When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing . The calculation uses a correction factor for body size. Creatinine is used for this purpose since it is normally present in the body, and very little creatinine is reabsorbed after it is filtered. The amount of filtrate made in the kidney depends on the amount of blood that passes through the glomeruli and on the ability of the glomeruli to act as filters.
Diuretics can increase the creatinine clearance.
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.