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Creatinine is a product of
muscle metabolism. It is produced at a nearly constant rate and is
excreted in the urine.
Because of it's constant rate of production, the amount of creatinine
in the urine is an indirect measurement of kidney function (glomerular
filtration rate).
If kidney function is significantly reduced, the urine creatinine will
fall. With more severe degrees of kidney failure, the serum creatinine
will eventually rise.
A 24-hour collection of urine is the most accurate way to assess renal
function with creatinine. A blood (serum) creatinine is also measured
during this time and used to calculated the volume of urine needed to
"clear" the measured amount of creatinine from the blood and
into the urine. This is called the "creatinine clearance."
Vigorous exercise or muscular trauma occurring during the collection
will cause an increased amount of creatinine in the blood and may lead to
false creatinine clearance results.
Serum proteins are normally extracted from urine prior to it
being released from the kidney.
Because the renal glomeruli are imperfect, normal individuals may
occasionally have "trace" amounts of protein in their urine.
Larger amounts (1+ or more) are considered abnormal and may reflect an
underlying kidney problem.
Urine protein can be measured on any urine sample (a "spot
urine"), but the most accurate measure is with a 24-hour collection
of urine.
During pregnancy, due to the increased renal blood flow, some
additional protein may be lost in the urine. This increased protein loss
should not normally be in quantities exceeding 300 mg in 24 hours. If more
than 300 mg in 24 hours is found, this may signal the development of pre-eclampsia.
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