Erythrocyte Sedimentation Rate - ESR
The erythrocyte sedimentation rate (ESR) was first developed as a pregnancy test in the
early 1900's. As such it was not a very reliable test but was then shown to have value as
a non-specific marker of illness. Two methods have been used - Westergren or Wintrobe with
the former method being the more commonly used method today.
How is Erythrocyte Sedimentation Rate - ESR Performed
The test is performed by diluting whole blood in citrate at a dilution of 4:1. This
whole blood may be anticoagulated with EDTA prior to dilution. The mixture is then
aspirated into a vertical tube, which has a uniform internal diameter, to a height of
200mm. The distance that the red cells sediment down through the plasma in 1 hour is taken
to be the ESR.
Sedimentation of red cells in this system is affected by forces both for and against
sedimentation. The forces resisting sedimentation are the negative charge on the red cell
surface (causing red cells to repel each other (zeta potential)), the upflow of plasma
displaced by falling red cells, and the rigidity of red cells. The forces accelerating
sedimentation are anaemia, and plasma proteins. Plasma proteins bind to red cell membranes
thereby reducing the zeta potential thus allowing rouleaux formation to occur. There are a
number of artifactual causes of an elevated ESR. These include vibration of the ESR tube,
the tube being non-vertical, and the age of the sample (increasing age decreases the ESR).
The degree to which proteins reduce the zeta potential can be rated on a scale of 1-10:
fibrinogen 10, beta-globulin 5, alpha globulin 2, gamma globulin 2, albumin 1.
How is Erythrocyte Sedimentation Rate - ESR Interpreted
The ESR is a non-specific test and so can be difficult to interpret. Recent trials of
the ESR have demonstrated no value in screening asymptomatic individuals, because not only
is the number of abnormals low but also in most cases the abnormal test returns to normal
over several months without any significant diagnosis being made.
There is also little evidence of value in screening symptomatic patients because a
complete history and physical examination is a much better tool for detecting
abnormalities. Older text books suggest that an extensive search should be made for the
cause of an elevated ESR but provide little evidence of the benefits of such a search.
Recent cost-benefit analysis has suggested that tests in addition to a complete history
and physical examination are not cost effective.
However, there are several groups of patients where the ESR is important, viz. patients
suspected of having temporal arteritis or polymyalgia rheumatica. In these cases treatment
is often initiated after an elevated ESR result is known and prior to a definitive biopsy.
In these patients the diagnosis is difficult to sustain, but not excluded, if the ESR is
normal.
The ESR can also be useful in monitoring certain groups of patients viz. rheumatoid
arthritis, temporal arteritis, polymyalgia rheumatica, and Hodgkin's Disease (H.D.), where
disease activity is mirrored by changes in the ESR.
There is debate as to whether this test is useful in distinguishing between organic and
psychosomatic disease.
There is no evidence of any diagnostic value to the ESR when attempting to evaluate
acutely ill patients already known to have acute or chronic infections, or cancer (except
H.D.). Even in screening patients with possible myeloma the ESR has been replaced by
measurement of total protein and globulin fraction. . |
