Background Previous studies have shown the phenomenon of clustering of autoimmune diseases (AD) in families of patients with Systemic Lupus Erythematosus (SLE).
Objectives The purpose of this study was to evaluate the incidence, prevalence and risk to have an AD in first degree relatives (FDR) of patients with SLE and to investigate if a history of AD in FDR is a risk factor for SLE.
Methods A multicentre case-control study was carried out in Italy. Cases and controls were randomly selected and interviewed using a detailed questionnaire. We conducted stratified analyses according to sex of relatives and/or proband to evaluate whether the relation between SLE in FDR and AD differs by sex. We used the extended generalised estimating equations (EGEE) to measure familial aggregation. The strenght of the association between family history of AD and SLE was measured as an odds ratio (OR). The adjusted OR was obtained by unconditional logistic regression model and the potentially confounding covariates were family size, age and sex of proband.
Results The prevalence of AD among SLE and control FDR was respectively 6.1% and 1.5% (p < 0.01). The incidence rate of AD and control FDR was 1.35 × 1000 py and 0.32 × 1000 py respectively. The relatives of SLE cases have 4 time probability of having AD compare to relatives of control (RR 4.26; 95% CI 2.25- 8.03). Using EGEE the adjusted OR taking into account the non-independence of family members was 4.77 (95 CI 2.07–11). A risk of 8.57 (95% CI 3.18–23.1) was detected if the proband is female and 1.40 (95% CI 0.22–8,78) if proband is male. The risk of AD is significantly greater for female with a female FDR with SLE (OR 10.36; 95% CI 2.90- 36.9). No AD was found among SLE spouses, while one case was found among spouses of controls.
Conclusion This case-control study defines the risk to have an autoimmune disorder for SLE first degree relatives. A family history for AD represents a risk factor for SLE.
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