The ESR test (erythrocyte sedimentation rate), often called a sed rate, is a simple blood test that looks for inflammation somewhere in the body. It does not diagnose a specific disease by itself. Instead, it helps your clinician understand whether an inflammatory process may be present and how active it might be over time.

In the lab, a sample of your blood is placed in a tall, thin tube. Red blood cells (erythrocytes) naturally sink to the bottom. The ESR measures how far the red blood cells fall in one hour, reported as millimeters per hour (mm/hr). When inflammation is present, certain blood proteins increase and can cause red blood cells to clump together and settle faster—leading to a higher ESR.

What the results can mean:

Higher ESR (high sed rate): Suggests inflammation may be present. A high ESR can be seen with infections, autoimmune diseases, inflammatory arthritis, some kidney conditions, and certain cancers. ESR can also be mildly elevated from non-disease factors such as older age, pregnancy, anemia, or some medications—so your clinician interprets it along with symptoms and other labs (often C-reactive protein/CRP).

Normal ESR: Makes significant ongoing inflammation less likely, but it does not fully rule it out. Some inflammatory conditions can still occur with a normal ESR, especially early in the illness.

Low ESR: Usually not concerning. It may occur with conditions that affect red blood cell shape or number (for example, some blood disorders), but it is less commonly used to guide care.

Clinicians typically order an ESR blood test when symptoms suggest inflammation or an immune-related condition, or to help monitor a known diagnosis. Because ESR is a broad “screening” marker, it’s often used together with other tests and your medical history.

Common reasons your doctor may order ESR include:

Unexplained symptoms that could reflect inflammation, such as:

fever of unknown origin, persistent fatigue, unintended weight loss, generalized aches, or prolonged malaise.

Joint and muscle symptoms, such as:

joint pain/swelling/stiffness (especially morning stiffness), suspected inflammatory arthritis, muscle pain and weakness, or evaluation for polymyalgia rheumatica.

Headache and vision-related red flags in older adults, where rapid evaluation is important, such as:

new severe headaches, scalp tenderness, jaw pain with chewing, or vision changes (to help assess for giant cell arteritis/temporal arteritis).

Suspected or ongoing infection when inflammation tracking may help, especially in longer-lasting infections (ESR often changes more slowly than CRP).

Monitoring chronic inflammatory conditions to assess disease activity or response to treatment, including many autoimmune disorders and inflammatory arthritis. Trends over time (rising or falling ESR) are often more informative than a single result.

Clarifying other abnormal results (for example, anemia, elevated inflammatory markers, or concerning symptoms) to provide additional context.

• Rheumatoid arthritis
• Polymyalgia rheumatica
• Giant cell arteritis (temporal arteritis)
• Systemic lupus erythematosus (lupus)
• Vasculitis (inflammation of blood vessels)
• Inflammatory bowel disease (Crohn’s disease, ulcerative colitis)
• Chronic or significant infections (e.g., osteomyelitis, endocarditis, tuberculosis)
• Certain cancers (including some lymphomas and multiple myeloma)
• Chronic kidney disease and some inflammatory kidney disorders
• Autoimmune thyroid disease (in inflammatory phases, in some cases)

• Checking for underlying inflammation when symptoms are unexplained (fatigue, fever, body aches)
• Monitoring autoimmune disease activity and treatment response over time
• Tracking inflammatory arthritis control (e.g., rheumatoid arthritis) alongside symptoms and exams
• Supporting evaluation of possible infection or prolonged inflammatory illness when paired with other tests (like CRP and CBC)
• Following recovery or flare patterns in chronic inflammatory conditions to help guide follow-up care
• General health evaluation when your clinician suspects a systemic inflammatory process