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Disclaimer: long read, but lots of information about bloodwork related to kidney function for those who would like to enhance their understanding. Has nothing to with the OPs intent to abuse, which I cannot condone.
I'm definitely not saying you're wrong, but I've been trying to wrap my head around this (GFR) for a long time and was unable to process it sufficiently until I was involved heavily in the medical field. So I'm just putting this out there to help add knowledge to anyone reading that may be interested in their own bloodwork.
The GFR number they give you on your blood work is calculated directly from your serum creatinine. They developed this equation in an effort to get away from using the 24hr urine analysis for obvious reasons (time, cost savings, people just generally don't want to do it). With that being said...studies have shown that athletes and especially lifters have higher serum creatinine due to a higher amount of lean mass.
Most of these calculations use an estimated body surface area (at best) which doesn't take into account the greater lean body mass. Those that don't use the estimated area still don't improve the numbers much. I have seen this in my own personal bloodwork for years and years, both "on" and "off" with the GFR numbers never having more than a 10pt difference over a span of 4 years (so far), but none of them being in an "ideal" range.
So a better estimate of kidney function (for bodybuilders) might be the cockcroft-gault equation, which shows creatinine clearance rates based on serum creatinine (those reading this may have to convert bloodwork from mg/dl to mmol/L, plenty of calculators online just google). When I've taken this step, my creatinine clearance is in the high-end of normal which for most of us, it should be--athletes and bodybuilders tend to have higher creatinine clearance.
Anyways, the best method of course is to ask for the 24hour urine collection analysis if one is worried about kidney function. It's a way more precise measure than any equation ever could be. So if anyone is really freaking out about their kidney function they should be ordering this from their doctor.
Cliffs
-GFR on bloodwork is calculated from serum creatinine values
-bodybuilders / athletes tend to have higher serum creatinine values due to increased lean body mass
-GFR on bloodwork does not take into account this increased LBM and probably won't be an accurate indicator of kidney function
-creatinine clearance using the cockcroft-gault equation is a better estimate of kidney function for bodybuilders (you can calculate this using your serum creatinine values, but may have to convert from mg/dl to mmol/L)
-ultimately if one is worried about kidney function, they need to have a 24hr urine collection analysis performed
I posted about this back in september http://www.professionalmuscle.com/forums/2376535-post406.html