A standardized test of renal concentrating capacity in adults with some results in essential hypertension.

THREE GROUPS OF PATIENTS: A with normal glomerular filtration rate, B with moderate and C with advanced renal damage, were dehydrated and fasted for 30 hours. At regular intervals measurements were taken of urine osmolality, urine specific gravity and serum osmolality. The time required to reach maximum urine osmolality varies with the degree of dehydration and inversely with the severity of kidney damage. In patients with normal glomerular filtration rate, maximum urine osmolality is not attained by 30 hours of dehydration. Thus, for shorter periods, all "normal ranges" of concentrating capacity must be related to specific durations of dehydration. Carefully measured urine specific gravities parallel urine osmolalities closely, especially when proteinuria and glucosuria are absent. The measurement of U/P osmolality ratio offers no clinical advantage in the assessment of renal concentration capacity over the measurement of urine osmolality alone. In Group A, hypertensives achieved higher urine concentrations than did the nonhypertensives under identical test conditions. A normal range for renal concentrating capacity has been presented.