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Glomerular Filtration Rate (GFR) Testing

By Dr Liji Thomas, MD

The glomeruli are tiny tubules that filter blood in the kidneys. Muscles produce a waste product called creatinine, which is removed from blood plasma by the kidneys during this glomerular filtration process. Creatinine is then passed into the urine and not reabsorbed by the glomeruli to any significant degree.

The GFR Test

The glomerular filtration rate (GFR), also known as estimated glomerular filtration rate (eGFR), represents a test designed to determine how effective this renal clearance of creatinine is in order to assess kidney function. A blood test is taken to determine how much creatinine is in the blood. This result is taken into consideration along with the patient’s age, gender, body size and race.

If the eGFR is found to be low, kidney function is said to be impaired or reduced. In general, the serum or blood creatinine level does not increase until renal function has become significantly impaired. It must be noted that it is not possible to make an accurate assessment if people have high levels of creatinine as a result of a large amount of muscle tissue.

An eGFR test should be used to monitor people with chronic kidney disease (CKD) and people with risk factors for CKD such as diabetes, high blood pressure, a family history of the condition or cardiovascular disease. A doctor can use the eGFR to establish the stage of kidney disease and the best treatment plan. The earlier renal disease is diagnosed, the greater the chance of slowing or preventing its progression.

GFR and Disease Stage

For adults, a healthy GFR is more than 90, although this gradually declines with age, even in the absence of kidney disease. When a person does have a kidney disease, the GFR changes depending on the stage of the condition. The different kidney disease stages and associated GFRs are listed below.

  • Increased risk of kidney disease – Risk factors for kidney disease such as diabetes, hypertension, older age, ethnic group or family history of the condition and a GFR of more than 90.
  • Stage 1 – Kidney damage, but normal kidney function, as indicated by a GFR of 90 or above.
  • Stage 2 – Kidney damage and mildly impaired function, as indicated by a GFR of between 89 and 60.
  • Stage 3a – Mild to moderately impaired function, as indicated by a GFR of 59 to 44
  • Stage 3b – Moderate to severely impaired function, as indicated by a GFR of 43 to 30
  • Stage 4 – Severely impaired function, as indicated by a GFR of 29 to 15
  • Kidney failure – A GFR of less than 15

Chronic Kidney Disease

An eGFR of below 60 for a period of three months or an eGFR of 60 with the presence of kidney damage indicates that a patient has CKD. In this case, a doctor will start to investigate what is causing the kidney disease, monitor the kidney function and start to plan an adequate treatment.

A urine test is carried out to check the urine level of a protein called albumin. Albumin leaks into the urine when a person has kidney disease and this is referred to as albuminuria.

A doctor may also recommend further testing (such as an ultrasound scan or computed tomography scan) to help determine whether there are problems such as a kidney stone or tumor or abnormal structure of the kidney and urinary tracts. In some cases, a biopsy may be taken to assess how much damage has occurred and to aid treatment decisions.

Kidney health

Regardless of a person’s eGFR result, steps that can be taken to keep the kidneys as healthy as possible are described below:

  • Maintaining a healthy blood pressure, which is around 120/80 mmHg
  • Ensuring diet is low in salt and fat
  • Maintaining good control of blood sugar in cases of diabetes
  • Exercising for a minimum of 30 minutes on most days of the week
  • Maintaining a healthy weight
  • Discussing with the doctor drugs that may protect the kidneys
  • Refraining from smoking or using tobacco products

Reviewed by: , MD, PhD

Further Reading

Last Updated: Aug 17, 2016

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