- Polycythemia vera is a myeloproliferative disorder characterized by an increased red blood cell mass, which leads to increased blood viscosity and decreased plasma volume.
- This reduces the tendency of red cells to aggregate and settle, resulting in a normal or decreased ESR.
- In contrast, conditions like rheumatoid arthritis, tuberculosis, and acute bacterial infections cause inflammation, increasing fibrinogen and other acute-phase proteins that promote red cell aggregation and elevate the ESR.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Chapter 40, p. 237