A sedimentation rate, formally called an erythrocyte sedimentation rate or ESR, is a blood test that measures how quickly your red blood cells sink to the bottom of a thin glass tube over one hour. The result, reported in millimeters per hour (mm/hr), serves as a general marker of inflammation in your body. A faster rate suggests more inflammation; a slower rate suggests less. It’s one of the oldest and simplest lab tests still in routine use, ordered to help detect or monitor conditions ranging from infections to autoimmune diseases.
How the Test Works
The concept behind an ESR is straightforward. A lab technician draws a standard blood sample, then transfers it into a tall, narrow tube called a Westergren tube, which stands upright for exactly one hour. During that time, gravity pulls red blood cells downward, leaving a column of clear plasma at the top. The technician measures how many millimeters of clear plasma have appeared. That number is your sedimentation rate.
When your body is fighting inflammation, your liver produces higher levels of certain proteins, especially fibrinogen and immunoglobulins, that circulate in the blood. These proteins cause red blood cells to stick together in stacks, sometimes described as looking like a roll of coins. These clumps are heavier than individual cells, so they fall faster. The more inflammation you have, the more clumping occurs, and the higher your ESR reading.
Normal Ranges by Age and Sex
Normal ESR values aren’t the same for everyone. They vary by age, sex, and even pregnancy status. The standard reference ranges using the Westergren method are:
- Children: 10 mm/hr or lower
- Men under 50: 15 mm/hr or lower
- Women under 50: 20 mm/hr or lower
- Men over 50: 20 mm/hr or lower
- Women over 50: 30 mm/hr or lower
ESR naturally rises with age, which is why the thresholds shift upward after 50. Women tend to have slightly higher baseline values than men. Pregnancy can also raise the rate significantly, which is expected and not a sign of a problem on its own.
What a High ESR Can Mean
A high sedimentation rate tells you that something in the body is producing inflammation, but it doesn’t tell you what. It’s a nonspecific test, meaning dozens of conditions can push the number up. That’s why it’s almost always ordered alongside other tests to narrow down the cause.
Conditions commonly associated with an elevated ESR include:
- Autoimmune diseases: rheumatoid arthritis, lupus, polymyalgia rheumatica
- Blood vessel inflammation: giant cell arteritis (which affects arteries in the scalp, neck, and arms) and other forms of vasculitis
- Inflammatory bowel disease: Crohn’s disease and ulcerative colitis
- Infections: bacterial infections in particular can cause significant elevations
- Kidney disease
- Heart disease
- Certain cancers
Giant cell arteritis is one condition where ESR plays a particularly important diagnostic role. In someone over 50 with new, severe headaches and scalp tenderness, a very high ESR can be one of the strongest clues pointing toward this diagnosis and the need for rapid treatment to prevent vision loss.
What a Low ESR Can Mean
A low or normal ESR is usually reassuring, since it suggests your body isn’t dealing with significant inflammation. However, certain conditions can keep the ESR artificially low even when inflammation is present. Polycythemia, a condition where you have too many red blood cells, makes them settle more slowly. Sickle cell disease and other conditions that change the shape of red blood cells can also suppress the rate, because abnormally shaped cells don’t stack into clumps the way normal ones do. These situations can make the test misleading.
ESR vs. CRP Testing
If you’ve had an ESR ordered, you may also see a CRP (C-reactive protein) test on your lab work. Both measure inflammation, but they behave differently in your bloodstream. CRP rises within hours of an infection or inflammatory flare and returns to normal within three to seven days once the problem resolves. ESR is slower to climb and stays elevated for a longer period.
Because of this timing difference, CRP is often the better choice for detecting new or rapidly changing inflammation. ESR is more useful for tracking chronic conditions over time, since its slower response gives a broader picture of how active a disease has been over days or weeks rather than hours. There’s no firm consensus on which test is superior overall, and many physicians order both to get complementary information.
Why ESR Has Limits
The biggest limitation of an ESR is that it’s nonspecific. A high number can mean anything from a mild viral illness to cancer, and a normal number doesn’t guarantee nothing is wrong. Several factors unrelated to disease can skew results. Anemia raises the ESR because fewer red blood cells in the tube means less resistance to settling. Obesity, older age, and female sex all nudge the baseline higher. On the other end, taking anti-inflammatory medications can lower the rate and potentially mask ongoing inflammation.
Because of these limitations, an ESR is rarely used alone to diagnose anything. It’s a screening and monitoring tool. If your ESR comes back elevated, your doctor will typically combine that information with your symptoms, a physical exam, and more targeted blood work to figure out the underlying cause. If you’re already being treated for a condition like rheumatoid arthritis, a declining ESR over time is a good sign that treatment is working, while a rising number may signal a flare.