Serum Creatinine (Cr, S) in this formula (Cockroft) is an estimated value from the GFR, if you have the 24h-Urine creatinine level and the 24h urine volume, go to Creatinine Clearance --click on the hyperlink below--. See also GFR.
Notice that (in this equation), smaller or older individuals have a lower GFR for any level of serum creatinine.