Creatinine (Cr) is used to measure kidney function.

Creatinine is produced by muscle metabolism and removed by the kidneys.  In general, the more effectively your kidneys are filtering your blood, the lower your levels of creatinine will be.

People with greater muscle  mass will produce more creatinine so will have higher levels even if their renal function is normal.  To help with this, creatinine is also reported with an eGFR (estimated glomerular filtration rate) which takes into account age and sex which are the largest determinants of muscle mass and so gives a better and more easily interpreted result.

In general, lower creatinine levels and higher eGFR numbers represent better kidney function.

Creatinine is a very commonly requested test and is useful because a lot of conditions can alter kidney function.  Diabetes and high blood pressure can cause kidney disease so doctors will usually check a creatinine level once per year or occasionally more often if you have these conditions.  Some medications can damage kidneys requiring monitoring and a great many medications have a greater effect if kidney function is poor so tests may be needed before starting medicines.

The test is also commonly added to others when there is no suspicion of kidney disease because it is quick and cheap to perform so there is often no reason not to check and a baseline result can be useful.

Almost everyone will lose kidney function as they age.  Chronic Kidney Disease (CKD) appears to be almost inevitable.  It is staged, with stages 1 and 2 being pretty much the norm in the population over 70 years of age.  We generally use this as an indicator of the need for more frequent monitoring and closer management of blood pressure - lower kidney function means a lower target blood pressure and a greater likelihood that you will need medication.  Don't be alarmed if you see the term 'Chronic Renal Failure' on our portal.  This is the old-term for CKD and simply reflects the time it takes to modernise medical software. 

As creatinine levels can fluctuate a fair bit depending on your level of activity, recent fluid intake and of course illnesses, it is important not to worry about small changes in your creatinine level.  Longer term trends are far more relevant.