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The Erythrocyte Sedimentation Rate (ESR or Sed Rate) is a very simple test that the lab can run on a fresh blood sample. The test is run by putting a few milliliters of whole, anticoagulated blood into a long slender tube. The tube is then placed in a rack and allowed to stand for 60 minutes. After the 60 minute incubation, the distance that the erythrocytes (red blood cells, or RBC) have settled is measured. Everyone’s blood will show some settling during the 60 minute period.
The normal values for the Sed Rate are usually accepted to be 0-15 mm/hr for males, and 0-20 mm/hr for females. However, age can also cause a slight increase in ESR, so some labs use these formulas: [(age divided by 2)=normal ESR] for males and [(age+10) divided by 2=normal ESR] for females.
The rate at which the blood cells settle is actually a very intricate set of interactions between the cells and the proteins in the blood plasma. The proteins in the plasma increase the viscosity of the plasma, so the more proteins, the slower the Sed Rate. During inflammatory reactions, many plasma proteins move out of the blood stream to the tissues that are inflamed. The lower levels of plasma proteins lead to an increased Sed Rate. Anemia can also cause the Sed Rate to increase, because if there are fewer red blood cells in the whole blood, they will settle faster. Some labs use a correction factor to correct the Sed Rate for severely anemic patients
Elevation of the Sed Rate usually indicates that an inflammatory process is happening somewhere in the body. The Sed Rate does not indicate any specific disease process, nor does it indicate the prognosis for the patient. Multiple Sed Rates measured during and after the inflammatory process may be helpful in predicting remission from the inflammation. The ESR is elevated in a lot of different conditions.
Often the C-Reactive Protein (CRP) is also elevated. (I’ll write a separate page on CRP.)
Some labs are looking at a test that measures plasma viscosity directly. The advantage of the viscosity test is that it is not subject to interference from anemia or age.
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