It actually measures the flow of filtered fluid through the kidney. The functioning of the kidneys can be estimated by various methods like 24 hour urine collections, isotope scans etc. The GFR can also be calculated using several formulae. The MDRD (Modification of Diet in Renal Disease) equation is commonly used. Several modifications of the MDRD equation are available. The abbreviated (used here) MDRD equation takes into account 4 variables: serum creatinine, age, race, and gender. Serum Creatinine is expressed in mg/dL. By entering your serum creatinine, age in years, race (African-American or not) and sex (male or female) you can estimate your kidney function (GFR).
Normal GFR for healthy women is 88 to 128 mL/min and 97 to 137 mL/min in males. Say, roughly around 100. GFR declines with age. From the age of 40 years, it decreases by about 10 – 12 mL every decade. So at the age of 90, GFR would be about 40. And for the same age as a male, a female would have 15% lesser GFR.