The test begins by taking blood from a person's veins. The most common places are the vein in the elbow and the veins on the back of the hands. People usually feel a sharp stinging pain when the needle enters their skin. After the blood is drawn out, it is sent to a medical laboratory where a laboratory technician calculates the GFR. It is possible to calculate an exact value for the GFR but this is rarely done any more. Most of the time, the value is estimated (this is called an eGFR). The basis for this estimate is a chemical in the blood called creatinine. The calculation may also include things such as gender, age, height, weight, and race. Getting an exact measurement needs more tests and special equipment. Doctors now normally use the estimated GFR with signs, symptoms and the medical history given by the patient to come to a diagnosis. A final diagnosis of kidney disease may sometimes be made by a pathologist using tests such as a kidney biopsy.