ESR (erythrocyte sedimentation rate) measures how quickly red blood cells settle to the bottom of a vertical tube over one hour. When inflammation is present, proteins called acute phase reactants (particularly fibrinogen) cause red blood cells to clump together (form rouleaux), making them heavier and causing them to fall more quickly.

ESR is a non-specific marker of inflammation — a raised ESR indicates that something is causing inflammation somewhere in the body, but cannot identify the cause. It is elevated in autoimmune conditions (including rheumatoid arthritis, lupus, and polymyalgia rheumatica), infections, malignancy, and anaemia. ESR is often measured alongside CRP — the two tests are complementary, as ESR responds more slowly and can remain elevated longer after the acute inflammatory stimulus has resolved.