Sed rate, or erythrocyte sedimentation rate (ESR), is a blood test that can reveal inflammatory activity in your body. A sed rate test isn't a stand-alone diagnostic tool, but it can help your doctor diagnose or monitor the progress of an inflammatory disease.

When your blood is placed in a tall, thin tube, red blood cells (erythrocytes) gradually settle to the bottom. Inflammation can cause the cells to clump. Because these clumps are denser 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 descended, the greater the inflammatory response of your immune system.

Sed rate tests might be useful when evaluating unexplained fever, some types of arthritis and symptoms that affect your muscles. Also, they can help confirm a diagnosis of certain conditions, including:

• Giant cell arteritis
• Polymyalgia rheumatica
• Rheumatoid arthritis

A sed rate test can also help determine the severity of your inflammatory response and monitor the effect of treatment.

Because a sed rate test can't pinpoint the problem that's causing inflammation in your body, it's usually 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.

A phlebotomist, nurse or medical assistant will use a needle to draw blood from a vein, most likely a vein in your arm. After the test, the site on 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 descended in one hour (hr). The normal range is 0 to 22 mm/hr for men and 0 to 29 mm/hr for women. The upper threshold for a normal sed rate value may vary somewhat from one medical practice to another.

Your sed rate is one piece of information to help your doctor check your health. Talk to your doctor about what your sed rate results mean in light of your symptoms and the results of your other diagnostic tests.

Accuracy of test results

A number of conditions can affect the properties of blood, thereby affecting how quickly red blood cells sink in a sample of blood. So information about inflammatory disease — what your doctor intends to learn from the sed rate test — can be obscured by the influence of other conditions. These complicating factors include:

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

Your doctor will take into account possible complicating factors when interpreting the results of your sed rate test.