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Erythrocyte Sedimentation Rate Test

by Robert S. Dinsmoor

A blood test that measures the general level of inflammation in the body. The erythrocyte sedimentation rate test &mdash: also known as the ESR or “sed rate” test — is used to help diagnose and monitor a number of inflammatory conditions, including rheumatoid arthritis (RA), systemic lupus erythematosus, anemia, kidney disease, and thyroid disease.

The ESR test makes use of a special property of the blood. In the presence of inflammation, certain proteins in the blood interact with the membranes of the red blood cells, making the cells clump together in stacks. Those stacks tend to sediment — or sink to the bottom — in a vial of blood. To perform an ESR test, blood is drawn from a person, placed in a vertical tube, and left to sit for an hour. To calculate the ESR, the technician measures the distance between the top of the liquid part of the blood and the top of the sedimented red blood cells. The greater the distance, the greater the level of inflammation. When a standard Westergren tube is used, normal results are typically up to 20 millimeters per hour in women younger than 50 and up to 15 millimeters per hour in men younger than 50. (Results for older people are generally higher.) Doctors may also determine normal upper limits for ESR levels by dividing a person’s age in half, adding 5 to the final number for women. An ESR above these levels may point to inflammation in the body. People with a serious infection can have an ESR over 100 millimeters per hour.

Doctors frequently use the ESR test when they suspect RA or another type of inflammatory arthritis. While most (though not all) people with RA have an elevated ESR, many other conditions, including other types of arthritis (but not osteoarthritis), can elevate the ESR as well. The ESR is thus not helpful for distinguishing between types of inflammatory arthritis. It is only one part of the process of making a diagnosis, which includes a full medical history and physical exam as well as other, more RA-specific blood tests.

In addition, the ESR may be used to monitor the activity of a person’s RA or gauge how well a drug is working to treat RA. However, because the ESR changes slowly in response to changes in inflammation, it is not ideal for this use. The C-reactive protein (CRP) test, another test for general levels of inflammation, offers a better snapshot of the activity of RA.

Last Reviewed on May 2, 2012

Robert S. Dinsmoor is a medical writer and editor based in Massachusetts.

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