Different GFR calculators - can anyone explain the difference?

[myleftkidney]

I've come across two GFR calculators, each based on blood creatinine, that give very different results.

The Cockcroft-Gault Calculator http://www.nephron.com/cgi-bin/CGSI.cgi which includes weight as part of the calculation.
The MDRD GFR Calculator http://www.kidney.org/professionals/KDOQI/gfr_calculator.cfm which does not use weight as part of the calculation.

The differences in GFR calculated between the two are very different, at least for me, with the Cockcroft calculation giving me a significantly higher number than the MDRD calculator (26 vs 18).  Apparently my transplant team uses the Cockcroft calculation and thus I'm not yet on THE list while I've seen several blood tests that use the MDRD calculator that put me below 20.

Can anyone explain the difference?  Why would one be chosen over the other? 

Thanks
 

[paris]

Interesting.   They do give two different numbers.  The second one agrees with my neph's and the tranplant centers numbers for my GFR.    I have never seen the first one before.  I hope someone has an answer for us.   I am going to go do some research.