Objective: To obtain information on the rate of concordance for ankylosing spondylitis (AS) in a population-based series of twins.
Methods: AS cases were identified by record linkage of the population-based Finnish Twin Cohort and the nationwide registry for fully reimbursed medications. A clinical examination was performed to establish concordance for AS.
Results: There were 6 monozygotic (MZ) pairs and 20 dizygotic (DZ) pairs with at least 1 member affected by AS. Three MZ pairs and 3 DZ pairs were concordant for the disease. All affected subjects were HLA-B27 positive. The pairwise concordance rate was 50% in MZ twins and 20% in HLA-B27 positive DZ twins (95% confidence intervals 11.8-88.2% and 4.3%-48.1%, respectively).
Conclusion: These results indicate that AS disease expression is largely, but not entirely, genetically based, with a gene or genes other than B27 probably playing a role.