RLO: Clinical impact of changes in drug clearance

Age: The elderly

Total body clearance in elderly patients is reduced through a reduction in the efficiency of both hepatic and renal mechanisms.

Both the liver volume and its blood flow decrease with increasing age, being around 35% lower in elderly compared to young adults. Similarly, there is a decrease in the activity of the metabolising enzyme systems in the liver. This results in a decrease in hepatic drug clearance which may be important when determining appropriate dosages for drugs which rely on hepatic clearance to remove them from the body. Tricyclic antidepressants, for example, should be used at a lower dose in the elderly.

In terms of renal clearance, there is a reduction in both renal mass and renal perfusion with increasing age. Moreover, the rates of glomerular filtration and tubular secretion also decrease with increasing age, such that, at age 75, the glomerular filtration rate is only 50% of that of a young adult. Indeed, the British National Formulary recommends assuming at least mild impairment of renal function when prescribing for the elderly.

 

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