What Is a Sed Rate (Erythrocyte Sedimentation Rate)?

Among First Tests Ordered When Arthritis Is Suspected

Person drawing blood
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A sed rate, short for erythrocyte sedimentation rate, is a blood test that detects nonspecific inflammation in the body. Nonspecific means that the sed rate test does not identify the source of the problem or illness which is causing the inflammation. An elevated (abnormally high) sed rate does suggest that there is an ongoing inflammatory process, but not how or why.

Why It's Used

If you consult a doctor because you are experiencing persistent joint pain, the doctor will perform a physical examination and order diagnostic tests. You should expect your doctor to order x-rays of the affected joints, as well as certain blood tests. A sed rate is typically among the first blood tests ordered when arthritis is suspected. A sed rate can also be elevated when infection or tumor is present. Illnesses associated with the breakdown or decreased function of tissue or organs can cause an elevated sed rate. Pregnancy is yet another condition linked to an elevated sed rate.

Aside from diagnostic purposes, a sed rate is often ordered periodically to check on disease activity. As your arthritic condition improves and becomes less active, it is expected that your sed rate would decrease and approach the normal range. Sed rate is actually part of the DAS28 measure for disease activity. While there are several versions of DAS28, they all measure disease activity using patient self-assessment, tender joint counts and swollen joint counts (up to 28), as well as the sed rate or CRP (C-reactive protein) test. Interestingly, studies have shown that there was often a discrepancy between the results of sed rate based DAS28 and CRP based DAS28. The sed rate based DAS28 often showed higher disease activity than CRP based DAS28. The concern was that the differing results could affect treatment decisions.

How It's Performed

When a sed rate is performed, the blood sample is placed in a tall, thin, vertical tube. The test measures how fast the erythrocytes (red cells) settle in one hour. The result is reported as millimeters per hour—that is, millimeters of plasma on top of the sediment of red cells.

Under normal conditions, when there is no inflammatory process or illness, red cells fall slowly. Increased levels of abnormal proteins in the blood or other proteins called acute phase reactants, such as fibrinogen or immunoglobulins which are increased in inflammation, cause red cells to fall more quickly.

Normal Reference Ranges

The usual method used for sed rate is known as the Westergren method. As mentioned above, results are reported in mm/hr (millimeters per hour). Typically, the sed rate increases with age and is higher in women. Many laboratories do not adjust for gender or age and consider that a normal sed rate is:

  • Men: 0-15 mm/hr
  • Women: 0-20 mm/hr

When adjusted for age and gender, the accepted reference range for the test is:

Adults (Westergren method):

  • Men under 50 years old: less than 15 mm/hr
  • Men over 50 years old: less than 20 mm/hr
  • Women under 50 years old: less than 20 mm/hr
  • Women over 50 years old: less than 30 mm/hr

Children (Westergren method):

  • Newborn: 0 to 2 mm/hr
  • Newborn to puberty: 3 to 13 mm/hr

The Bottom Line

While no single test can diagnose arthritis, the sed rate helps to confirm inflammatory types of arthritis, such as rheumatoid arthritis, lupus, ankylosing spondylitis, and psoriatic arthritis. On the other hand, osteoarthritis is typically associated with a normal or modestly elevated result for sedimentation rate. The sed rate is often ordered in conjunction with the CRP test which also detects nonspecific inflammation.

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