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Glomerular filtration rate (GFR) describes the flow rate of filtered fluid through the kidney. Creatinine clearance rate (CCr) is the volume of blood plasma that is cleared of creatinine per unit time and is a useful measure for approximating the GFR. Both GFR and CCr may be estimated by formulas using just a blood test result (eGFR and eCCr).
The results of these tests are important in assessing the excretory function of the kidneys. For example, grading of chronic renal insufficiency and dosage of drugs that are primarily excreted via urine are based on GFR (or creatinine clearance).
The most advocated formula for calculating the GFR is the one that was developed by the Modification of Diet in Renal Disease Study Group.
The most commonly used formula is the "4-variable MDRD" which estimates GFR using four variables: serum creatinine, age, race, and gender. The formula underestimates the GFR in healthy patients with GFRs over 60 mL/min/1.73 m².
(Reference: Wikipedia, National Kidney Foundation)