geriatrics
Pharmacokinetics in Older Adults
Renal elimination
One of the most important pharmacokinetic changes associated with aging is decreased renal elimination of drugs. After age 40, glomerular filtration rate (GFR) decreases an average of 8 mL/min/1.73 m2/decade (0.1 mL/sec/m2/decade); however, the age-related decrease varies substantially from person to person. Serum creatinine levels often remain within normal limits despite a decrease in glomerular filtration rate (GFR) because older adults generally have less muscle mass and are generally less physically active than younger adults and thus produce less creatinine. Maintenance of normal serum creatinine levels can mislead clinicians to assume those levels reflect normal kidney function. Decreases in tubular function with age parallel those in glomerular function. These changes decrease renal elimination of many drugs (see table Effect of Aging on Metabolism* and Elimination of Some Drugs ). Clinical implications depend on the extent that renal elimination contributes to total systemic elimination and on the drug’s therapeutic index (ratio of maximum tolerated dose to minimum effective dose). Creatinine clearance (measured or estimated using computer programs or a formula, such as Cockcroft-Gault—see 