A calculation of how efficiently the kidneys are filtering wastes from the blood. A traditional GFR calculation requires an injection into the bloodstream of a fluid that is later measured in a 24-hour urine collection. A modified GFR calculation requires only that the creatinine in a blood sample be measured. Each laboratory has its own normal range for measurements. Generally, the normal range for men is 97 to 137 mL/min/1.73 m2 of body surface area. The normal range for women is 88 to 128 mL/min/1.73 m2.