What is the GFR?
The eGFR is short for Estimated Glomerular Filtration Rate. This rate indicates the rate of flow of filtered fluid through the kidney. This is a number based on a blood test for creatinine content, where creatinine is a waste product. The eGFR is a test of renal function, which means it tells how well your kidneys are working. Your kidneys are important since they help in the production of red blood cells, clean waste and keep your bones strong.
The renal system is responsible for clearing waste from blood plasma, which is then excreted. This filtration happens in the kidneys. As the body cleans the fluid, it returns the clean liquids to the blood and excretes waste and excess water as urine.
Kidneys contain millions of nephrons, which are microscopic tubes that filter the fluid entering the kidneys through capillaries, or tiny blood vessels. As the fluid passes through the nephrons, plasma protein and waste are removed. The cleaned fluid, containing water and electrolytes, is passed back into the blood stream through the capillaries. The rate is, in part, a measurement of pressure from the renal glomerular capillaries into the Bowman’s Capsule within a certain time, where the Bowman’s capsule is the point where the fluid is transferred from the capillaries into the nephrons. This is the first step in the process of ultrafiltration, or glomerular filtration which removes the waste for excretion as urine.
The GFR effectively measures the creatinine levels in your blood. If you have high levels of waste in your blood, then it is likely that your kidneys are not working optimally.
A linked measure, the Creatinine Clearance Rate (CCR) is the volume of blood plasma which is effectively cleaned of the waste product, creatinine in a certain time. Creatinine is a waste product released by the muscles. Where there is too much creatinine in the blood, this can be an early indication of kidney disease. A blood test is used to calculate your CCR.
Measuring GFR
In clinical practice, this the CCR serves as the best measure for calculating GFR. The GFR considers the result of the CCR, your age and your gender. There are also more complex measures of GFR.
The unit of measurement for GFR is ml/min/1.73m2
The value 1.73m2 is a standardized measurement for the volume of a human body. This test is thus only applicable for people 19 years and over. There is another tool to calculate GFR for children.
There are actually 4 ways to calculate the GFR, and our calculator will provide you with all four results for comparison.
1. The CKD-EPT Creatinine Equation
This is the most widely used, and highly recommended measure. This uses one blood test result for serum creatinine. The formula includes a coefficient for gender and race.
2. The CKD-EPI Creatinine-Cystatin Equation
This equation is a version of the first, but also considers Cystatin, a protein produced by cells. When kidneys are functioning well, the Cystatin levels are maintained at a normal level. This equation considers both Cystatin and Creatinine levels.
3. CKD-EPI Cystatin C Equation
This equation uses only serum Cystatin C and does not consider Creatinine levels. Using the Cystatin C measure alone is particularly useful where a simple creatinine test is inconclusive, or when you are particularly muscular, so your levels of creatinine may be naturally high (one example is a body builder).
4. MDRD GFR Equation
This method was developed by the Chronic Kidney Disease Epidemiology Collaboration. The Modification of Diet in Renal Disease (MDRD) is only used in cases where chronic kidney disease has been diagnosed. This test has proved to be very accurate for people of all races, between 18 and 70 with impaired kidney function. This measure also uses a test for standardised serum creatinine in mg/dl of blood.
It should be noted that the MDRD has been shown to potentially underestimate the actual GFR in healthy patients by as much as 29%.
GFR Results
The results of the GFR test relate to stages of Chronic Kidney Disease (CKD).
Stage I: no kidney damage, or normal kidney function
Stage II: Kidney damage with mild loss of kidney function
Stage III: Mild -Moderate-severe loss of kidney function, on a spectrum from 30 – 59.
Stage IV: Severe kidney damage
Stage V: Kidney failure
The following table outlines the GFR levels according to each stage of chronic kidney disease.
GFR is thus used to measure stages of chronic kidney disease, within about a 10% error margin, considered to be acceptable.
A normal eGFR is over 60, thus stage I and II are considered healthy, perhaps with some minor kidney damage. In general, kidney function decreases with age, and wear-and-tear of the body.
It is also advised to test frequently, to measure the change in GFR over time, in order to assess whether a low GFR is an indication of decreasing kidney function. A once off low result is not sufficient to determine whether you are at risk of kidney disease.
In any case, if your GFR is low and remains low for a period of 3 months, it is a good idea to consult your doctor.
