Kidney function is measured by the Glomerular Filtration Rate (GFR), the rate at which each kidney performs its continual processing and cleansing of blood. The GFR indicates the volume of liquid that the kidneys filter from the blood per minute (primary urine) and it is the best overall indicator of the kidney function. The normal GFR is about 100 ml/min.
A persistently reduced GFR can signify kidney disease. Based on the GFR chronic kidney disease (CKD) is categorized into five stages, from 90+ ml/min to < 15 ml/min. Persons with CKD stage 4 have advanced kidney damage (GFR of 15 to 29 ml/min) and might need dialysis or a kidney transplant in the near future. If the GFR is only 5 ml/min or less, hyperkalemia may result, particularly when high-potassium foods are consumed.

In the kidney the glomerulus (plural glomeruli) is a tiny ball-shaped structure composed of capillary blood vessels actively involved in the filtration of the blood to form urine. The glomerulus is one of the key structures that make up the nephron, the functional unit of the kidney.