Creatinine is a waste product produced by skeletal muscle tissue. Creatinine is produced at a constant rate (dependent upon muscle mass) and is freely filtered at the glomerulus (with a small fraction of creatinine being excreted in the distal nephron). Because of these properties, creatinine is a useful biomarker of glomerular filtration.
Creatinine clearance is a calculated parameter that indicates the volume of plasma from which creatinine is cleared per unit time. With worsening renal function, the creatinine clearance decreases (i.e. the kidney is unable to clear as much creatinine in a given time).
Creatinine clearance is calculated from the urine creatinine and plasma creatinine concentrations, and requires a 24 hour urine collection. It has largely been replaced by the estimated glomerular filtration rate as a biomarker of glomerular function, but may still be requested in specific settings (for example, when dosing nephrotoxic drugs).
This test requires collection of a 24 hour urine specimen, as well as a serum/plasma collection for creatinine, taken during the same 24 hours that the urine is collected.
A decreased creatinine clearance may indicate an impairment in renal function/glomerular filtration. However, there are several caveats to consider when interpreting creatinine clearance results:
– Creatinine clearance will overestimate glomerular filtration rate by approximately 10% as the calculation does not take into account the fraction of plasma creatinine that is secreted in the distal nephron. This overestimation tends to increase with worsening renal function as secreted creatinine takes up a higher proportion of the overall urine creatinine.
– Creatinine clearance has a relatively large within-subject biological variation (i.e. results for an individual may vary significantly from day-to-day).
Alternatives to creatinine clearance include estimation of eGFR from plasma creatinine or cystatin C, both which can be measured on a single blood sample and do not require a 24 hour urine collection.
Adult reference interval: >1.5 ml/second
Paediatric reference intervals are not available.
Creatinine clearance = (Urine Cr x Urine volume(mL))/Plasma Cr
(For a 24 hour collection, the result is divided by 86,400 to produce a result in mL/second)
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