OK, I managed to find some supporting research. First problem with the eGFR is muscle mass.
The use of creatine supplements and high protein diets
Lydia Williamson, David New. How the use of creatine supplements can elevate serum creatinine in the absence of underlying kidney pathology. BMJ Case Rep. 2014 Sep 19;2014.
This report describes a case in which the consumption of the bodybuilding supplement creatine ethyl ester resulted in raised serum creatinine in the absence of true underlying kidney pathology. The abnormalities reversed after discontinuation of the supplement. A case of pseudo renal failure was recognised and kidney function was concluded to be normal.
As rawdeal posted
Alessandra Calábria Baxmann, Marion Souza Ahmed, Natália Cristina Marques, Viviane Barcellos Menon, Aparecido Bernardo Pereira, Gianna Mastroianni Kirsztajn and Ita Pfeferman Heilberg. Influence of Muscle Mass and Physical Activity on Serum and Urinary Creatinine and Serum Cystatin C. Clinical Journal of the American Society of Nephrology. Vol. 3, Issue 2. March 2008.
Conclusions: Cystatin C may represent a more adequate alternative to assess renal function in individuals with higher muscle mass when mild kidney impairment is suspected.
Forbes GB, Bruining GJ. Urinary creatinine excretion and lean body mass. Am J Clin Nutr 1976;29:1359-1366.
Creatinine is produced nonenzymatically in the skeletal muscle, and the amount of creatinine production, and therefore the 24 h excretion of creatinine, is directly related to muscle mass.
Cystatin C may be preferable to creatinine to assess kidney function in individuals with higher muscle mass. An increase in muscle mass would be expected to result in an increase in serum creatinine level in the absence of change in kidney function. Because serum creatinine is derived from the metabolism of creatinine produced by muscle, a significant increase in muscle mass would be expected to increase serum creatinine. An elevation in serum creatinine could also occur with creatine supplements, which he is not taking. This patient has a normal urinalysis and no proteinuria, all of which indicate no evidence of underlying kidney disease. Cystatin C, which is cleared by the kidney, is produced by all nucleated cells; therefore, levels are less dependent on muscle mass. Cystatin C can also be used for more accurate glomerular filtration rate estimation in these patients as a component of the Chronic Kidney Disease Epidemiology Collaboration equation.
Creatinine is produced nonenzymatically in the skeletal muscle, and the amount of creatinine production, and therefore the 24 h excretion of creatinine, is directly related to muscle mass.
So I would say as rawdeal has educate your doctor about this and ask for the Cystatin C test instead of eGFR. If he won't do that just refuse to see a specialist and monitor your eGFR in the future.