Creatinine is the anhydride of creatine, which is found in muscle and inter-converted to phosphocreatine during muscle contraction. A proportion of muscle creatine undergoes spontaneous hydrolysis to creatinine, which is excreted.
A 70 kg man of normal muscle mass excretes 1 - 2 grams of creatinine per day: hence the practice of correcting urinary metabolites to a concentration "per gram of creatinine" to give an indication of the daily excretion rate.
This is of course an approximation: plasma creat
24 hr urine / Mid- stream urine
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