Functional Reserve of the Kidney.

The exploration of the normal limits of physiological responses and how these responses are lost when the kidney is injured are rarely used in clinical practice. However, the difference between "resting" and the "stressed" responses identify an adaptive reactiveness that is diminished before baseline function is impaired. This functional reserve is important in the evaluation of prognosis and progression of kidney disease. Here we discuss stress tests that examine protein-induced hyperfiltration, proximal tubular secretion, urea-selective concentration defects and acid retention. We discuss diseases in which these tests have been used to diagnose subclinical injury. The study and follow-up of abnormal functional reserve may add considerable understanding to the natural history of chronic kidney disease.