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To date, there have been no studies systematically comparing red blood cell distribution width (RDW) among rheumatoid arthritis (RA), ankylosing spondylitis (AS) and osteoarthritis (OA). Therefore, this study aimed to make comparisons and to explore whether erythrocytopenia and hemoglobin (Hb) reduction could influence RDW level and its association with conventional inflammatory or immune markers in RA, AS and OA.A total of 222 patients with RA, 150 with AS, 78 with OA and 126 healthy controls (HC) were enrolled. Clinical and laboratory data of all subjects were extracted from electronically stored medical records.Increased RDW level was found only in RA patients and showed significant diagnostic value for RA. It was much higher in those with erythrocytopenia and Hb reduction. However, those without Hb reduction did not show significant difference of RDW from HC. RDW positively correlated with CRP and ESR respectively in RA and OA patients. However, when the patients were divided into Hb reduction and non-Hb reduction groups, the correlations became insignificant.RDW level is increased only in RA patients, but not in those with AS and OA. However, increased RDW and its association with CRP may be mainly due to Hb reduction. Therefore, whether RDW could be used as useful inflammatory index for RA, AS and OA remains to be evaluated.