Sedimentation Rate (ESR) Test

What is a sedimentation rate (ESR) blood test?

The erythrocyte sedimentation rate test is a blood test providers use to detect inflammation in your body. Providers usually refer to it by the shortened name “sed rate test” or “ESR test.”

Your healthcare provider might use a sed rate test to help diagnose or monitor inflammatory conditions. Inflammation happens when your immune system activates special cells after an injury or to fight substances that shouldn’t be in your body (like viruses, bacteria or toxins).

An ESR test can help your provider screen for lots of inflammatory conditions, including:

• Autoimmune diseases
• Blood disorders
• Infections
• Inflammatory arthritis
• Some types of cancer

An ESR blood test doesn’t confirm or rule out any diseases or conditions — it can’t make a specific diagnosis. But it can show your provider whether or not there’s inflammation inside your body. They’ll use that result along with other tests, a physical exam and any symptoms you’re experiencing to home in on a diagnosis.

How does an ESR test work?

The ESR test is a simple blood test. A phlebotomist or healthcare provider will remove a small amount of your blood from a vein (usually in your arm).

ESR is a measure of your erythrocyte sedimentation rate. Erythrocyte is the medical name for red blood cells. A sedimentation rate is how long it takes a substance to fall to the bottom of a container of liquid. So, your erythrocyte sedimentation rate (ESR) is how long it takes your red blood cells to fall to the bottom of a sample of your blood in a vertical container.

Picture a lava lamp where the gel inside floats up and down as the light at the bottom heats and cools it. In this case, a vial of your blood is the lamp, and there’s no heating element. Your provider will time how long it takes your red blood cells to settle to the bottom of the test tube.

If it happens faster than usual, you might have an inflammatory condition.

How do I prepare for an ESR test?

You don’t need to do anything special to get ready for a sed rate test. You should be able to eat, drink fluids and take all of your usual medications and supplements.

What happens during the test?

A provider or phlebotomist will wipe an antiseptic on your arm and insert a needle into your vein. They may ask you to make a fist to help blood flow to that arm.

You might feel a sharp pinch when your provider inserts the needle. Tell them if you’re nervous about needles or blood tests. They’ll help you relax and stay calm.

Does a sedimentation rate test have any risks?

The sed rate test is extremely safe and doesn’t have any risk of complications. It doesn’t cause side effects.

Your arm might hurt for a few hours after your test. Your provider will put a bandage on the spot where they inserted the needle. It’s normal to have minor bleeding or bruising after a blood draw.

What are the results of an ESR blood test?

An ESR test shows how quickly your red blood cells sink to the bottom of your blood sample. Most providers use the Westergren method. They’ll put a sample of your blood into a special vial called a Westergren tube. Then, they’ll see how long it takes your red blood cells to sink to the bottom.

Your provider will measure the rate of red blood cell sedimentation as a speed — millimeters per hour (mm/hr). It’s like a tiny version of your car’s speedometer measuring how fast you’re driving in miles or kilometers per hour.

What is a normal sed rate for my age?

A normal range for ESR depends on your age and sex:

Typical ESR ranges can vary from person to person. Some testing labs have different standard ranges, too. Your provider will share your test results and help you understand them.

High sed rate

Having a higher-than-usual sedimentation rate may mean there’s inflammation somewhere in your body. Your provider will talk to you about which other tests or treatments you’ll need if you have an unusually high sed rate.

Low sed rate

Having a low sed rate isn’t usually a cause for concern. In fact, people usually have a sedimentation rate that’s lower than the thresholds listed above. But everyone’s body is different. Some people have a sed rate that’s naturally higher or lower than usual.

What is a sed rate test done by the modified Westergren method?

Most providers do sed rate (ESR) tests using the Westergren method, where they put a sample of your blood in a special tube and then measure how long it takes the red blood cells to sink to the bottom.

Other sed rate test techniques are collectively known as “modified Westergren methods.” Some use less of your blood, mix your blood with a sterile solution or use a centrifuge (an automatic machine that spins very fast to separate the parts of your blood) to get results faster. Which method your provider uses is usually up to their personal preference and which type of test they think will get the most accurate results.

Most providers still use the Westergren method because it has the longest, most successful track record for giving accurate results. Experts are still studying different test techniques to find ways to improve on ESR test speed and accuracy.