Learning Objective
Explain the physiological basis of the erythrocyte sedimentation rate (ESR), identify conditions that increase or decrease ESR, and understand its clinical use and limitations.
Definition & Mechanism
The erythrocyte sedimentation rate (ESR) measures the rate at which red blood cells (RBCs) settle in a vertical tube over time.
Normal physiology:
- RBCs remain dispersed due to negative surface charges (zeta potential) that cause mutual repulsion.
During inflammation:
- Acute-phase proteins (especially fibrinogen) coat RBCs
- → ↓ zeta potential
- → ↑ RBC aggregation (rouleaux formation)
Result:
- Larger, denser RBC aggregates settle faster
- → increased ESR
Clinical Use
- Nonspecific marker of inflammation
- Often measured alongside C-reactive protein (CRP)
- CRP: more specific and responds more rapidly to changes in inflammation
- ESR: rises and falls more slowly
Activity
Conditions Associated with Increased ESR (↑ ESR)
- Most anemias
- Infections
- Inflammatory conditions
- Giant cell (temporal) arteritis
- Polymyalgia rheumatica
- Malignancy
- Metastatic cancer
- Multiple myeloma
- Chronic kidney disease
- Especially end-stage renal disease
High-Yield USMLE Step 1 Summary
- Fibrinogen ↑ → zeta potential ↓ → rouleaux → ESR ↑
- ESR = sensitive but nonspecific
- CRP = more specific, faster response
- Very high ESR → think temporal arteritis, PMR, or malignancy
