Medical professionals calculate glomerular filtration rate by checking the amount of creatinine in a blood sample, then making mathematical adjustments according to the patient's age, sex, ethnicity, height and weight. Creatinine is a waste product filtered by the kidneys, so monitoring it is useful for measuring kidney function, per MedlinePlus.
Creatinine is left in the blood after its parent chemical, creatine, supplies energy to power muscles. Body composition and metabolism vary according to physical and demographic factors. For example, women have lower muscle density than men, resulting in generally lower creatinine levels. Customizing the calculation for each patient yields more accurate GFR results, according to MedlinePlus.