Twenty-eight elderly inpatients with severe
hyponatremia were investigated prospectively to determine if fractional
uric acid clearance was increased significantly in the syndrome of inappropriate secretion of
antidiuretic hormone (
SIADH) compared with other causes of
hyponatremia. The patients were divided into three groups: Group A--
hyponatremia due to
SIADH, Group B--
hyponatremia due to
diuretic use, and Group C--
hyponatremia possibly due to a number of causes. Serum
uric acid was 3.2 +/- 0.4 and 5.3 +/- 0.6 in Groups A and B, respectively (P less than 0.05). Fractional clearance of
urate in Groups A and B were 43 +/- 5.1 and 16.7 +/- 1.9, respectively (P less than 0.001). In Group C six of the nine had abnormally increased fractional clearance of
uric acid (31.6 +/- 3.8) in addition to accepted biochemical criteria of
SIADH. In 10 patients with
SIADH,
urate clearance measured before water restriction was 40.2 +/- 9.0%, and after serum
sodium returned to normal 11.6 +/- 1.5 (P = 0.05). Of the total 28 patients 17 had increased fractional
uric acid clearance with biochemical criteria of
SIADH, suggesting that this syndrome is a common cause of the increased susceptibility to
hyponatremia among older patients.