proteins, changes in pH on drug elimination, and disease states in answering the question.
An understanding that drug dosing should be based on estimating creatinine clearance and plasma concentration monitoring was essential. Loading dose is usually unaltered. However, maintenance dose and dosing interval need to be adjusted owing to an increased half-life. Many candidates did not emphasise the need to increase dosing interval as well as reduce maintenance dose in renal
Syllabus: II2d, D12h
References: Goodman and Gilman's the Pharmacological Basis of Therapeutics
p10-14. Foundations of Anaesthesia Basic and Clinical Science, Hemmings p107.
Basic and Clinical Pharmacology, Katzung p35, 48-49.
As far as college commentary goes, this one is particularly good. Much of the renal clearance chapter has been constructed using this as a framework. The next time this question showed up ( ), the pass rate went from 0% to 52%.
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