Some elderly patients show gradual increases in creatinine even when hypertension, blood glucose levels, etc., are well controlled. When should these patients be referred to nephrology or come in for a 24-hr urine test?
—KIM CRUM, MSN, FNP, RNC, Cutler, Ohio

Is the age-related decline in renal function that causes a decrease in creatinine clearance a result of normal aging or disease? That depends on how you define elderly. Normal aging has been difficult to study since the population of elders is so varied in their history of environmental exposures and the existence of underlying, often occult, medical conditions. Generally, lean body mass (which is the source of creatinine) decreases with age. But renal function also declines, resulting in less creatinine clearance. Serum creatinine in the elderly will not increase until 50% of nephrons are no longer functional. Because of these age-related changes, any elevation of creatinine is significant, particularly when determining dosage for drugs primarily excreted by the kidney. Since serum creatinine is not a reliable indicator of renal function in the elderly, renal function is best assessed and monitored with a 24-hr urine test for creatinine clearance. For more information, see J Nephrol. 2007;20:617-625.
—Virginia Joslin, PA-C, MPH (118-11)