Glomerular Filtration Rate (GFR)

Glomerular Filtration Rate (GFR)
What is GFR and how do we calculate it
What are the normal ranges
Clinical importance of the GFR

Management and further investigation of reduced GFR
eGFR
What is the evidence behind reporting of the eGFR

Glomerular Filtration Rate (GFR)

Kidney disease affects many people worldwide, and a great number of people will have milder degrees of kidney damage. This can go undetected and untreated for many years. An acute or chronic problem of the kidneys can be discovered when an abnormal result is detected on routine urine or blood tests. If any abnormalities are detected, it is important that the kidneys are further investigated to rule out any reversible causes and to reveal the likely cause of your kidney deterioration. Once this has been determined, the doctor can initiate management and optimise treatment for your kidney condition.

You may have heard of the glomerular filtration rate (GFR) – a value that is becoming increasingly useful to help measure your level of kidney function. The GFR can give the doctor an indication of how much damage has been done to the kidneys and determine your stage of kidney disease. By monitoring the trends and changes in the GFR over a period of time, the course of your disease can be observed. The earlier kidney disease is detected, the better the chances of slowing progression of disease and restoring function of the kidneys.

Multiple studies have been performed in patients with kidney disease and the diagnosis and further investigation of kidney disease has been guided by results of these studies. For example, the Kidney Disease Outcomes Quality Initiative (K/DOQI) released a comprehensive set of guidelines, which support the use of the GFR in helping diagnose kidney impairment. A reduction in the GFR has been shown to correlate with the amount of kidney damage present.

The blood creatinine level is one of the most commonly used measures in assessing the overall kidney function. Creatinine is a waste product in your blood that is produced after muscle activity. It is normally removed by the kidneys, but when the kidney fails to function to it’s normal capacity, the creatinine level rises. However, creatinine is affected by many factors and varies between patients due to differences in age, sex and muscle mass. Compared to blood creatinine levels, the GFR provides a much more accurate picture of kidney function, taking into consideration these factors of patient age, gender and lean weight.

What is the GFR and how do we calculate it?

Your GFR tells you how much kidney function you have. It is defined as the sum of the filtration rates of all your functioning nephrons (filtering units making up the kidneys). An estimate of the normal value for your GFR can be obtained by using the equation: GFR = 140 – [your age]. For example, if you are a fit, healthy 30 yr old, your GFR should be approximately 110mls/min.

It is also important to remember that your GFR is affected by other factors such as gender, age and your body size. There is considerable variation even amongst people in the population who have similar characteristics. There are many formulae that can be used to calculate the GFR, but you will hear of commonly used formulas such as the Cockcroft & Gault and the MDRD formulas.The Cockcroft-Gault equation, accounts for age, sex and muscle mass (this value is multiplied by 1.23 for men, to account for a greater fraction of the body weight being composed of muscle): Creatinine clearance = [140 – age (years)] x LBW (kg) x 1.23 (if male) (mls/min)
Serum creatinine (umol/L)
Automatic calculators are now available that can directly calculate the GFR.
To gain an idea of your GFR, you should input your age, Lean Body Weight (LBW) and you also need creatinine levels from your blood tests. Your lean body weight is approximately: Height (cm) – 100, and also equal to the expected serum creatinine if you are female and have normal kidney function. If your actual weight is less than the values calculated for the lean body weight, you should use this weight.
MDRD (modification of diet in renal disease). The abbreviated MDRD equation has been developed, to facilitate calculation of the GFR:GFR (mL/min per 1.73 m²;1.21) = 186.3 x Serum Creatinine (exp[-1.154]) x Age (exp[-0.203]) x (0.742 if female) x (1.21 if african american)To use this formula to gain an idea of your GFR, you should input your age, gender, race and you also need creatinine levels from your blood tests.

Click here to calculate your Glomerular Filtration Rate.

What are the normal ranges?

GFR values can be classified as follows:

Normal: 100-140 mls/min,
Mild Kidney Failure < 90 mls/min,
Moderate Kidney Failure < 60mls/min,
Severe Kidney Failure < 30 mls/min, and
End-stage Kidney Failure < 15mls/min, which is incompatible with life, without dialysis or transplantation.

Your GFR will naturally decrease by 0.5-1ml/min as a part of the normal ageing process. For example, a healthy 60-70 year-old will have a GFR of 60-70 mls/min (half their young adult normal).

Clinical importance of the GFR

Trends in the GFR provide us with an overview of how the kidneys are functioning. A decrease or decline in the GFR implies progression of underlying kidney disease or the occurrence of a superimposed insult to the kidneys. This is most commonly due to problems such as dehydration and volume loss. An improvement in the GFR may indicate that the kidneys are recovering some of their function. Whereas we might deduce that a stable GFR implies stable disease, this is not always the case. There is not a directly proportional relationship between the loss of kidney function and loss of kidney mass. The kidney can compensate for loss in renal function by increasing it’s reabsorption in the remaining nephrons, thus temporarily maintaining a particular GFR value. With a reduction in the GFR, the elimination of waste products such as urea and creatinine is impaired. Failure to adjust doses of drugs that are excreted by the kidneys such as aminoglycoside antibiotics and heparins may lead to toxic levels of these drugs accumulating.

Management and further investigation of reduced GFR

If you are found to have a decreased GFR level, the doctor may ask you some more questions, perform a clinical examination and certain investigations to work out the underlying cause of your abnormal result.

Have there been any causes of insults to the kidneys? For example:
- have you been taking any new medications (anti-inflammatories, diuretics or fluid tablets, antibiotics),
- have you had any procedures that involve administering contrast and dyes (such as angiograms, CT),
- have you been sick or dehydrated and vomiting?
If the kidneys are not functioning optimally, there may be some complications that can occur. Have you noticed excess fluid in the body (swelling of the ankles, abdomen), difficulty breathing, excessive lethargy, confusion or abnormalities in your heart beat / rhythm?
Your doctor may choose to carry out some further investigations:
- Blood tests may be taken to see what your haemoglobin levels and white cell counts are, to monitor the balance of your electrolytes and waste products the kidney normally excretes.
- Urine samples are obtained, that are tested with a dipstick and also analysed to see if there is any protein or other abnormalities present.
Imaging of the kidneys through procedures such as an ultrasound test. This can provide an idea of the size of the kidneys, how your urinary tract looks and whether there is any obstruction present.
Some drugs depend on the kidneys to be excreted from the body. These medications should all be monitored closely and adjusted as necessary. Medications that are known to be toxic to the kidney should be avoided.
Lifestyle factors that contribute to your risk of disease of the heart and major blood vessels should be identified and optimised. These include: quitting smoking, adhering to a healthy diet low in fats and salts, limiting alcohol consumption and regular exercise. You should have your blood pressure checked regularly by your local GP. Your local doctor may start some blood pressure medications that can be beneficial to your kidneys.

If there are any abnormalities in blood tests or results of other investigations, you may be referred to a kidney specialist called a nephrologist.

eGFR

You may have heard about a value called the eGFR, which stands for estimated glomerular filtration rate. As the name suggests, this is an estimated value of the GFR, based on an individualised formula. When the doctor requests certain blood tests to investigate your kidney function, the eGFR is also now routinely provided. Kidney Health Australia has more information on this value, after recognising it’s usefulness in routine clinical settings. The eGFR is calculated based on your age, sex, level of creatinine and your race. This formula does not require any body measurements, thus can be easily calculated in the laboratory. However, there may be falsely high eGFRs for patients with very low body muscle mass, or falsely low eGFRs if you have a large muscle mass. When the eGFR approaches the near normal GFR range, it is less accurate. Therefore it is simply reported as ‘greater than 60mls/min’. Your creatinine levels are used in the calculation – in situations where the level is not steady (eg acute kidney failure or muscle breakdown), the eGFR will also be less accurate. To account for the effect of race, the formula increases the GFR in African Americans in view of their greater average muscle mass than compared to Caucasians. However, there are no corrections for other populations such as the Aboriginal and Torres Strait Islanders (ATSI), Maori or Asian subcultures.

What is the evidence behind reporting of the eGFR?

As a result of the automatic reporting of eGFR, the clinical detection of kidney disease has significantly increased in the community.

In 2004, a clinical study involving 324 outpatients was performed to evaluate the effect of automatic laboratory reporting of eGFR with each request for serum creatinine. The effects on detection of chronic kidney disease were assessed. This automatic reporting was combined with education aimed at doctors involved in the study. The presence of an eGFR value, in conjunction with education significantly increased the detection of chronic kidney disease from 22.4% to 85.1% of patients. However, further research needs to be conducted regarding the significance and longer term outcomes resulting from this improved detection of chronic kidney disease. Last but not least, if the eGFR is normal, but there are other abnormal investigation results (eg abnormal urine findings or uncontrolled, high blood pressure), the GFR should be calculated.

References

Akbari A, Swedko PJ, Clark HD et al. Detection of chronic kidney disease withlaboratory reporting of estimated glomerular filtration rate and an educationalprogram. Arch Intern Med 2004; 164: 1788–92.
Anavekar N, Bais R, Carney S. Chronic kidney disease and automatic reporting of estimated glomerular filtration rate: a position statement, MJA 2005; 183 (3): 138-141
Bradley M. Denker, Barry M. Brenner, Assessment of Glomerular Filtration Rate, Harrisons Principles of Internal Medicine. New York: The McGraw-Hill Companies; 2006
Kidney Health Australia, Estimated Glomerular Filtration Rate [online] 2007 [cited 13th August 2007].
Available from URL: http://www.kidney.org.au/HealthProfessionals/eGFRSupportResources/tabid/96/Default.aspx
The CARI Guidelines – Caring for Australians with Renal Impairment, Use of estimated glomerular filtration rate to assess level of kidney function, [online] 2005 [cited 26th September 2007].
Available from URL: www.cari.org.au/guidelines.phd
Thomas M. Detecting and Managing Early Kidney Failure, Department of Nephrology, RPH, 2007.
Thomas M. What’s the Hoo-Har about eGFR? Department of Nephrology, RPH, 2007.
Thomas MC. What does an eGRF <60 really mean? Baker Heart Research Institute, Melbourne, 2007.