Renal functional reserve in elderly patients.

The concept of renal functional reserve (RFR) is used in the evaluation of renal function in health and disease. RFR is defined as the difference between glomerular filtration rate (GFR) at rest and the maximum GFR for example under stimulation by an amino acid infusion or a protein rich meal. GFR decreases normally with age by about 1 ml/min per year. The purpose of this study was to establish the effect of the GFR decrease with age on absolute and relative values of RFR. We investigated 12 hospitalized patients 60 to 85 years of age (mean 74 +/- 2.0 SEM) recovering from nonrenal disease. On two consecutive days, inulin and paraaminohippuric acid (PAH) clearance was performed over a period of 4 hours. On day A patients received 1 liter of a 5% glucose solution i.v. On day B an equal volume of an amino acid solution (2 mg/kg/min) was given. In these elderly patients baseline GFR without stimulation measured 73.2 +/- 7.9 ml/min per 1.73 m2 and stimulated GFR was 99.1 +/- 8.5 ml/min, a 35.3% increase (p < 0.025). Thus, renal functional reserve measured 25.9 +/- 8.7 ml/min. PAH clearance increased from 329.0 +/- 41.9 ml/min per 1.73 m2 to 438.8 +/- 50.3 ml/min (p < 0.025). These data confirm that baseline GFR is lower in the elderly compared to young adults. However, renal functional reserve is well maintained in elderly human subjects. This suggests that glomerulosclerosis of aging may not be the only explanation for the low baseline GFR in the elderly.(ABSTRACT TRUNCATED AT 250 WORDS)