Sedimentation rate, often called ESR or “sed rate,” is a blood test that measures how quickly your red blood cells settle to the bottom of a tall, thin tube over one hour. The result is reported in millimeters per hour (mm/hr). A faster settling rate signals that something in your body is driving inflammation, though the test can’t tell you exactly what. It’s one of the oldest and simplest lab tests still in routine use, ordered to help detect and monitor conditions ranging from autoimmune diseases to serious infections.
How the Test Works
When inflammation is present anywhere in your body, your liver ramps up production of certain proteins that circulate in your blood. The most important of these is fibrinogen. These proteins coat the surface of red blood cells and reduce the natural electrical charge that normally keeps the cells repelled from each other. With less repulsion, red blood cells begin sticking together in stacks, somewhat like a roll of coins. These clumps are heavier than individual cells, so they drop to the bottom of the test tube faster.
The standard method, called the Westergren method, uses a vertical tube filled with a blood sample mixed with an anticoagulant. After exactly 60 minutes, a technician reads how many millimeters of clear plasma sit above the layer of settled red blood cells. A higher number means faster settling, which generally means more inflammation.
Normal Ranges by Age and Sex
ESR naturally varies based on who you are. The commonly used reference ranges are:
- Men under 50: 0 to 15 mm/hr
- Men over 50: 0 to 20 mm/hr
- Women under 50: 0 to 20 mm/hr
- Women over 50: 0 to 30 mm/hr
Women tend to have slightly higher baseline values than men, and ESR gradually rises with age in both sexes. A simple rule of thumb sometimes used: the upper limit of normal for men is roughly age divided by two, and for women it’s age plus ten, divided by two. These are approximations, and your lab may list slightly different cutoffs on your results.
What a High ESR Can Mean
An elevated sedimentation rate is a red flag for inflammation, but it’s deliberately nonspecific. It casts a wide net. Your doctor uses it alongside symptoms, physical exam findings, and other tests to narrow down what’s going on. The conditions linked to elevated ESR fall into three broad categories.
Autoimmune and Inflammatory Diseases
Rheumatoid arthritis, lupus, polymyalgia rheumatica, and various forms of blood vessel inflammation (vasculitis) all raise ESR. Two conditions rely heavily on ESR for diagnosis: polymyalgia rheumatica, where an ESR of 40 mm/hr or higher is a typical diagnostic threshold, and giant cell arteritis (also called temporal arteritis), where values above 100 mm/hr are common. That said, about 4 to 13 percent of people with these conditions have a normal ESR at the time of diagnosis, and roughly 5 percent start with a normal value that rises later.
Infections
Bacterial infections, particularly deep-seated ones like bone infections (osteomyelitis) or joint infections, tend to push ESR up significantly. In children, ESR can help flag invasive bacterial infections that need aggressive treatment. The test is especially useful for detecting low-grade bone and joint infections that other inflammatory markers sometimes miss.
Cancers
Certain malignancies produce proteins or trigger immune responses that accelerate red blood cell settling. Lymphoma, multiple myeloma, and metastatic cancers of the breast, prostate, and colon are all associated with elevated ESR. One particular blood cancer, Waldenström macroglobulinemia, can drive ESR above 100 mm/hr because it floods the blood with abnormal antibody proteins.
What Affects ESR Besides Inflammation
One of the biggest limitations of this test is that many things can push the number up or down without any true inflammatory disease being present. Anemia raises ESR because fewer red blood cells settle differently in the tube. Kidney failure, obesity, and pregnancy all elevate it. Your menstrual cycle, regular alcohol use, and recent vigorous exercise can shift results too. Certain medications and supplements also interfere, so it’s worth mentioning everything you take before the test.
On the other end, conditions that change the shape or number of red blood cells can artificially suppress ESR. Polycythemia (too many red blood cells), sickle cell disease, and very high white blood cell counts can all produce a misleadingly low reading, potentially masking inflammation that’s actually present.
ESR Compared to CRP
C-reactive protein (CRP) is the other major blood test for inflammation, and doctors often order it alongside or instead of ESR. CRP is a more sensitive and faster-reacting marker. When inflammation starts, CRP rises within hours and drops quickly once the cause resolves. ESR, by contrast, responds sluggishly. It can take days to rise after inflammation begins and weeks to normalize after it ends. This lag means ESR is more likely to give a false negative early in an illness and a false positive after one has resolved.
About 12.5 percent of patients show a mismatch between the two tests. When CRP is elevated but ESR is normal, infection or acute tissue damage (like a heart attack or blood clot) is the usual explanation. When ESR is high but CRP is normal, the cause is often a condition without obvious systemic inflammation, such as certain cancers. In rheumatoid arthritis, patients vary considerably in which marker tracks their flare-ups better, so doctors sometimes measure both at the start to see which one is more reliable for that individual.
There are two situations where ESR remains clearly superior to CRP: detecting low-grade bone and joint infections, and monitoring disease activity in lupus.
What to Expect During the Test
The sedimentation rate test is a simple blood draw, no different from any other routine lab work. No fasting is required. A small sample is taken from a vein in your arm, and results are typically available within a day or two. The draw itself takes a few minutes, and there’s no special preparation beyond letting your provider know about medications and supplements you’re currently taking.
How Doctors Use the Results
A single ESR result rarely leads to a diagnosis on its own. Doctors treat it as one piece of a larger puzzle. A mildly elevated result in someone with no symptoms may simply reflect age, weight, or another benign factor. A very high result, particularly above 100 mm/hr, narrows the possibilities significantly toward giant cell arteritis, serious infection, or certain blood cancers, and typically prompts more targeted testing.
Where ESR is especially valuable is in tracking a known condition over time. If you have rheumatoid arthritis or polymyalgia rheumatica, your doctor may recheck your ESR periodically to see whether your treatment is controlling inflammation. A falling ESR generally means things are heading in the right direction. A rising one, even before symptoms worsen, can signal a flare that needs attention.