ESR (Sedimentation Rate) Test Explained
Content last reviewed 2026-09-12 · Educational information — not medical advice
Quick answer
ESR measures how fast red blood cells settle in a tube — inflammation makes them clump and fall faster. It's a blunt but useful inflammation marker: normal results help rule OUT inflammatory disease; elevated results are non-specific and need context from symptoms and other tests.
What ESR physically measures
A blood sample sits upright for one hour; red cells settle by gravity. Inflammation alters plasma protein ratios (rising fibrinogen, globulins), promoting cell clumping (rouleaux) that SETTLES FASTER. The result (mm/hour) reflects this sedimentation — an indirect, slow-changing inflammation gauge.
Reference ranges by age
- <50 years: men <15 mm/hr, women <20
- >50 years: men <20, women <30 (rises with age physiologically)
- Marked elevation >100 mm/hr: strong inflammation signal — check myeloma, vasculitis, infection, malignancy
What ESR does well
- Rule-out power: normal ESR makes active polymyalgia rheumatica, temporal arteritis, and bacterial infection LESS likely
- Disease monitoring: rheumatoid arthritis, giant-cell arteritis, polymyalgia — declining ESR tracks treatment response
- Chronic mild elevation: hints at slowly-inflamed conditions (connective tissue, IBD)
What ESR fails at
It's NON-SPECIFIC — elevated in pregnancy, anemia (below-normal ESR unexpectedly), infection, autoimmunity, malignancy, kidney disease, and advancing age. It also rises weeks-slowly and falls slowly, useless for acute dynamic decisions where CRP outperforms it. Falsely LOW in sickle cell, polycythemia, and very high fibrinogen situations alter results mechanically.
ESR vs CRP
| Feature | ESR | CRP |
|---|---|---|
| Response speed | days (slow) | hours (fast) |
| Specificity | low | moderate-high |
| Monitors chronic disease | good | moderate |
| Reflects acute infection | poor | good |
| Use in temporal arteritis | essential | supportive |
They complement rather than substitute.
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Frequently asked questions
ESR 45 with no symptoms — worry?
Often incidental elevation from age, anemia, or subclinical inflammation. Repeat with CRP and clinical context before extensive investigation.
How long after starting steroids does ESR drop?
Giant-cell arteritis/polymyalgia ESR typically falls within days-weeks of steroid initiation — serial monitoring guides tapering.