A sed rate (erythrocyte sedimentation rate, or ESR) tests for inflammation in your body. It doesn’t pinpoint a specific disease. Instead, it measures how quickly red blood cells sink to the bottom of a thin tube of blood over one hour, and a faster-than-normal rate signals that something is driving inflammation, whether that’s an infection, an autoimmune condition, or sometimes cancer.
How the Test Works
When inflammation is present, your liver produces higher levels of certain proteins that cause red blood cells to clump together. Heavier clumps sink faster. A lab technician draws a standard blood sample, mixes it with a substance that prevents clotting, places it in a graduated tube, and lets it stand upright for exactly one hour. The result is simply how many millimeters of clear plasma appear at the top of the tube. More millimeters means more inflammation.
Because so many different conditions trigger this protein response, the sed rate is considered an indirect and nonspecific marker. It tells your doctor that inflammation exists somewhere, but not where or why. That’s why it’s almost always used alongside other tests, symptoms, and imaging rather than on its own.
Normal Ranges by Age and Sex
Results are reported in millimeters per hour (mm/hr), and what counts as “normal” shifts with 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 before puberty: 3 to 13 mm/hr
A result slightly above these cutoffs isn’t automatically cause for alarm. Mildly elevated numbers can reflect temporary situations like a recent cold. The higher the number climbs, the more likely a significant condition is involved.
Conditions It Helps Diagnose
Doctors order a sed rate most often when they suspect one of a handful of inflammatory or autoimmune diseases. Polymyalgia rheumatica and giant cell arteritis (a type of blood vessel inflammation that typically strikes people over 50) are two conditions where the sed rate plays a central role in diagnosis. In studies of giant cell arteritis, 99% of confirmed cases had an ESR above 30 mm/hr, and the average was above 90 mm/hr.
The test is also commonly used to help evaluate rheumatoid arthritis, lupus, and osteoarthritis. For joint and bone infections like septic arthritis and osteomyelitis, a sed rate above 15 mm/hr, combined with symptoms like fever and inability to bear weight, adds useful diagnostic information. In children, a threshold of 20 mm/hr detected bacterial bone or joint infections with 94% accuracy at the time of hospital admission.
Other conditions where the sed rate provides diagnostic value include inflammatory bowel disease, certain lymphomas, multiple myeloma, and vasculitis (widespread inflammation of blood vessels).
What a Very High Result Means
Values over 100 mm/hr are considered extremely elevated and are often tied to serious underlying disease. The most common causes at that level include severe infection, multiple myeloma, giant cell arteritis, polymyalgia rheumatica, and certain types of lymphoma. A result this high will almost certainly lead to additional testing to identify the specific cause.
Tracking Disease Activity Over Time
Beyond initial diagnosis, one of the sed rate’s most practical uses is monitoring. If you’ve been diagnosed with rheumatoid arthritis or polymyalgia rheumatica, for example, your doctor may order periodic sed rates to see whether your inflammation is improving with treatment. A falling number generally means your condition is responding. A rising number may signal a flare or the need to adjust your treatment plan.
How It Compares to a CRP Test
C-reactive protein (CRP) is another common inflammation test, and doctors frequently order both at the same time. The key difference is speed. CRP rises within hours of an infection or inflammatory event and drops back to normal within three to seven days once the problem resolves. The sed rate rises more slowly and stays elevated longer. That makes CRP better for catching early or rapidly changing inflammation, while the sed rate is often more useful for monitoring chronic conditions over weeks or months.
CRP is also a more direct measurement. It quantifies a specific protein the liver produces during inflammation. The sed rate, by contrast, is influenced by a wider range of factors, which makes it less precise but still valuable as a broad screening tool. There is no consensus on which single test is better overall, which is why many doctors order both.
What Else Can Affect Your Results
Several factors unrelated to disease can push your sed rate higher or lower, which is one reason the test is never used in isolation. Pregnancy commonly raises the sed rate. So do anemia, kidney problems, and obesity. Older age naturally increases it, which is why normal ranges are higher for people over 50. Certain medications can also shift results in either direction.
On the other hand, conditions that change the shape or number of red blood cells, like sickle cell disease or polycythemia (an unusually high red blood cell count), can artificially lower the sed rate even when inflammation is present. Your doctor interprets your result in the context of your full health picture, not as a standalone number.