Background: Recent research has verified that blood group or Rh factor can influence susceptibility to various cardiovascular, neoplastic and infectious diseases including COVID-19. While a number of studies have looked at correlations between blood group and various rheumatological diseases, findings have been inconsistent, often because many of these studies suffered from small sample size issues. In order to better understand the potential relationships between blood group/Rh factor and rheumatological diseases, we performed a large-scale self-report pilot study of blood type distributions in five autoimmune diseases.
Methods: Five autoimmune diseases were included in the study: systemic sclerosis, systemic lupus erythematosus, rheumatoid arthritis, psoriasis, and ankylosing spondylitis. We also included a control group in which participants did not have any autoimmune diseases. The participants were recruited through social media and organizations such as the Lupus Foundation and the National Psoriasis Foundation. Respondents who met the inclusion criteria were asked only two questions by anonymous survey: blood type and country of birth.
Results: Each autoimmune disorder group included between 570 and 951 US participants. While there was little difference in blood type distribution patterns among the five diseases, unexpectedly, all five disease groups showed a consistent pattern where Rh negative was almost twice as high as US population norms. A post-hoc non-autoimmune control group was added in order to determine if this anomalous finding was an artifact of the study design. The control group displayed a similar unexpected increase in the Rh-negative blood type prevalence, suggesting that the very high Rh-negative frequency among the tested disease groups was likely to be an artifact of the study design.
Conclusions: Overall, our preliminary study results show no meaningful differences between the disease groups and the post-hoc control group, suggesting that neither ABO type nor Rh factor affects susceptibility to the development of any of the five studied autoimmune diseases. Nevertheless, the unexpected observed difference in Rh factor distribution between the studied groups/control group and the corresponding US population norms has important implications for any research study using self-selected subjects. Our results suggest that such studies may be subject to unanticipated biases, requiring meticulous controls to confirm impartiality and exclude any artifacts of the study design.