The kidney plays a vital role in maintaining total body homeostasis by having both excretory and endocrine functions. The Kidney Disease Outcome Quality Initiative (K/DOQI)of the National Kidney Foundation was established to define and classify chronic kidney disease (CKD) to assist the clinician in earlier recognition and treatment of CKD and its complications. These K/DOQI clinical practice guide lines identified GFR as the keystone for the definition and staging of CKD.Inulin has long been regarded as the 'gold standard' of exogenously administered markers of GFR. However, its scarcity and high cost have greatly diminished its usefulness and it is now generally of historical interest only. Also 51Cr-EDTA is considered the most reliable method, with pure glomerular filtration, a high and stable labeling yield and only little protein binding. However, it has limited availability.