Glomerular Filtration Rate and Renal Functional Reserve

Glomerular filtration rate (GFR) is usually accepted as the best overall index of kidney function in health and disease. Normal GFR varies according to age, sex, and body size. In young adults it is approximately 120-130ml/ min/1.73 m2 and declines with age1. A decrease in GFR precedes the onset of clinical kidney failure; therefore, a persistently reduced GFR is a specific indication of chronic kidney disease (CKD), while an abrupt reduction of GFR possibly transient in nature may be used to describe acute kidney injury (AKI). Below 60 ml/min/1.73 m2, the prevalence of complications and the risk of cardiovascular disease seem to increase both in CKD and in AKI2-4 (see Table I and II for classification of CKD according to e-GFR). The physiological mechanism of glomerular filtration is generally clearly understood. A more complex issue however, is the measurement of GFR in clinical practice and especially the definition of “normal” renal function. In fact, one cannot define “renal function” just relying on glomerular filtration rate since the convective transport of solutes in Bowman’s space is just one of the many functions of the kidney. Furthermore, the measurement of GFR or its calculation from derived equations can be complex and faulty. Finally, GFR may not be a fixed function, but may rather display significant variations among individuals or even in different moments within one individual. All these aspects have an important impact on the diagnosis and staging of chronic kidney disease, but they are similarly important in the evaluation of kidney function in ICU patients with or without acute kidney injury. We will try to elucidate some of the aspects related to glomerular filtration rate in the clinical setting.