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S2
National Kidney Foundation. Glomerular Filtration Rate. Available online: www.kidney.org/kidneydisease/ckd/knowGFR.cfm

S3
National Kidney Foundation. Kidney Learning System: GFR. Available online: http://www.kidney.org/professionals/KLS/gfr.cfm

S4
Levey et al. A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group. Annals of Internal Medicine. 1999; 130(6):461-470.