Thanks.
What I wrote came from Brenner & Rector's The Kidney (6th ed):
".... Prolonged storage of the urine can introduce error in the creatinine
clearance determination by perturbing urine creatinine levels. High
temperature and low urine pH enhance the conversion of creatine to
creatinine in urine. Indeed, storing urine under adverse conditions for 24
hours was shown to cause a 20% increase in the amount of measured urine
creatinine [Ref]. This problem can be obviated by refrigerating urine
samples and by measuring the urine creatinine level without undue delay. ...
[Ref]: "Carrie BJ, Golbetz HV, Michaels AS, Myers BD: Creatinine: An
inadequate filtration marker in glomerular diseases. Am J Med 1980; 69: 177-82"
But I take your point: though serum creatinine and creatine concentrations
are almost identical in normal individuals, I just discovered that urine
creatine is <0.35 mmol/L in normals.
Thank you very much.
Reza
P.S. Part of some books are available through 'MD Consult'
<http://www.mdconsult.com/>. You can get 10-day free trials which are
usually long-enough to serve the purpose of finding information - right or
wrong!
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