Relation Between Red Blood Cell Distribution Width and Inflammatory Biomarkers in Rheumatoid Arthritis

To the Editor.—We read with interest the article by Lippi and colleagues1 on the relation between red blood cell distribution width (RDW) and inflammatory biomarkers in a large cohort of unselected outpatients. In that article, the authors stated that there are strong, graded associations of RDW with high-sensitivity C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) in adult outpatients. Red blood cell distribution width is a parameter representing the heterogeneity of red blood cell size, which is calculated by the automated blood analyzer. Red blood cell distribution width has recently shown to be a risk factor for cardiovascular disease when increased and is related to mortality in the general population.2,3 C-reactive protein and ESR are nonspecific tests for acute phase inflammation and are widely used for monitoring disease activity in rheumatoid arthritis (RA).4 Rheumatoid arthritis, a common systemic rheumatic disease, and atherosclerosis are both chronic inflammatory diseases, and RA is also an independent cardiovascular risk factor.5We measured RDW, CRP, and ESR in 561 patients with RA and 131 patients with osteoarthritis (Table 1). The RDW was determined using an XE-2100 (Sysmex, Kobe, Japan), and the ESR was measured with Westergren method (StaRRsed III, Mechatronics Instruments, Hoorn, the Netherlands). C-reactive protein concentration was quantified using a nephelometer (BNII, Dade Behring/Siemens Healthcare Diagnostics, Marburg, Germany). Because CRP and ESR were not normally distributed, they were logarithmically transformed, and we used linear regression analysis. There were correlations between RDW and both CRP and ESR in patients with RA. However, in patients with osteoarthritis, RDW did not show any correlation with ESR but showed correlation with CRP (Table 2). Because most of the CRPs measured in patients with osteoarthritis patients were at values below detection limits (0.3 mg/dL), the results were not significant. Anemia affects the value of ESR; therefore, we divided RA cases into anemic and nonanemic groups. Correlations between RDW, CRP, and ESR were determined using correlation coefficients for 341 cases in the anemic group (men, hemoglobin <13 g/dL; women, hemoglobin <12 g/dL) and 220 cases in the nonanemic group. Regardless of the presence of anemia, RDW showed correlation with CRP but no correlation with ESR (Table 3).In the RA cases grouped according to RDW quartiles, there were graded associations of RDW with CRP and ESR, but when divided into anemic and nonanemic cases, there was graded association of RDW with CRP but not with ESR in both groups. Unlike the results measured by Lippi and colleagues1 in unselected outpatients, RDW is thought to be affected by anemia in patients with RA.The authors have no relevant financial interest in the products or companies described in this article.