To quantify the sibling risk of RA, SLE and AS. To analyse the concordant and discordant associations between RA, SLE and AS.
Follow-up study of all individuals and their siblings born in or after 1932 and hospitalized for RA, SLE or AS between 1973 and 2004 (32 yrs). Data were retrieved from a comprehensive dataconstructed by using several national Swedish data registers, including the Total Population Register, the Swedish Hospital Discharge Register and the Multigeneration Register. Standardized incidence ratios (SIRs) were used to estimate sibling risks.
For males, the overall significant SIRs were 4.72, 4.35 and 4.14 for RA, SLE and AS, respectively, if a sibling was affected by any inflammatory disease. The corresponding significant SIRs for females were 4.12, 3.73 and 4.73. The concordant significant SIRs in siblings were 5.12, 17.02 and 17.14 for RA, SLE and AS, respectively. There were also discordant associations between RA and SLE, whereas AS was only associated with AS.
This study was able objectively to quantify the sibling risk of RA, SLE and AS, which represents useful knowledge for clinicians and geneticists. The analysis of concordant and discordant associations may be useful in future studies aimed at finding specific genes associated with these diseases.