Erythrocyte Sedimentation Rate (ESR)

Erythrocytes are red blood cells. The sedimentation rate is the time it takes for your red blood cells to settle at the bottom of a test tube. An erythrocyte sedimentation rate (ESR) is a blood test that can show if you have inflammation in your body.

Inflammation is your immune system's response to injury, infection, and many other conditions, including immune system disorders, certain cancers, and blood disorders. To do an ESR test, a sample of your blood is sent to a lab. A health care professional places the sample in a tall, thin test tube and measures how quickly your red blood cells settle or sink to the bottom of the tube.

Typically, red blood cells sink slowly. But inflammation makes red blood cells stick together in clumps. These clumps of cells are heavier than single cells, so they sink faster.

If an ESR test shows that your red blood cells sink faster than expected, it may mean you have a medical condition causing inflammation. How quickly the red blood cells in your sample fall to the bottom of the test tube is a sign of how much inflammation you have. Faster ESR rates mean higher levels of inflammation.

But an ESR test alone cannot diagnose which condition is causing the inflammation.

Why this test is done

Your health care provider may order an ESR if you have symptoms of a condition that causes inflammation. Your symptoms will depend on the condition you may have, but they may include:

• Headaches
• Unexplained fever
• Unexplained weight loss
• Joint stiffness
• Neck or shoulder pain
• Loss of appetite
• Anemia

How to prepare

You don't need any special preparations for this test. But if your provider ordered other tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your provider will let you know if there are any special instructions to follow.

Sample type

Blood

Other names for this test

ESR, SED rate sedimentation rate; Westergren sedimentation rate