Q. Could you please explain the ESR?

A. The erythrocyte sedimentation rate (ESR), also called a sedimentation rate or sed rate, is a test that is performed by putting blood in a tube (called a Westergren tube – there are a bunch in the above photo) and measuring the rate at which the red cells settle in the tube (in mm/hr). It’s commonly used (and probably over-used) as a measure of inflammation.

The physics of how blood settles has to do with the zeta potential between the red cells. The zeta potential is the normal, negative force that exists between red cells and pushes them apart from each other. Things that disrupt the zeta potential make it easier for the red cells to come close to each other, and therefore the cells settle faster in the tube (and the ESR goes up). Things that increase the zeta potential between the red cells (making them more repellant than usual) will cause the red cells to settle at a slower rate, and the ESR will therefore go down.

In inflammation, for example, there is an increase in fibrinogen in the blood. Fibrinogen is a big molecule that gets in between red cells, decreasing the zeta potential, and making them settle at a faster rate in the ESR test. All kinds of inflammatory diseases (for example, temporal arteritis, rheumatoid arthritis, and lupus) will make the ESR go up. The ESR also goes up in multiple myeloma because of all the immunoglobulin floating around in the blood.

There are many factors that can affect the ESR (for example, pregnancy, old age and anemia all make the ESR go up). Also, since there are so many diseases that may show an elevated ESR, just having a high ESR alone is a rather non-specific finding. You need to take into account the patient’s clinical picture and other lab tests to get a handle on what’s making the ESR go up.

Perhaps a better marker of inflammation is C-reactive protein (CRP), which is not affected by as many factors as is the ESR.