Renal function testing and liver function testing share many of the same problems. In both the kidney and the liver a multiplicity of enzyme and transport systems coexist—some related, others both spatially and physiologically quite separate. Processes going on in one section of the nephron may or may not directly affect those in other segments. Like the liver, the kidney has not one but a great many functions that may or may not be affected in a given pathologic process. By measuring the capacity to perform these individual functions, one hopes to extract anatomical and physiologic information. Unfortunately, the tests available to the clinical laboratory are few and gross compared with the delicate network of systems at work. It is often difficult to isolate individual functions without complicated research setups, and it is even more difficult to differentiate between localized and generalized damage, between temporary and permanent malfunction, and between primary and secondary derangements. One can measure only what passes into and out of the kidney. What goes on inside is all-important but must be speculated on by indirect means. A tremendous handicap that results from this situation is the inability of function tests to reveal the etiology of dysfunction; the only information obtained is whether or not a certain degree of dysfunction is present and a rough estimate of its severity. Therefore, useful information can be obtained only through knowledge of the physiologic basis for each test and by careful correlation with other clinical and laboratory data.

Renal function tests fall into three general categories: (1) tests predominantly of glomerular function, (2) tests reflecting severe glomerular or tubular damage (or both), and (3) tests of predominantly tubular function.