From the article:
In 2004 Ward analyzed 63 rheumatoid arthritis (RA) clinical trial studies with 90 active disease modifying antirheumatic drug treatment arms and demonstrated that erythrocyte sedimentation rate (ESR) was more sensitive to change than C-reactive protein (CRP) at 12 weeks and 24 weeks of treatment, with mean effect size differences of 0.09 to 0.11 units1. It is puzzling that Crowson, et al, authors from the Mayo Clinic and Centocor, address the issues of ESR versus CRP again, and with correlation analyses2. If we get their point, it is that it doesn’t matter which test is used in a clinical trial as they are both about as effective (or ineffective). This conclusion comes primarily from finding similar levels of correlation between swollen joint count and ESR and CRP, and through analysis of normal values. But correlation analysis, unfortunately, cannot address sensitivity to change, so we cannot assume that the tests are equal, particularly in view of Ward’s report1. And there are problems with “normal values,” as we note below.
The authors then recommend the use of the CRP in clinical trials and clinical practice settings because it is easier and less time-consuming to perform (at the Mayo Clinic). It should be noted that small clinics can perform the simple ESR in their laboratories, but must send CRP determinations to specialty laboratories at additional cost and delay. The central laboratory advantage of the CRP is important to clinical trials, but it is a mythical advantage in clinical practice. There are a number of other myths that are underscored by the current study.
Whatever the value of knowing normal values for ESR and CRP in the general population, the values have little meaning in patients with RA who are not representative of the general population3. In addition, many people with active RA have normal values of ESR/CRP and many with inactive RA have abnormal values4. In the article by Crowson, et al, the normal/abnormal cutoff of ESR and CRP were not determined on paired samples in the same population, virtually guaranteeing non-agreement in normal/abnormal categories for the 2 tests.
Rather than distinguishing healthy persons from those with RA, ESR, and CRP are most often used as measures of RA activity. The usual clinical trial activity cutpoints for ESR are 28–30 mm/h and for CRP range from 1.0 to 2.0. However, there is no clear rational cutoff for activity (or for normality) of ESR/CRP in RA4,5. Many people with active RA have inactive values of ESR/CRP and many with inactive RA have active values.