Glomerular Filtration Rate (GFR) is a measure of the rate at which kidneys filter fluids. The GFR provides an indication of the underlying kidney function, hence is useful for the diagnosis and testing of kidney disease. 2.The glomerular filtration rate (GFR) is the rate at which an ultrafiltrate of plasma is produced by glomeruli per unit of time. The GFR gives the best estimate of the number of functioning nephrons or functional renal mass.