Blood tests have many different uses. They can be used to diagnose a condition, to monitor a particular organ in the body such as the kidney or liver, and they can also be used to give measurements of bodily processes such as the level of sugar in your blood or current levels of inflammation.
Doctors refer to the ESR, which stands for erythrocyte sedimentation rate, to test for inflammation or infection. The laboratory tests the speed at which red blood cells fall to the bottom of a test tube. If there are higher levels of inflammation, the cells fall faster, and the test produces a higher result. It is a good test at showing an objective level of infection or inflammation but doesn't indicate where it is coming from.
If you had inflammatory conditions such as inflammatory bowel disease, or infective endocarditis, your ESR levels would all be raised above normal in all these cases. It can be used to track response to treatment or check in on your baseline level of inflammation or infection.
If your doctor is ordering an ESR test as part of your blood investigations, they want to get some information on levels of infection or inflammation. If you attended your doctor with joint pain symptoms, for example, a normal ESR level would be reassuring that there was not a serious amount of inflammation or infection, as in inflammatory arthritis.
In itself, it is difficult to interpret, it needs to be matched with your symptoms. For example, if you have swelling in multiple joints, it may indicate an autoimmune condition and suggest further investigations may be needed. The level of ESR can also point to how much inflammation is present. Once treatment is started, the level can be used to guide how effective treatment is over time, as you would expect the anti-inflammatory medication to reduce these levels if it's working as it should. Hopefully, your symptoms will also be improved.
ESR takes a few days to rise in response to an episode of inflammation, and stay elevated while inflammation continues. This compares to another inflammatory marker, CRP, which rises quickly within a day or two and falls quickly once inflammation or infection improves. For this reason, ESR is more useful for long term inflammatory conditions and their treatment, and CRP is a better guide for an acute infection and its treatment.
You do not need to be fasting for this blood test, there are no special requirements. It can be taken like any normal blood test and is usually taken in a gold or yellow colour bottle.
Remember to press hard for a good few minutes after the needle has been removed and keep your elbow straight, to prevent a nasty bruise!
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