A sed rate test measures how quickly your red blood cells settle to the bottom of a thin tube over one hour. It’s a simple blood test used to detect inflammation in your body. The faster your red blood cells sink, the more likely it is that something is driving inflammation, whether that’s an infection, an autoimmune condition, or something else. It can’t pinpoint exactly what’s wrong, but it’s a useful signal that something needs investigating or monitoring.
How the Test Works
Your red blood cells normally repel each other because they carry a negative electrical charge on their surface. This keeps them evenly spread out in your blood. When inflammation is present, your liver ramps up production of certain proteins, especially fibrinogen and immunoglobulins. These proteins stick to the surface of red blood cells and reduce that electrical repulsion, causing the cells to clump together into stacks that look like coins piled on top of each other (called rouleaux).
Clumped cells are heavier than individual cells, so they drop faster through the tube. The result is reported in millimeters per hour: the more millimeters the cells have fallen in 60 minutes, the higher the sed rate. It’s an indirect measure. The test doesn’t detect a specific disease. It detects the protein changes that come with inflammation.
Why Your Doctor Ordered It
A sed rate is most commonly ordered when your doctor suspects an inflammatory condition but isn’t sure what it is yet. It helps narrow the search. It’s also used to track how well treatment is working in conditions already diagnosed.
The test plays a particularly important role in diagnosing two conditions: polymyalgia rheumatica and temporal arteritis (also called giant cell arteritis). In temporal arteritis, one study found the average sed rate exceeded 90 mm per hour, with 99% of patients having values above 30. Polymyalgia rheumatica causes severe stiffness and aching in the neck, shoulders, or hips, and an elevated sed rate is one of the key criteria doctors use to diagnose it.
Beyond those two conditions, doctors use the sed rate alongside other tests to evaluate a wide range of problems: rheumatoid arthritis, lupus, inflammatory bowel disease, kidney disease, infections, vasculitis (inflammation of blood vessel walls), and certain cancers. It’s also used to monitor relapse in conditions like Hodgkin’s lymphoma.
Normal Ranges by Age and Sex
What counts as “normal” depends on your age and sex. Using the standard Westergren method:
- 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 up to puberty: 3 to 13 mm/hr
Sed rates naturally creep upward with age, and women tend to run slightly higher than men at every age. A result just above the cutoff in an older adult doesn’t necessarily signal disease, which is why the number is always interpreted alongside symptoms and other lab work.
What a High Result Means
A high sed rate tells you inflammation is present somewhere, but not where or why. The list of possible causes is long. In one study of patients referred to rheumatology for elevated sed rates who had no prior diagnosis, the most common explanations were infections (about 25% of cases), new autoimmune conditions (with polymyalgia rheumatica and giant cell arteritis being the most frequent), and cancers (about 9%).
Infections that commonly raise the sed rate include pneumonia, urinary tract infections, bone infections, and endocarditis. Autoimmune conditions include rheumatoid arthritis, lupus, Sjögren’s disease, and ankylosing spondylitis. Among cancers, lymphoma, multiple myeloma, and ovarian cancer have all been associated with significantly elevated levels.
The higher the number, the more likely a serious condition is involved, but there’s no clean threshold that separates “mild” from “concerning.” A value of 100 mm/hr demands urgent workup. A value of 25 in a 55-year-old woman might be completely normal.
Factors That Skew Results
Because the sed rate depends on blood composition, several things besides inflammation can push it up or down. Anemia raises the sed rate because fewer red blood cells settle differently in the tube. Pregnancy does the same, particularly in the second and third trimesters, due to changes in blood protein levels. Obesity and older age also tend to elevate results without any underlying disease.
On the other side, conditions that increase the number of red blood cells (like polycythemia) or change their shape (like sickle cell disease) can keep the sed rate artificially low, potentially masking real inflammation. This is one reason the test is never used alone to rule something in or out.
How It Compares to CRP
C-reactive protein (CRP) is the other common blood test for inflammation, and doctors often order both at the same time. The key difference is speed. CRP rises faster when inflammation starts and drops faster when it resolves. In one study of bone and joint infections, CRP returned to normal in about 10 days after treatment began, while the sed rate took about 24 days.
That makes CRP more useful for tracking acute infections or seeing whether a treatment is working in real time. The sed rate, because it changes more slowly, is better suited for monitoring chronic conditions over weeks and months. It’s also more sensitive to certain conditions like polymyalgia rheumatica, where CRP may not rise as dramatically. Using both tests together gives a fuller picture than either one alone.
What to Expect During the Test
The sed rate is a standard blood draw from a vein in your arm. No fasting is required, and you don’t need to stop any medications beforehand. The sample is placed in a tall, narrow tube and left to sit for exactly one hour. A technician then measures how far the red blood cells have fallen. Results are typically available within a day or two, and your doctor will interpret them alongside your symptoms, physical exam, and any other lab tests that were ordered at the same time.