ESRErythrocyte sedimentation rate
How fast red cells settle in a tube, an old but useful inflammation marker.
The erythrocyte sedimentation rate measures how far red blood cells fall through a vertical column of blood in one hour. Inflammation raises the level of proteins that make red cells stick together, and heavier clumps sink faster.
It matters as a slow, general marker of inflammation. It rises and falls over days to weeks, where CRP responds within hours, so the two answer different questions: ESR is better suited to following a long-running condition.
In practice ESR rises naturally with age and with anaemia and pregnancy, so it is interpreted with those in mind and read alongside CRP rather than instead of it.