IRIS has been discussing methods for estimation of GFR in the context of standardization of methods. Both in human medicine and small animal medicine, is estimation of GFR considered the most accurate measure available for assessment of renal function. GFR is estimated by measurement of clearance of a filtration marker, which is a drug that is freely filtered in glomeruli, without significant tubular reabsorption or excretion. GFR will be reduced before abnormal increases in serum urea and creatinine occur. Estimation of GFR can be of great value for research and in clinical practice, but traditional methods have been to cumbersome to come into widespread use outside research institutions. Renal clearance of inulin has been regarded as the “gold standard” method, but validation of simpler methods, without urine sampling (plasma clearance) or with a reduced number of samples, are being evaluated.