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ESRs have limited clinical utility
(Aotea News, August 2009)
Dr Ken Romeril
ESR and CRP are amongst the most commonly requested tests with ESR tests being performed more often. It is now recognised that in most situations CRP gives more valuable information to the requesting clinician.
The two tests are used to assess the inflammatory response with fibrinogen and alpha + beta globulin the major contributors to the ESR, and immunoglobins to a lesser extent.
There is a significant lag between the changes at a clinical level and variations in the ESR. If the haemocrit is reduced, the ESR is of limited use due to changes in plasma leading to increased red cell aggregation.
CRP belongs to the Pentraxin family of proteins, which has 5 identical sub-units. Plasma levels of CRP begin increasing within 4-6 hours following an inflammatory stimulus, and the half life of CRP is 5-7 hours. CRP has several functions including anti-infective by opsonisation for phagocytosis and also anti inflammatory by aiding in neutrophil release from blood vessels.
There are few studies that compare the use of CRP and ESR, hence there are only a few conditions for which we can make clear recommendations. The best approach is to consider the various clinical conditions that may be posed so as to choose either ESR or CRP.
In the diagnosis of polymyalgia rheumatica (PMR), for example, both tests can be used, remembering that 10% of cases can have a normal ESR. CRP measurement will also give a baseline and it is the best test for monitoring response to therapy. The same comments can be made for GCA (giant cell arteritis).
Neither CRP or ESR are of use in diagnosing rheumatoid arthritis, as other defined diagnostic criteria are better. CRP is consistently a better measure of the disease activity and it is known that sustained high levels of CRP are associated with worse outcomes.
In terms of response to antibiotic treatment, CRP is good and ESR tends to have a slow response. Most malignant tumours, especially if metastatic, induce an acute phase response, in particular lymphoma and renal cell carcinoma.
Neither CRP or ESR however, should be used as a screening test for malignancy in the general population, as any increase is nonspecific.
In summary then, ESR is of limited clinical utility; CRP should be the first choice on most clinical occasions, and seldom should one request both ESR and CRP simultaneously.
bPAC guidelines are the source of these comments and the references are available at www.bpac.org.nz