Q. Could you please explain the ESR and why it decreases with polycythemia and increases with anemia?
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.
You asked about polycythemia vera and anemia . In polycythemia vera, there are a lot more red cells present in the blood. Consequently there is less serum (and less serum protein) between the cells – so the red cells stick together easier and fall at a faster rate in the ESR test (and the ESR goes up). In anemia, there are fewer red cells, with more protein in between, and a slower rate of settling (so the ESR goes down).