Most kidney diseases attack the nephrons, causing them to lose their filtering capacity. Damage to the nephrons may happen quickly, often as the result of injury or poisoning. But most kidney diseases destroy the nephrons slowly and silently. Only after years or even decades will the damage become apparent. Most kidney diseases attack both kidneys simultaneously.

Glomerular Filtration Rate (GFR) based on measurement of blood concentration of Creatinine and/or Cystatin C. GFR is a calculation of how efficiently the kidneys (working together as a team) filter wastes from the blood.  It is the term that kidney specialists use to judge if kidney function is impaired, and if so, how much. 

Creatinine is a waste product created by the normal breakdown of muscle cells during activity, whereas Cystatin C is generated by all cells in the body.   Both these substances are filtered from the blood by the kidneys.  When the kidneys are not working well, creatinine and Cystatin C build up in the blood.  The worse the kidney function (the lower the GFR), the higher the blood concentration of creatinine and Cystatin C will be. 

GFR below as 90 is considered "impaired" kidney function.  Renal replacement therapy (dialysis or kidney transplantation) is required if and when GFR declines to less than 15.  People with GFR impairment due to chronic kidney disease feel quite well as long as the complications of GFR impairment are recognized and treated.  These include: