Serum Creatinine

Kidney function

Creatinine is a waste product made by the normal breakdown of muscle. Healthy kidneys filter it out of the blood into the urine, so the level in the blood rises when the kidneys are not working well. It is the main blood test of kidney function, and many laboratories use it to calculate the estimated glomerular filtration rate (eGFR), a measure of how well the kidneys filter.

• To check kidney function if you have diabetes or high blood pressure
• Before and during treatment with medicines that can affect the kidneys
• If you have swelling of the legs or face, or pass much more or less urine than usual
• During severe infection, dehydration or illness, to watch for strain on the kidneys
• Before a scan with contrast dye or an operation

Normal meals and drinks are fine. A large meat meal shortly before the test, or heavy exercise the day before, can raise creatinine a little, so avoid both if you can. Keep taking your usual medicines unless your doctor has told you otherwise.

Sample

Blood sample

How it works

The collector comes to your home and takes a small blood sample from a vein in your arm, which takes a few minutes. The tube is labelled at your door and sent for testing.

Report

Within 24 hours of collection.

The report shows your creatinine next to the laboratory's reference range, and often an estimated GFR. A higher creatinine usually means the kidneys are filtering less well, which can be temporary, for example with dehydration, or long-term. Creatinine is naturally higher in people with more muscle and lower in people with less. Your doctor will interpret the result, often by comparing it with earlier ones.

One test: Serum Creatinine, from one blood sample.