A decline in the function of the kidneys may affect the excretion of certain drugs. Due to a decreased cardiac output, there will be a smaller blood flow in the kidneys. Also the number of functioning nephrons declines during ageing, as does the glomerular filtration rate and the tubular function.
The creatinine serum concentration is not an adequate tool in elderly patients to determine renal function. Serum creatinine concentrations reflect a balance between creatinine production by muscle tissue and creatinine clearance by the kidney. In older persons, the creatinine production is decreased as a result of decreased muscle mass. So, "normal" creatinine concentrations in elderly may not reflect a true decline in renal function.
The Cockroft-Gault equation corrects for age-associated changes in creatinine production based on population parameters:
creatinine clearance = (140-age) x weight (kg) / 0.81 x serum creatinine (μmol/L)
(for women the calculation has to be multiplied by 0.85)
Example: antibiotics in elderly: a lower volume of distribution and reduced clearance would lead to higher concentrations (from low Vd) and accumulation. Therefore it is typically best to decrease the frequency of administration.
An 84 year-old, 48-kg female with a serum creatinine of 186 needs a renally eliminated antibiotic. What is her estimated creatinine clearance?
Extra info: The calculation: creatinine clearance = [((140-84) x 48) / (0.81 x 186)] x 0.85 = 15 ml/min