Background/Purpose: Experimental models suggested both erythrocytes and platelets activated immune cells and enhanced inflammation in inflammatory arthritis. Although previous cross-sectional studies showed the form and number of erythrocytes and platelets were associated with disease activity, the cross-sectional design cannot assess the causal relationship between erythrocytes/platelets and disease activity. Other limitations include no adjustment with confounding factors and small sample size. The objective of this multi-center longitudinal large cohort study is, therefore, to identify the effects of erythrocytes and platelets on disease activity in patients with RA.

Methods: Hemoglobin concentrations, mean corpuscular volume (MCV), red blood cell distribution width (RDW), platelet counts, and mean platelet volume (MPV) were measured as exposures. Disease activity was assessed using clinical disease activity index (CDAI) for primary outcome. Considering the estimation of effects of time-varying exposures and clustering effects by individual, linear mixed-effect models were used to examine the association between the indices of erythrocytes and platelets on the previous visit and disease activity on the next visit. The indices of erythrocytes and platelets, and disease activity were used as time-dependent variables while participant identification number and time from baseline were included as random factors. Age, sex, disease duration, RF, ACPA, prednisolone, and DMARDs were included as covariates. Sensitivity analyses were also conducted based on secondary outcomes of DAS28-CRP and simplified disease activity index (SDAI).

Results: A total of 63913 samples (median sampling interval: 66.2 days) from 3973 patients was included. The median age at baseline was 63.0 years with 78.3 % of women and mean disease activity were moderate (CDAI:10.9). Regarding erythrocytes, the next disease activity was significantly higher in patients with lower hemoglobin concentrations (-0.53/mg/dL, P < 0.001), MCV (-1.18/fL, P < 0.001), and RDW (-0.09/fL, P < 0.001) on the previous visit after adjusting by potential confounding factors. For platelets, higher platelet counts and lower MPV were associated with the next higher disease activity (0.20/μL, P < 0.001 and -0.55/fL, P < 0.001, respectively). These results were similar using DAS28-CRP and SDAI.

Conclusion: These results suggest the number and form of erythrocytes and platelets capture disease activity that remain unmeasured by established disease activity measures in RA. These may also support erythrocytes and platelets influence immune and inflammatory processes in patients with RA as well as the established opposite causal relationship.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-effects-of-erythrocytes-and-platelets-on-disease-activity-in-patients-with-ra-answer-longitudinal-cohort-study/