How does the sed rate (erythrocyte sedimentation rate or ESR) test work?

The blood is placed into a tall thin tube and the test measures the rate of fall (sedimentation) of erythrocytes (red blood cells). The red cells settle faster than normally if there is alteration of blood proteins, which indicates inflammation.

The results are the measurement, in millimeters, of the clear fluid (plasma) left at the top of the tube after a period of 1 hour.

Last reviewed by a Cleveland Clinic medical professional on 03/14/2018.


  • Lab Tests Online. Erythrocyte Sedimentation Rate. Accessed 5/7/2018.
  • Cush JJ. Cush J.J. Cush, John J.Approach to Articular and Musculoskeletal Disorders. In: Kasper D, Fauci A, Hauser S, Longo D, Jameson J, Loscalzo J. Kasper D, Fauci A, Hauser S, Longo D, Jameson J, Loscalzo J Eds. Dennis Kasper, et al.eds. Harrison's Principles of Internal Medicine, 19e New York, NY: McGraw-Hill; 2014.
  • Common Laboratory Tests. In: LeBlond RF, Brown DD, Suneja M, Szot JF. LeBlond R.F., Brown D.D., Suneja M, Szot J.F. Eds. Richard F. LeBlond, et al.eds. DeGowin’s Diagnostic Examination, 10e New York, NY: McGraw-Hill; 2014.

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