The diagnosis of chronic kidney disease (CKD) includes testing for kidney disease in urine by looking for blood, protein and infection. It also includes the measurement of the estimated glomerular filtration rate (eGFR) by using an equation that was developed for estimation on the basis of scientific research. That equation can be found at our web site at: https://www.kidney.org/professionals/kdoqi/gfr
This particular equation is only accurate for eGFR’s that are less than 60 milliliters per minute per 1.73 meters squared. There are other tests such as the creatinine clearance, urea clearance, inulin clearance and iothalamate clearance that can be used to measure the GFR that are more accurate for a clearance greater than 60. These clearance methods are very difficult to perform and subject to some error as well. Stage 1 CKD is defined as a clearance greater than 90 and abnormal findings in the urine or on the basis of such tests as an ultrasound test or a kidney biopsy. If urine or kidney tests are abnormal for greater than 3 months and the clearance is greater than 90, this person is said to have Stage 1 CKD. If the urine and kidney tests are abnormal for greater than 3 months and the GFR is between 60 and 89, this person is said to have Stage 2 CKD. However, if the eGFR or the GFR are less than 60 but greater than 30for greater than 3 months, then the person is said to have Stage 3 CKD. Stage 4 CKD is classified as a person having an eGFR or GFR between 15 and 29 for greater than 3 months. Stage 5 CKD is then an eGFR or GFR less than 15 for greater than 3 months. These are the means by which we classify CKD.
The classification system does not diagnose the cause of CKD, only the Stage. It was developed to have a greater understanding of the progression of CKD and not for specific diagnosis of cause.
It is necessary that a nephrologist then arrives at a specific diagnosis for specific treatment of the underlying kidney disease.
I would not consider an eGFR of 59 as “panic stations” but rather an indication of progression for me to then properly diagnose the cause of the underlying disease. These numbers are not meant to instill fear, but rather allow for meaningful discussion of progression and then diagnosis followed by treatment.
A normal GFR is, indeed, greater than 90 milliliters per minute per 1.73 meters squared. A person can have a normal GFR and still have CKD.
I hope this is helpful as an explanation as to the usefulness of this classification system that was developed by the National Kidney Foundation in the early 1980’s. Additional information can be found at the links provided on our web site at: https://www.kidney.org/professionals/KDOQI/gfr_calculator