Reply to Niv Ben-Shabat: Mortality in AS according to treatment.

We read with great interest the article by Niv Ben-Shabat et al. in Arthritis Care and Research titled "Mortality in ankylosing spondylitis according to treatment: a nationwide retrospective cohort study of 5900 patients from Israel" (1). The study concludes that ankylosing spondylitis (AS) is associated with reduced life expectancy, but not amongst the small subgroup of AS patients treated with TNF-inhibitors. We have concerns regarding the study design and analyses that underpin these findings.