Renal concentration tests: Osmotic pressure, specific gravity, refraction and electrical conductivity compared

Abstract

Concentration tests were performed in patients with varying degrees of renal insufficiency and in normal control subjects, comparing measurements of urinary specific gravity, osmotic pressure, electrical conductivity, refraction and related concentrative properties. No one concentrative property was found to be of unique diagnostic value. Concentration tests by any measure are highly variable physiologically and the results are poorly reproducible. Such tests can be usefully quantified for several concentrative properties by means of boundary values, above which renal function is assumed to be normal and below which it is assumed to be impaired, with a zone of doubt between. Suggested values for this purpose are tabulated. Selection of one or another concentrative property for diagnosis should be determined largely upon methodologic considerations. Osmotic pressure and refraction are methods of choice in respect to accuracy; the latter, in addition, combines rapidity and simplicity of measurement. A theoretic discussion of effects of osmotic diuresis on concentration tests is presented, together with corrections for the effects of glycosuria and proteinuria on measurements of urinary refraction and specific gravity. It is recommended that specific gravity measurements of urine for clinical purposes be abandoned as a laboratory procedure in favor of the more accurate determinations of refraction or osmotic pressure.