Assessment of renal function in the old. Special considerations

The most important clinical renal function to monitor with age is glomerular filtration rate (GFR), as all other changes in renal function tend to parallel changes in GFR. The serum creatinine and creatinine clearance have become the clinical tests most frequently used to measure GFR. The creatinine clearance decreases at a rate approximately 1% per year after age 40 years. Mean serum creatinine values, however, fail to increase with age because creatinine production, which is dependent on muscle mass, falls at nearly the same rate as the renal clearance of creatinine. Serum creatinine concentrations must be interpreted with this in mind when used to determine or modify dosages of drugs cleared totally, for example, the aminoglycoside antibiotics, or partially, for example, digoxin, by the kidney.