What Is ESR? Understanding the Inflammation Blood Test

The erythrocyte sedimentation rate, or ESR, is one of the oldest blood tests still in everyday use. Despite being over a century old, it remains a helpful and inexpensive way to detect inflammation in the body. It is often requested alongside CRP, and understanding how the two differ helps make sense of your results.

What ESR measures

ESR measures how quickly red blood cells (erythrocytes) settle to the bottom of a thin tube over one hour. When inflammation is present, certain proteins in the blood cause red cells to clump together and sink faster, so a higher sedimentation rate points to inflammation. The result is reported in millimetres per hour.

Unlike a test that measures a specific chemical, ESR is an indirect measure — it reflects changes in blood proteins rather than pinpointing any single cause. That makes it a broad, general indicator rather than a precise one.

How ESR differs from CRP

Both tests flag inflammation, but they behave differently, which is why doctors sometimes use them together:

• Speed — CRP rises and falls within hours, giving a real-time picture. ESR changes more slowly, over days, so it can reflect longer-standing inflammation.
• What affects them — ESR is influenced by age, sex, anaemia and pregnancy, which can nudge the result up without any active disease. CRP is less affected by these factors.
• Together — using both can give a fuller sense of whether inflammation is new, ongoing or settling.

When ESR is raised

A raised ESR simply signals that inflammation is likely present somewhere. Common associations include:

• Autoimmune and inflammatory conditions — such as rheumatoid arthritis, polymyalgia rheumatica and giant cell arteritis, where ESR can be particularly useful.
• Infections — especially those lasting more than a few days.
• Some cancers — occasionally ESR is raised, though it is never used alone to look for cancer.

Because normal ranges shift with age and sex, a mildly raised ESR is not always a cause for concern and must be interpreted by a clinician.

What a normal ESR tells you

A normal ESR is broadly reassuring, but like any single inflammation marker it does not exclude every condition. Some inflammatory illnesses can be active with a normal ESR, particularly early on, which is one reason doctors often pair it with CRP and repeat testing if symptoms continue.

When to get checked

ESR can be worth checking if you have persistent symptoms such as unexplained joint or muscle pain, stiffness, fever or general malaise, particularly if these have lasted more than a couple of weeks. It is best requested as part of an assessment rather than in isolation. If your result is abnormal or your symptoms persist, a clinician can decide what — if anything — needs further investigation.

Written by Dr Ibrar Ahmed — MBChB MRCP MRCGP PhD PG Dip (Cardiology)
Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic