Glomerular filtration rate
Glomerular filtration rate (GFR) is the rate at which an ultrafiltrate of plasma is produced by glomeruli of the kidney per unit of time. It is the best estimate of renal function. There are various means of estimating it, based on substances that do not undergo active transport across urinary tract interfaces. One such is inulin clearance while creatinine clearance is easier to directly measure but has more potential for inaccuracies. Renal plasma flow is slightly different.
See also Estimated Glomerular Filtration Rate page.