The estimated glomerular filtration rate (eGFR) that you describe is calculated based on the serum creatinine test. There are several different equations that can be used to calculate the eGFR. Some use age, some use gender and some consider race. This is because that the serum creatinine will change based on age, race gender and body size. Hence, the equations must be corrected for each of these factors and then a calculation is used to estimate kidney function (eGFR) that is the other variable that affects the serum creatinine. I suggest you review our web site for a discussion of the kidney function estimating equations at: https://www.kidney.org/professionals/kdoqi/gfr
Remember, that these equations are estimates and the variability comes from the accuracy of the laboratory measurement of the serum creatinine. Hence, the eGFR can vary by 5 to 10% and still be a fair estimation.