Clinical evaluation of renal function
Current clinical practice requires that the physician understand and evaluate renal function. Traditionally, the functions of the kidney have been considered as being either glomerular or tubular; modern concepts of renal physiology would require the inclusion of endocrine and metabolic functions as well. The intent of this discussion is to review current methods of evaluating renal tubular and glomerular function.
Evaluation of renal tubular function cannot be accomplished by any single test. This is because the tubules have such varied and complex functions. Testing of certain tubular functions is of clinical importance because tubular dysfunction may be responsible for certain clinical disorders and among the earliest indicators of renal parenchymal disease. Renal concentrating and diluting ability as well as renal acidification are among the most useful indicators of tubular health.
Tests of concentrating ability
Impaired renal concentrating ability is one of the earliest indicators of many different types of renal parenchymal disease and may be a clue to a renal parenchymal disorder before any abnormality in blood urea nitrogen (BUN) or serum creatinine (Cr) is detected.1
To test concentrating ability one must have an adequate stimulus for water conservation (a period of fluid restriction) and a means for measuring the tonicity or concentration of the urine. Measurement of urine osmolality by freezing point depression is one of the most reliable methods of determining urine tonicity. After a 24-hour period of fluid restriction, the urine osmolality will be greater than 900 mOsm in healthy humans.2 These values will be. . .