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 the ability of the glomeruli to act as filters.
Because a small amount of creatinine is secreted by the kidney tubules, creatinine clearance is not exactly equivalent to the GFR. In fact, creatinine clearance usually overestimates the GFR. This is particularly true in patients with advanced renal failure, where the percentage of secreted creatinine in the urine means a greater percentage of the overall urine creatinine.
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