An ESR, or erythrocyte sedimentation rate, is a blood test that measures how quickly your red blood cells settle to the bottom of a thin glass 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 pinpoint exactly what. It’s one of the oldest and most commonly ordered lab tests, used to help detect and monitor conditions ranging from infections to autoimmune diseases to certain cancers.
How the Test Works
Red blood cells normally repel each other because they carry a negative electrical charge on their surfaces. This keeps them evenly dispersed in your blood plasma. When inflammation is present, your liver ramps up production of certain proteins, particularly fibrinogen and immunoglobulins. These proteins coat the surface of red blood cells and reduce that electrical repulsion, allowing the cells to clump together into stacks that resemble coins piled on top of each other. These stacks are heavier than individual cells, so they sink faster.
The test itself is straightforward. A blood sample is placed in a tall, narrow tube and left upright for exactly one hour. A technician then measures how far the red blood cells have fallen, in millimeters. The more inflammation-related proteins circulating in your blood, the more clumping occurs and the higher the number.
Normal Ranges
ESR values naturally vary by age and sex. The commonly used rule of thumb for an upper limit of normal is:
- Men: age divided by 2
- Women: (age + 10) divided by 2
So a healthy 60-year-old man might have a normal ESR up to about 30 mm/hr, while a 60-year-old woman could be normal up to about 35 mm/hr. Women tend to have slightly higher baseline values, and ESR naturally creeps upward with age. People over 50 often have higher readings even when nothing is wrong. This is one reason the test is always interpreted alongside symptoms and other lab results, never in isolation.
What a High ESR Means
A high ESR tells your doctor that inflammation is present somewhere in your body, but it doesn’t say where or why. It’s a nonspecific marker, meaning dozens of different conditions can raise it. Common causes include infections (bacterial, viral, or fungal), autoimmune diseases like rheumatoid arthritis and lupus, and cancers such as lymphoma. Tissue injury, anemia, kidney disease, and even pregnancy can also push the number up.
Values above 100 mm/hr are considered very high and narrow the diagnostic possibilities somewhat. Conditions most frequently linked to ESR readings that extreme include giant cell arteritis (inflammation of the blood vessels near the temples), polymyalgia rheumatica (widespread muscle pain and stiffness), chronic infections, certain cancers, and a rare blood disorder called Waldenström macroglobulinemia. Severe COVID-19 has also been associated with ESR values above 100.
What a Low ESR Means
An unusually low ESR is less common but can also signal a problem. Conditions that slow red blood cell sedimentation include polycythemia (when your body makes too many red blood cells, thickening the blood), sickle cell disease (where misshapen red blood cells don’t stack normally), and leukocytosis (an extremely high white blood cell count). Heart failure and certain kidney or liver problems can also produce low readings.
Where ESR Is Most Useful
The ESR isn’t recommended as a general screening tool for acute inflammation. For that purpose, another blood test called CRP (C-reactive protein) is preferred because it responds faster, rising within hours of an inflammatory event and returning to normal within three to seven days once the problem resolves. ESR, by contrast, rises slowly and stays elevated for much longer.
Where ESR really earns its place is in monitoring chronic inflammatory conditions over time. The College of American Pathologists identifies several specific situations where ESR testing remains valuable:
- Giant cell arteritis: ESR is a cornerstone of diagnosis and ongoing monitoring. Most patients show both elevated ESR and CRP, though the two tests can occasionally disagree, so ordering both together improves diagnostic sensitivity.
- Polymyalgia rheumatica: ESR helps support the diagnosis and track whether treatment is working.
- Rheumatoid arthritis: Serial ESR measurements help gauge disease activity over months and years.
- Lupus flares vs. infection: In lupus patients who develop a fever, the ratio between ESR and CRP can help distinguish a disease flare from an infection. Each unit increase in the ESR-to-CRP ratio is associated with a 17% increase in the odds that the fever is from a lupus flare rather than an infection.
- Orthopedic infections: Using ESR and CRP together can improve accuracy, particularly by reducing false positives when the likelihood of infection is low.
Outside of these specific scenarios, routinely ordering both ESR and CRP together is discouraged because discrepant results can create confusion rather than clarity.
Factors That Skew Results
One of the ESR’s biggest limitations is that many things besides inflammation can alter the result. Age and sex are the most obvious: older adults and women naturally run higher. Pregnancy raises ESR because fibrinogen levels increase significantly during gestation. Anemia also elevates ESR because fewer red blood cells in the sample means less resistance to settling. Obesity has less effect on ESR than it does on CRP, but kidney problems, liver disease, and certain medications can all push the number in either direction.
Immunoglobulin therapy (IVIG) is a specific situation worth noting. Because ESR is directly influenced by immunoglobulin levels, it will remain artificially elevated in people receiving this treatment, making CRP the better choice for tracking inflammation in those patients.
ESR Is a Clue, Not a Diagnosis
The ESR is best understood as a flag that something is happening in your body. It’s sensitive enough to pick up inflammation from many sources, but that same sensitivity means it can’t tell you which source is responsible. A study of 80 patients with polymyalgia rheumatica and giant cell arteritis found that 22.5% had a normal ESR at the time of diagnosis, and relying too heavily on the test actually delayed diagnosis in 10 of those patients. Serious complications occurred in 4 patients whose ESR was normal or only minimally raised.
That study also found that changes in ESR during follow-up didn’t consistently parallel changes in symptoms. Your ESR might drop while you still feel terrible, or remain elevated after you’re feeling much better. This is why doctors use it as one piece of a larger puzzle, combining it with your symptoms, physical exam findings, imaging, and other blood work to build a complete picture.