13.7: Pharmacokinetics in Geriatric Patients: Effect of Age on Drug Excretion

In geriatric patients, renal physiology undergoes significant changes, including diminished renal blood flow and a lower glomerular filtration rate (GFR), leading to alterations in medication clearance. Drugs such as aminoglycoside antibiotics, lithium, and digoxin, which rely on glomerular filtration for removal from the body, particularly impact pharmacokinetics. These drugs tend to have slower clearance rates in older adults, necessitating careful dosage considerations.

Evaluation of renal function in this demographic is commonly conducted by estimating GFR from serum creatinine levels. This is achieved using the Cockcroft–Gault, or CG, and the Modification of Diet in Renal Disease, or MDRD, equations. The CG formula suggests that GFR decreases linearly with age, while the MDRD indicates a more complex, nonlinear pattern of decline.

Despite their utility, the CG and MDRD equations have limitations, particularly in individuals over 70, where data is sparse. Moreover, in the elderly, serum creatinine levels are often not a reliable indicator of renal function due to a natural reduction in muscle mass. This can lead to underestimations of kidney function when using creatinine-based assessments.

Furthermore, renal function in the aged population is influenced by intrinsic renal changes and extrinsic factors such as hypertension and cardiovascular disease, which can independently impact kidney health. These dynamics underscore the need for a nuanced approach to assessing and managing renal health in older patients.

Geriatric patients have reduced renal blood flow and glomerular filtration rate, or GFR, which affects renal drug clearance.

Drugs like aminoglycoside antibiotics, lithium, and digoxin, eliminated primarily via glomerular filtration, show notably reduced clearance rates in this population.

Renal function can be assessed by estimating creatinine clearance and GFR based on serum creatinine levels using the Cockcroft–Gault or CG and the Modification of Diet in Renal Disease or MDRD equations, respectively.

The CG equation predicts a steep linear decrease with age, while the MDRD equation suggests a nonlinear decline.

Both equations provide accurate estimations but are limited by the lack of significant data for individuals over 70 years.

Also, serum creatinine levels may decrease in the elderly due to the natural reduction in lean muscle mass or malnutrition, making it an inadequate screening test for renal failure.

Additionally, renal function may be affected by other conditions like hypertension and cardiovascular disease.