LAUSR

Introduction Glomerular filtration rate (GFR) is accepted as the best overall index of kidney function. GFR varies with age, sex, body size and even diet [1]. A persistently reduced GFR is an indication of chronic kidney disease (CKD), whereas an abrupt reduction of the GFR may be used to describe acute kidney injury (AKI). A GFR of <60 mL/ min/1.73 m2 increases the risk of complications and cardiovascular disease [2]. The concept of “normal” renal function is complex. GFR is not fixed; to the contrary, it shows dynamic physiological variation and can be assessed by clearance [3]. Technically, clearance is a tool for comparing renal function among different individuals independently (at least in large part) of urine flow, body size and solute concentration in the blood. In the steady-state condition, clearance reflects GFR. The widely used creatinine clearance is only a surrogate of GFR because of its proximal tubular excretion. However, serum creatinine correlates with the reciprocal of GFR, making it possible to estimate steadystate GFR [4]. The estimated GFR (eGFR) can then be calculated with the Modification of Diet in Renal Disease (MDRD) study equation [5], which includes age, sex, and race as variables to obtain the eGFR and mitigate the limitations of using serum creatinine alone. Nonetheless, eGFR is not equivalent to GFR and not immediately related to the physiology of glomerular function at a specific moment in time. Moreover, it cannot estimate the capacity of the kidney to respond to physiological demands—i.e., the so-called renal functional reserve (RFR). At rest, the kidney operates at baseline capacity, and this capacity, like the heart during exercise, can be increased to a certain maximum level. This renal capacity to increase function is a fundamental property that allows it to adjust in response to physiological stimuli. For example, a protein meal or the intravenous infusion of amino acids markedly and rapidly increases GFR in healthy humans [6]. In this review we describe the role and importance of the RFR in physiology and disease.