Creatinine is a waste product produced by skeletal muscle tissue. Creatinine is usually measured in the plasma as a biomarker of renal function.
Because creatinine is produced and excreted at a constant rate, urine creatinine concentration can be measured to assess the concentration of a urine specimen. This is particularly relevant when measuring the concentration of another analyte in urine, which can be corrected for urine concentration by expressing that analyte as a ratio to creatinine concentration (e.g. albumin/creatinine ratio).
Whilst plasma creatinine is normally measured to assess renal function, urine creatinine is measured when calculating creatinine clearance, a largely outdated method of assessing glomerular filtration.
A low urine creatinine may indicate an incomplete collection of a 24 hour urine sample, whereas an elevated urine creatinine may indicate an over-collection.
Creatinine concentration is related to muscle mass, therefore lower-than-expected results may be seen in those with low muscle bulk, and results above the reference interval may be seen in those with large muscle mass. Consumption of creatine supplements will also increase the urine creatinine concentration, as the assay is unable to differentiate between creatine and creatinine.
The reference intervals below relate to a 24 hour timed urine collection
|Age||Lower ref limit (mmol/24hr)||Upper ref limit (mmol/24hr)|
|6 months – 2 years||0.2||2|
|> 15 years (M)||9||18|
|> 15 years (F)||7||12.5|
Enzymatic spectrophotometry on a Beckman Coulter AU5822 analyser using Beckman Coulter reagents.
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