If you are a member of the kidney community – whether as a patient, caregiver, advocate or knowledge seeker – it is likely that you have come across the term “Glomerular Filtration Rate” or more commonly, the GFR. Perhaps you are also aware that this term broadly relates to your kidney’s functional capacity.
But do you completely understand what GFR really is?
How does your medical team clinically assess the GFR for your kidneys?
How valid is it throughout your CKD journey?
And is it always indicative of the true health status of your kidneys?
All these are some very pertinent questions that each member of the Kidney Community, especially the patients & caregivers must thoroughly know the answers to. And that is exactly what we are here to help you with, today.
GFR: Dissecting the concept
GFR stands for “Glomerular Filtration Rate”.
What does “Glomerular” mean though?
In good health, every kidney comprises of over 1 million filtration units called “Nephrons”.
Each of these nephrons is a complex, twisted tube (image below) where approximately 10 thousand cells of 26 different types arrange themselves in sophisticated patterns that make-up separate specialized filtration zones. Of these, the very first filtration zone of each nephron that literally separates out waste products like Urea and Creatinine from whole blood is called the “Glomerulus” (pronounced as “glow-mare-you-lus”). It is also the zone that prevents any blood cell or plasma protein like albumin from leaking out into the nephrons along with the wastes.
Basically, the Glomerulus is a tuft of very fine blood vessels wrapped by specialized “slit cells” of nephrons, such that the whole set-up acts as a sieve. Take a look at the image below to understand this better:
The word “Glomerular” refers to anything pertaining to the Glomerulus.
So, the quantity of blood “cleansed” of its wastes and excess components per minute at the level of the Glomerulus is referred to as the “Glomerular Filtration Rate” or “GFR”.
In perfect health, kidneys filter between 110 to 140 millilitres of blood per minute.
This means, in normal health, it takes between 35 to 45 minutes for your kidneys to “cleanse” all of your 5 litres of blood and “restore” healthy blood biochemistry. So, healthy kidneys run about 35 “cleanse-cycles” of your blood throughout the day on an average!
Since the glomerulus is the very first “gatekeeper” that separates the entire waste-load of the blood, it is this segment of blood filtration process by kidneys, that serves as a reliable measure of the “blood-cleansing” capacity of kidneys in the clinic.
Assessing GFR
There are two methods for assessing your glomerular filtration rate. These are:
Inulin Clearance Test
Calculating the estimated GFR (eGFR) with a blood test
INULIN CLEARANCE TEST
Inulin Clearance test is a “direct” method of estimating your kidneys’ blood filtration function.
What is Inulin:
Inulin (NOT Insulin) is a naturally-occurring “polysaccharide” substance that is sweet to taste but does not get absorbed, metabolized or newly-produced within our body. This makes it useful in making artificial sweeteners that are safe for consumption by diabetic patients.
Rationale of the test:
Inulin is “metabolism-neutral”, not additionally produced within the body and in good health, passes out via urine in entirety without undergoing any chemical change during the kidney filtration process. These characteristics make it ideal for studying how well kidneys can “clear-out” a substance from the blood by means of filtration at the glomerulus.
Procedure:
Inulin solution is administered directly into the patient’s bloodstream either via injection or slow infusion with a drip-set over 3-4 hours. Then, serial samples of blood and urine are assessed every 30 minutes, over 4 hours after Inulin administration to check for the time it takes to clear-out the Inulin from blood. These observations help your medical team to record in real-time, the quantity of Inulin (in milli-litres) that passes out in urine per unit time (in minutes) or in other words, your true GFR.
Pros:
- 100% of inulin is excreted out unchanged, hence this test is a fully-reliable method for measuring “blood-cleansing” ability of kidneys.
- It is the gold standard for measuring GFR (kidney filtration function) in the “academic research setting”.
Cons:
- Tedious
- Resource-heavy
- Time-consuming
- Not all medical labs have the infrastructure hence impractical for a regular clinical setting.
ESTIMATED GFR (E-GFR) CALCULATION
Estimated GFR calculation is an indirect method. It analyzes the ability of your kidneys to clear out toxic wastes such as Creatinine from the blood.
What is eGFR?
Your eGFR number represents how efficiently your kidneys remove toxic metabolic wastes from your bloodstream.
- In healthy volunteers and kidney patients not on dialysis, eGFR gives you the filtration function of your native Kidneys only.
- For Kidney patients on dialysis, the eGFR number indicates how efficiently, the joint effort of your unwell kidneys and the process of dialysis is cleaning your blood.
- In-case of members who have had a kidney transplant, the eGFR number shows the extent to which the new kidney graft is cleansing your blood.
Procedure:
There are elaborate mathematical formulae (from prior research evidence) in place for calculating e-GFR.
There are multiple separate e-GFR formulae for Adults and Children.
- Of these, the GFR formula derived by the CKD-EPI (2009) Study yields the most accurate results for adults across a wide range of kidney function, up until it falls below the level of 15%.
- On the other hand, for paediatric cases (children), the “Bedside Schwartz” Equation for eGFR calculation has been found to yield the most accurate results.
These formulae account for factors like Serum Creatinine level, Age, Gender & Ethnicity in case of adult patients and additionally, height & weight in case of Children (below 19 years of age).
Some of these eGFR formulae are also based on the level of another blood component called Cystatin C. However, those formulae tend to overestimate your kidney function by at least 20%. Hence, formulae based on “Creatinine” levels namely the CKD-EPI equation (2009) for Adults and “Bedside Schwartz” Equation for Children are preferred to calculate your eGFR.
If you are a kidney patient, you have probably undergone blood work as part of “Renal Function Tests”. You will find your Creatinine (and in some cases, Cystatin C) values there.
If you know your Creatinine level, you can calculate your own kidney function with our eGFR Calculators here or click on the image link below.
Learn how to interpret GFR numbers with respect to kidney disease status further below.
If you are a professional in nephrology (physician, renal nurse, renal dietician), try out our Professional eGFR “Composite” Calculators for Adult patients & Pediatric cases here or click on the image links below. All clinically-approved eGFR formulae are available in the respective professional calculators.
Pros:
- Convenient
- Only one simple blood test can yield Serum Creatinine (and if needed, Serum Cystatin C values)
- Based on precise, evidence-based mathematical formulae that adjust for factors such as body surface area, age and ethnicity
- No additional special infrastructure required hence fits well in the daily clinical setting.
Cons:
- Once GFR values fall below 15ml/min or End-Stage Renal Disease (ESRD) sets-in, the accuracy of eGFR formulae starts to decrease.
At that point, serum creatinine levels directly serve as a point of reference for clinical decision making. Usually, an adult patient qualifies for Dialysis and/or Kidney transplant once Serum Creatinine rises above 6 mg/dL. This roughly corresponds to eGFR below 10ml/min.
- A low eGFR value does not necessarily indicate kidney disease.
eGFR estimation is an indirect method based on variables such as Serum Creatinine, age and ethnicity. So, high serum creatinine levels will yield a low eGFR value.
Creatinine is a metabolic waste product of muscle cells. Healthy kidneys then remove the excess creatinine via urine. Hence, Serum Creatinine could rise either with an “over-enthusiastic” production by muscles or a “poorly-functioning” exhaust system i.e. kidneys.
To understand further about this aspect, read more about Creatinine here.
Interpreting your GFR number
Chronic kidney disease presents with a gradual decline in kidney function over the course of several years. This corresponds to a progressively reducing waste-filtration capacity of ailing kidneys and hence, GFR value.
To allow for diagnosis & monitoring of the extent of kidney damage and deciding on an appropriate course of treatment, Chronic Kidney Disease has been classified based on the GFR number.
The table below elaborates how your GFR number relates to the extent of kidney disease. The values below are applicable for both Inulin clearance method and eGFR calculation.
NOTE:
- End-Stage Renal Failure (ESRD) does not mean your kidneys have stopped functioning altogether. ESRD represents the stage where medicines alone cannot help unwell kidneys with the cleaning of blood wastes adequately. Renal replacement therapies namely Dialysis and/or Kidney Transplant become mandatory for the purpose.
- A GFR below 5ml/min means your native kidneys have stopped functioning completely. The technical name for this is Renal Shutdown. It is an emergency situation where the patient needs to be stabilized & started on dialysis on an urgent basis to reverse the situation & ensure survival.
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