A careful assessment of renal function is essential to the diagnosis and management of patients with renal disease. The determination of glomerular filtration rate (GFR) provides a quantitative index of the functional nephron population. Glomerular filtration rate is an essential test to establish the rate of progress and the prognosis of renal disease. Serial determinations of GFR can be used to gauge the efficiency of therapeutic measures and to follow closely the evolution of disease processes. In selected instances, the evaluation of kidney size may be important to clarify information obtained from measurements of GFR. The time factor involved in changes in kidney size assumes importance when renal size is made a basis of measurement. Kidney size provides useful information as to the natural history of renal disease.
Neither test (determination of GFR or kidney size) excludes the necessity for a thorough work-up and a careful examination of the urine
Lubowitz H, Klahr S. Clinical Estimation of Functional Nephron Population. JAMA. 1968;203(9):657-658. doi:10.1001/jama.1968.03140090041011