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Creatinine

Definition

Creatinine is a by-product of muscle metabolism, which circulates around the body at a constant rate in blood serum and is excreted by the body, within the urine, through the kidney filters.¹

A serum creatinine result is used to calculate the estimated glomerular filtration rate (eGFR) using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation for the diagnosis of Chronic Kidney Disease (CKD).² Whilst serum creatine concentration is a good measure of eGFR, not all persons with CKD have a reduced eGFR, as such, health professionals use other creatinine calculations, which have been used to diagnose CKD.³ As an example, a urine creatinine result can used with a urine albumin result to calculate the albumin/creatinine ratio (ACR) for the diagnosis of macro and micro albuminuria and subsequent CKD.^1,4 Whilst serum creatine is created at a constant rate, urinary excretion levels are influenced by muscle mass, which differ by age, sex and ethnicity. As such, there are no recommended ethnicity-specific or age-specific cut-points for ACR.³

Further information on eGFR, ACR and CKD is available in the Chronic Kidney Disease, Albuminine/Creatinine ratio and Estimated glomerular filtration rate pages of this product.

In the National Health Measures Survey (NHMS), two creatinine tests were used to measure the amount of serum creatinine in the blood, and urinary creatinine in the urine, at the time of the test.

Population

Urine creatinine results were obtained for persons aged 5 years and over and serum creatinine results were obtained for persons aged 12 years and over, who participated in the NHMS and provided a urine and/or blood sample. Fasting was not required for this test.

Methodology

A urine and/or blood sample was collected from participants and creatinine levels were measured at the Douglass Hanly Moir (DHM) laboratory.

There is no consensus on the epidemiological cut off reference values for measuring creatinine in serum and/or urine as creatinine production varies pending muscle mass and varies throughout infancy and childhood.³ As such, no cut off points have been defined in the NHMS.

Note that the creatinine assay (test method) used to measure serum creatinine was traceable to Isotope Dilution Mass Spectrometry (IDMS) to improve assay precision. Further information about the analysis method and machines used to measure creatinine levels is available in Excel spreadsheet format in the Downloads page of this product.

Data items

The data items and related output categories for this topic are available in Excel spreadsheet format from the Downloads page of this product.

Interpretation

Points to be considered when interpreting data for this topic include the following:

• Creatinine test results do not confirm a specific diagnosis without consultation with a health professional.
• There are a number of different test methods to measure creatinine levels and each test method may produce different results. The data from this topic should therefore be used with caution when comparing creatinine results from other studies using a different test method.
• Whilst urinary creatinine has been collected from persons aged 5 years and over, note there are issues with the validity of using urinary creatinine excretion as an index of muscle mass.⁵