ESR (Erythrocyte Sedimentation Rate) Test

The Erythrocyte Sedimentation Rate, commonly shortened to ESR or 'sed rate', is a simple blood test that measures how quickly red blood cells settle to the bottom of a vertical test tube over the course of one hour.

It doesn't diagnose any specific disease on its own; instead, it acts as a general, non-specific marker of inflammation somewhere in the body, since inflammatory proteins in the blood make red cells clump together and fall faster than they normally would.

Because of this non-specific nature, ESR is most useful when combined with a patient's symptoms and other lab findings, and it's especially valuable for tracking how an inflammatory or autoimmune condition changes over time in response to treatment.

It's an inexpensive, well-established test that has been part of clinical medicine for nearly a century, and it remains widely used today precisely because of its simplicity and broad sensitivity to inflammation.