The sedimentation rate, or sed rate test is a blood test that measures inflammation in your body. It determines how quickly your red blood cells sink in a tube of blood. It’s also known as the erythrocyte sedimentation rate (ESR) test.
The sed rate test is used to diagnose or assess the progression of inflammatory diseases like arthritis, and autoimmune disorders like lupus. The sed rate can be high in certain types of cancers and infections. It can also show the effect of treatments for conditions that cause inflammation.
You might need a sed rate test if you experience symptoms of inflammatory conditions like arthritis or inflammatory bowel disease (IBD). These symptoms can include:
- joint pain or stiffness that lasts longer than 30 minutes in the morning
- headaches, particularly with associated pain in the shoulders
- abnormal weight loss
- pain in the shoulders, neck, or pelvis
- digestive symptoms, such as diarrhea, fever, blood in your stool, or unusual abdominal pain
When you’re experiencing inflammation, your red blood cells cling together, forming clumps. This clumping affects the rate at which red blood cells sink in a tube of blood.
The sed rate test lets your doctor see how much clumping is occurring. The faster the red cells sink to the bottom of a test tube, the more likely it is that inflammation is present.
The test can identify and measure inflammation in your body. However, it doesn’t help pinpoint the cause of inflammation.
Procedure and risks
You don't have to prepare for a sed rate test. You simply show up for your appointment and have blood drawn. You might feel the prick of the needle and mild pain or throbbing after the blood test is complete. If you’re uncomfortable at the sight of blood, you may also experience discomfort seeing the blood drawn from your body.
The blood sample will be placed in a thin tube before sitting for one hour. During and after this hour, your doctor will assess how far your red blood cells have sunk into the tube, how quickly they sink, and how many sink.
Your doctor may order a C-reactive protein (CRP) test at the same time as your sed rate test. CRP measures inflammation as well, but it can also help predict your risk for coronary artery disease and other heart disease.
The only risks of the sed rate test are those associated with the site of your blood draw, including:
- very light bleeding
As with any blood draw, these associated risks are minimal.
Your doctor will use one of two methods to measure your sedimentation rate.
- Your doctor will draw your blood into a Westergren-Katz tube until the blood level reaches 200 millimeters.
- The tube will be stored vertically and sit at room temperature for an hour.
- Your doctor will measure the distance between the top of the blood mixture and the top of the sedimentation of red blood cells.
The Wintrobe method is similar to the Westergren method, except the tube used is 100 millimeters long and thinner. An advantage of this method is that it’s less sensitive than the Westergren method, so less blood is used.
Clumped red blood cells sink lower and faster than individual cells. You doctor will look at how far the red blood cells sink during the hour of the test. This will indicate how much inflammation is present and what steps should be taken.
Factors that can influence your test results include:
- advanced age
- high cholesterol
- kidney issues
- multiple myeloma and other cancers
Some medications, such as dextran, methyldopa, or oral contraceptives can interfere with results by causing sed rates to be abnormally high. Other medications, such as cortisone or aspirin (Bayer), can cause sed rates to be abnormally low.
If high inflammation is confirmed, your doctor may want to do additional testing to find the cause of the inflammation.
Depending on your results, your doctor may want to order additional tests, including a second sed rate test to verify the results of the first one. These further tests can help your doctor identify the specific cause of your inflammation.
If your doctor suspects that an underlying condition is causing your high sed rate, they may refer you to a specialist who can properly diagnose and treat the condition. Conditions that may be causing your high sed rate include:
- polymyalgia rheumatica, which causes muscle pain and stiffness
- rheumatoid arthritis (RA), which leads to joint problems such as swelling and loss of function
- temporal arteritis, a condition in which your cranial arteries become inflamed or damaged
If your doctor detects inflammation, they may recommend one or more of the following treatments:
- taking a nonsteroidal anti-inflammatory drug (NSAID), such as ibuprofen (Advil, Motrin) or naproxen (Aleve)
- corticosteroids or "steroid sparing" medications
If an infection is causing your inflammation, your doctor will likely prescribe antibiotics to fight the infection.
An abnormally high sed rate can indicate the presence of cancerous tumors, especially if no inflammation is found.
A low sed rate can indicate that you have an underlying condition, such as:
- sickle cell anemia, a genetic disease affecting the red blood cells
- leukocytosis, or a high white blood cell count
- polycythemia vera (PV), a bone marrow disorder that leads to the production of excess red blood cells
If you do have one or more of these conditions, further sed rate tests can measure the effectiveness of treatments and keep track of your sed rate throughout your course of treatment.