Sed rate, or erythrocyte sedimentation rate (ESR), is a blood test that can show inflammatory activity in the body. Many health problems can cause a sed rate test result to be outside the standard range. A sed rate test is often used with other tests to help your health care team diagnose or check the progress of an inflammatory disease.

When blood is placed in a tall, thin tube, red blood cells, called erythrocytes, gradually settle to the bottom. Inflammation can cause the cells to clump together. Because these clumps are heavier than individual cells, they settle to the bottom more quickly.

The sed rate test measures the distance red blood cells fall in a test tube in one hour. The farther the red blood cells have fallen, the greater the inflammatory response of your immune system.

A sed rate test may be ordered if you have symptoms like unexplained fever, muscle pain or joint pain. The test can help confirm a diagnosis of certain conditions, including:

• Giant cell arteritis.
• Polymyalgia rheumatica.
• Rheumatoid arthritis.

A sed rate test also can help show the level of your inflammatory response and check the effect of treatment.

Because a sed rate test can't pinpoint the problem that's causing inflammation in your body, it's often accompanied by other blood tests, such as the C-reactive protein (CRP) test.

The sed rate is a simple blood test. You don't need to fast before the test.

During a sed rate test, a member of your health care team will use a needle to take a small sample of blood from a vein in your arm. This often takes just a few minutes. Your blood sample is sent to a lab for testing. After the test, your arm might be tender for a few hours, but you'll be able to resume most normal activities.

Results from your sed rate test will be reported in the distance in millimeters (mm) that red blood cells have fallen in the test tube in one hour (hr). Age, sex and other factors can affect the sed rate results.

Your sed rate is one piece of information to help your health care team check your health. Your team will also take into consideration your symptoms and your other test results.

Accuracy of test results

Many conditions can affect the properties of blood and change how quickly red blood cells sink in a sample of blood. Conditions that can affect the sed rate include:

• Advanced age.
• Anemia.
• Pregnancy.
• Kidney problems.
• Thyroid disease.
• Some cancers, such as multiple myeloma.
• Infection.
• Injury.

Your health care team will consider any other conditions you have when reviewing your sed rate test results. Talk to your rheumatologist or health care specialist about what your sed rate results mean and ask them any questions you may have.