What is Glomerular Filtration Rate (GFR)?

and please could you explain what glomerular filtration rate is? So I'll try and explain in simple terms. This is based on people's blood tests. This is a number that we calculate and use to quantify kidney function. In simple terms the higher the number the better. Some people think of it as a percentage so a number out of 100 and we say that a GFR, a glomerular filtration rate, greater than 60 is probably normal and then as that number goes down kidney function gets worse. We use this GFR number to categorize the stages of chronic kidney disease. So chronic kidney disease stage 3 is a GFR of 30 to 60, CKD stage 4 is a GFR of 15 to 30 and CKD chronic kidney disease stage 5 is when you have a glomerular filtration rate less than 15 and this is the stage at which you are developing end-stage kidney disease.