The sed rate, or erythrocyte sedimentation rate (ESR), is a blood test that measures how quickly your red blood cells settle to the bottom of a tall, narrow tube over one hour. The Westergren method is the standard technique used to perform this test. Results are reported in millimeters per hour (mm/hr), and a faster rate generally signals inflammation somewhere in the body. It’s one of the oldest and simplest lab tests still in regular use, often ordered alongside other bloodwork to help identify or track inflammatory conditions.
How the Test Works
A small blood sample is drawn from your arm and placed into a specialized Westergren tube, which is set upright. Over the next 60 minutes, gravity pulls the red blood cells downward while the liquid portion of your blood (plasma) stays on top. The distance the red blood cells fall in that hour is your sed rate.
Red blood cells normally repel each other because they carry a negative electrical charge on their surface. When inflammation is present, your liver produces higher levels of certain proteins, particularly fibrinogen and immunoglobulins. These proteins coat the surface of red blood cells and reduce that natural repulsion, causing the cells to stick together in stacks that look like piles of coins. These stacks are heavier than individual cells, so they sink faster. The more inflammation you have, the more protein in your blood, the bigger the stacks, and the higher your sed rate.
This makes the ESR an indirect measure of inflammation. It doesn’t detect a single substance the way some blood tests do. Instead, it reflects the combined effect of multiple inflammatory proteins on how your red blood cells behave physically.
Normal Ranges by Age and Sex
Normal values using the Westergren method vary based on age and sex:
- Men under 50: less than 15 mm/hr
- Men over 50: less than 20 mm/hr
- Women under 50: less than 20 mm/hr
- Women over 50: less than 30 mm/hr
- Newborns: 0 to 2 mm/hr
- Children through puberty: 3 to 13 mm/hr
Women tend to have slightly higher sed rates than men at every age, and the normal range increases as you get older. A result that falls within these ranges is typically considered normal, though your doctor will interpret it alongside your symptoms and other test results.
What a High Result Means
A high sed rate tells you that inflammation is present, but it doesn’t pinpoint the cause. Many conditions can drive it up, including rheumatoid arthritis, polymyalgia rheumatica, inflammatory bowel disease (Crohn’s and ulcerative colitis), infections, vasculitis (inflammation of blood vessel walls), giant cell arteritis, kidney disease, heart disease, and certain cancers.
Readings above 100 mm/hr are considered very high and tend to narrow the list of likely causes. At that level, the most common explanations are serious infections, autoimmune diseases, or malignancies. However, even extremely elevated results still require further testing to identify the specific condition responsible.
The test is particularly useful for monitoring diseases over time. If you’ve been diagnosed with polymyalgia rheumatica or giant cell arteritis, for example, your doctor may check your sed rate periodically to see whether treatment is keeping inflammation under control. A dropping number generally means things are improving.
What Can Skew Your Results
Several non-inflammatory factors can push your sed rate higher without any actual disease being present. Pregnancy raises fibrinogen levels significantly, which increases the ESR. Anemia causes red blood cells to settle faster simply because there are fewer of them, not because of inflammation. Obesity, kidney failure, and older age all tend to elevate results as well.
On the other hand, conditions that change the shape or number of red blood cells, such as sickle cell disease or a very high red blood cell count, can artificially lower the sed rate. Because of these variables, a single ESR reading is rarely used on its own to make a diagnosis. It’s almost always interpreted as one piece of a larger picture.
How ESR Compares to CRP
C-reactive protein (CRP) is another common blood test for inflammation, and it works differently. CRP measures a single protein produced by the liver during an inflammatory response. It rises faster when inflammation starts and drops faster when it resolves, making it a more responsive and sensitive marker.
The sed rate, by contrast, is slower to react. It can remain normal in the early hours of an acute infection, producing a false negative. And once elevated, it can take weeks to months to return to normal after the inflammation has resolved. CRP also produces fewer false positives because the ESR is influenced by so many non-inflammatory factors like anemia, age, and sex.
That said, the two tests aren’t interchangeable. Doctors often order both because each captures slightly different information. The ESR is better at reflecting chronic, ongoing inflammation, while CRP is better at catching acute changes. For conditions like polymyalgia rheumatica and giant cell arteritis, the sed rate remains a central part of diagnosis and monitoring.
What to Expect During the Test
The ESR is a standard blood draw. A technician takes a sample from a vein in your arm, and no fasting or special preparation is needed beforehand. The sample is then placed in a Westergren tube in the lab and left undisturbed for exactly one hour. Results are typically available the same day or the next. There’s no recovery time, and the only risk is the minor bruising or soreness that can come with any blood draw.