Creatinine is a metabolic byproduct generated from muscle breakdown. Creatinine is filtered from blood by the kidneys and is not reabsorbed in any appreciable amount, hence is a proxy of how well the kidneys filter blood.
Creatinine clearance (CrCl) is used to estimate Glomerular filtration rate (GFR) in mL/min. It is measured via a 24-hour urine collection.
Why does it overestimate? Due to tubular secretion of Creatinine. There will be filtered Cr + tubular secreted Cr in the urine collection. Making the GFR appear higher than it actually is when calculated.
NOTE: Certain drugs (i.e trimethoprim, cimetidine) reduce tubular secretion of Creatinine and actually provide a more accurate measure of GFR
RULE OF THUMB: Serum Cr will vary with the amount of muscle tissue. Hence in older patients and women, CrCl can appear normal while GFR is markedly reduced.
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