ABO and Rh blood group distribution in seropositive rheumatoid arthritis

The latest reports on ABO and Rh blood group distribution in patients with rheumatoid arthritis (RA) date from the 1960s. In a Japanese study on 70 subjects with RA, an increased frequency of AB and a decreased frequency of O blood groups were found. In a Norwegian study on 217 patients, no significant difference was seen in the frequency of ABO and Rh blood groups, compared to the normal population. This was also true for a study from Scotland made on 743 patients with definite or classical RA. After those studies were made, the American Rheumatism Association (ARA) 1987 criteria were published, rendering the classification of RA more precise. It has also been proposed that epidemiological RA surveys should be made in a homogeneous subgroup, consisting of patients who test positive for either rheumatoid factor or typical radiological erosions, in order to avoid dilution of RA groups by other inflammatory joint diseases. The early studies seem to be made on heterogeneous groups of patients with RA, which may have twisted the results of the blood group distribution. If blood group distribution were to differ from that of the normal population, it might promote our understanding of RA. This is why we studied the frequency of ABO and Rh blood groups in patients with RA.