GFR is a calculation of how efficiently the two 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. Many chronic kidney diseases get worse over time and kidney function, as measured by GFR, can decline progressively. Your doctor will measure the blood level of creatinine and/or Cystatin C to calculate your GFR at regular intervals. This information will help your doctor to evaluate your progress.

Creatinine is a waste product from muscle tissue, so it is somewhat dependent on body size and is affected by growth, whereas Cystatin C is independent of growth. Both these substances are filtered from the blood by the kidneys. When the kidneys are failing, creatinine and Cystatin C slowly build up in the blood. A formula is used to determine the GFR from the blood concentration of creatinine and Cystatin C. The higher their blood concentrations, the lower the GFR, the worse the kidney function.

Normal GFR varies between 90 and 130. GFR below 90 is considered “impaired” kidney function.

There are 5 stages of CKD, based upon the patient's GFR:

• stage I – GFR more than 90, but presence of risk factors for CKD, such as proteinuria or abnormal appearance of both kidneys on ultrasound or CT scan (e.g., cysts)
• stage II – GFR between 90 and 60
• stage III – GFR between 60 and 30
• stage IV – GFR between 30 and 15
• stage V – GFR less than 15; end-stage renal disease (ESRD), requiring renal replacement therapy (dialysis or kidney transplant)

People with GFR impairment due to CKD feel quite well as long as the complications of GFR impairment, which become more frequent and severe as CKD worsens, are recognized and treated.

Complications of GFR impairment include:

•  anemia (low red blood cell count)

•  poor growth

•  bone disease (the kidneys regulate blood calcium and phosphorous)

•  high blood pressure (hypertension)

•  acidosis (accumulation of acid in the blood)