Table of Contents
What is normal GFR?
GFR is the glomerular filtration rate. It is the amount of blood filtered by the glomeruli per unit time. We are born with about a million glomeruli. It is reported in milliliters per minute. From the age of 2 to the age of 25 a normal GFR is 125 ml/min. Like anything else “normal” there is a bell curve with standard deviations, but 125 ml/ min is normal.
After the age of 25 the GFR normally decreases by about 1 ml/min per year. That is the normal decrease in kidney function with aging with senescence.
What is eGFR?
E stands for estimated. eGFR is the estimated GFR from an estimating equation. The estimated GFR is a calculated number based on a number of factors including the serum creatinine, age and gender of the patient. It’s not an actual test run by the lab. The patient will most likely think the eGFR is an actual test run by the lab. This formula was developed and validated with a certain degree of precision and confidence intervals which are fancy ways of saying they are not exact.
The other thing is the formulas tend to be less accurate at higher levels of GFR. This makes sense because at lower levels of creatinine, a smaller change in creatinine means a bigger change in GFR. Every time the creatinine doubles the GFR decreases by 50%. A change in creatinine from 0.6 – 0.7 is a bigger change in GFR than a change in creatinine from 2.6-2.7.
Example let’s say for a given patient:
Creatinine 0.6 = GFR 120 – then
Creatinine 1.2 = GFR 60
Creatinine 2.4 = GFR 30
Creatinine 4.8 = GFR 15
Creatinine 9.6 = GFR 7.5
So as you can see, a “flukey” change in creatinine between 0.6 and 1.2 – lets say by 0.1 or 0.2 – because of hydration, if you exercised, what you ate the day before is going to result in a larger change in reported GFR than a change in creatinine by 0.1 – 0.2 in the 2.4 – 4.8 range. In either case your kidney function didn’t really change.
Patient’s often refer to their GFR as their percent kidney function. As you can see this is not exactly true as a normal GFR is 125 ml/ min, not 100. Also it is not clear what is meant by percent. Is it percent for a young person or percent for an older person? A normal GFR for a 75 year old may be 75, so if his eGFR is 50 then is his percent kidney function 50% or 66%?
What is chronic?
Chronic is 3 months. If someone’s GFR crosses stages on 1 measurement that is not worsening CKD. Needs to be for 3 months. I tend to give the patient credit for the best or highest GFR they had in the last 3 months. Not sure if this is right or not, but that’s what I do.
What are the CKD stages?
G (referring to GFR) may be added in the staging classification.
● CKD stage G1 – eGFR > 90
● CKD stage G2 – eGFR 60 – 89
● CKD stage G3a – eGFR 45 – 59
● CKD stage G3b – eGFR 30 – 44
● CKD stage G4 – eGFR 15 – 29
● CKD stage G5 – eGFR < 15
Uremic symptoms typically develop with a GFR in the 5-10 range. This staging system is an excellent way to freak out patients. The general lay person knowledge of stages comes from cancer. Stage 3 cancer is bad. Depending on your age and GFR, stage 3 kidney disease may be the mildest form of kidney disease.
CKD stage G1 – eGFR > 90: This is defined as normal or increased GFR. (GFR may be increased in conditions with hyperfiltration often seen in pregnancy and early diabetes). So we know that normal GFR is 125 and they say normal is > 90. That’s fine by me given there is a bell curve of normal and that the precision of eGFR is less at these levels.
Thing is this: Everyone who has normal kidneys and a normal GFR doesn’t have CKD stage 1. That would mean that everyone in the world has CKD. They don’t have anything, they have normal kidneys. So in order to have CKD stage 1 you need to have something else going on with the kidneys. This typically is either proteinuria (think diabetes or a glomerulonephritis with proteinuria and a normal creatinine) or a structural kidney disease (think polycystic kidney disease with a normal creatinine).
I typically just classify these patients by whatever condition they have. You should be doing this anyway. I find that calling it CKD stage 1 kidney disease as superfluous.
CKD stage G2 – eGFR 60-89: This is defined as a mildly decreased GFR. But, given the normal age-related decrease in GFR (by 1 ml/ min/ year) it may be normal depending on the age of the patient. For this reason and also because the estimating equations have less precision at higher GFRs, an eGFR in this range alone is not sufficient to classify someone as having chronic kidney disease. As in CKD stage 1 there has to be something else going on (such as proteinuria or structural kidney disease.
CKD stage G3 – eGFR 30-59: This is defined as a moderate decreased GFR. So based on the “normal” age related decline in kidney function/ decrease in GFR a decrease in GFR to this range is more than can be attributed to the normal aging process. Therefore an eGFR of < 60 for greater than 3 months. “buys” the patient a diagnosis of chronic kidney disease. The result of this classification system: A patient without proteinuria or structural kidney disease and an eGFR of 61 has nothing, the same patient has an eGFR of 59 for 3 months and now has stage 3 chronic kidney disease. Now the patient is understandably, but unnecessarily freaked out.
After the initial classification system was out for a while there was another observation: Patients with an eGFR in the high end of this range tended to end up okay particularly if they didn’t have proteinuria. There was a very low probability of progression to End Stage Kidney Disease. The staging system accordingly modified:
● CKD stage G3a – eGFR 45-59: Mild-Moderate decreased GFR
● CKD stage G3b – eGFR 30-44: Moderate-Severe decreased GFR
If you’re going to use a cutoff to stratify which patients are at higher risk use a cutoff of 45
CKD stage G4 – eGFR 15-29: This is defined as severely decreased GFR. This clearly is abnormal. Patient’s toward the lower end of this range should be counseled and prepared for the possibility of renal replacement therapy (dialysis or kidney transplant). A patient can be placed on a kidney transplant waiting list when the eGFR is less than 20. The discussions should include if the patient would want renal replacement therapy or conservative management and if so education and choice on dialysis modality (hemodialysis or peritoneal dialysis).
However, patient’s in this stage may have widely different risks for progression to ESRD. Not only based on their age, but also their age and degree of proteinuria. This risk can be predicted by the “Tangri” kidney failure risk calculator. Link here, further discussion below.
The Kidney Failure Risk Equation
CKD stage G5 – eGFR < 15: Defined as Kidney Failure. These patients are close to requiring renal replacement therapy. They should have made a decision regarding preferred modality or conservative care and have dialysis access preemptively placed.
End Stage Kidney Disease: This means the patient is dependent on renal replacement therapy (dialysis or kidney transplant). Sometimes it’s referred to as CKD stage G5D. Typically required when GFR is between 5-10.
Albuminuria Stages:
The degree of proteinuria influences the risk of progression. For this reason albuminuria was added to the staging system.
The GFR stages are referred to as G1 – G5. Albuminuria stages are:
● A1 – normal urinary albumin ( < 30 micrograms/ mg)
● A2 – moderately increased urinary albumin (30 – 300 micrograms/ mg)
● A3 – severely increased urinary albumin (> 300 micrograms/ mg)
Read about my opinion of this classification system here.
Suffice it to say that for any given GFR stage, the more proteinuria the worse the prognosis.
Tangri Kidney Failure Risk Equation
I find this very helpful, both in estimating risk for planning purposes and counseling the patient who may ask what the chance is that they will need dialysis. It provides a 2 year and 5 year risk percentage and can be done with just the eGFR and urinary albumin (additional lab information may add additional accuracy to the estimate).
I use it often. It can be found here:
The Kidney Failure Risk Equation
Summary
Understanding normal GFR including normal age related changes and risk for progression provides a foundation for counseling patients with their concerns:
● What does having CKD mean?
● What is the risk of needing dialysis or a kidney transplant?
