A product of creatine breakdown, it is predominantly derived from skeletal muscle and is also renally excreted. Low levels are found with malnutrition and high levels with muscle breakdown (rhabdomyolysis) and impaired excretion (renal failure). In the latter case, a creatinine value >120 pmol/l suggests a creatinine clearance <25ml/min.

The usual ratio for plasma urea (mmol/l) to creatinine (pmol/l) is approximately 1:10. A much lower ratio in a critically ill patient is suggestive of rhabdomyolysis whereas higher ratios are seen in cirrhosis, malnutrition, hypovolaemia and hepatic failure.

The ratio of urine:plasma creatinine may help distinguish between oliguria of renal or pre-renal origins. Higher ratios (>40) are seen in pre-renal conditions and low levels (<20) with direct renal causes.

Creatinine clearance is a measure of glomerular filtration. Once filtered, only small amounts of creatinine are reabsorbed. Normally it exceeds 100ml/min.

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