The pharmakokinetic change occurring in geriatric patient is due to

One of the most impoant pharmacokinetic changes associated with aging is decreased renal elimination of drugs. After age 40, creatinine clearance decreases an average of 8 mL/min/1.73 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 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 who assume those levels reflect normal kidney function. Decreases in tubular function with age parallel those in glomerular function. Refer katzung 11/e p1039