INTRODUCTION

It is known that chronic kidney disease (CKD) and senescence bring about a progressive reduction in glomerular filtration rate (GFR) and that in the former this is usually associated with an increase in the fractional excretion of calcium, phosphorus, magnesium, and uric acid. However, it has not yet been explained how these substances are excreted in the healthy oldest old. Thus, in the present study, we examined the renal handling of these substances in very aged people in comparison with CKD patients with similar GFR levels (stage III-CKD).

MATERIALS AND METHODS

Twenty volunteers were studied; 10 of them were healthy very old (VO) (≥ 75 years old) individuals and 10 were stage III CKD patients. Exclusion criteria were as follows: presence of altered (abnormally high or low) plasma calcium, phosphorus, magnesium and uric acid, as well as previous diagnoses of diabetes mellitus and obstructive uropathy and use of drugs that could alter plasma levels of the studied substances. All volunteers were on a diet with the same content of these elements (3-day dietary register). We measured calcium, phosphorus, magnesium, uric acid, creatinine in serum plasma and morning urine, as well as serum parathyroid hormone level, in each volunteer. From these data, fractional excretion (FE) of these substances was obtained. A statistical analysis was carried out using the Wilcoxon test.

RESULTS

Serum creatinine: 1.8 ± 0.4 mg/dl (CKD) versus 0.8 ± 0.2 mg/dl (VO), p = 0.0002; serum calcium: 9.1 ± 0.3 mg/dl (CKD) versus 8.7 ± 0.4 (VO), p = 0.022; serum magnesium: 2.3 ± 0.2 mg/dl (CKD) versus 2.0 ± 0.1 (VO), p = 0.05; serum phosphorus: 3.9 ± 0.5 mg/dl (CKD) versus 3.0 ± 0.4 mg/dl (VO), p = 0.002; serum uric acid: 6.6 ± 1.5 (CKD) versus 5.2 ± 1.4 mg/dl (VO), p = 0.04; FE of calcium: 2.5 ± 1 % (CKD) versus 0.8 ± 0.3 % (VO), p = 0.04; FE of magnesium: 7.2 ± 4.1 % (CKD) versus 2.9 ± 0.9 % (VO), p = 0.02; FE of phosphorus: 25 ± 9 % (CKD) versus 9.1 ± 5.7(VO), p = 0.001; FE of uric acid: 10 ± 3 % (CKD) versus 8 ± 5 % (VO), p = 0.05.

CONCLUSION

Serum levels and FE of calcium, phosphorus, magnesium and uric acid were significantly higher in CKD patients compared to healthy very old people with similar GFR, except for serum magnesium and FE of uric acid, which were similar in both groups.