Methods:~Experimental design: longitudinal study prospective in a single center.~The Ankylosing Spondylitis Assesment Study (ASAS) criteria are: (1) age <45 years, back pain ≥ 3 months, sacroiliitis on imaging and ≥ 1 associated minor criterion, or (2) HLA B27 positive and ≥ 2 minor criteria.~Minor criteria are: inflammatory back pain, arthritis, enthesitis, uveitis, Dactylitis, psoriasis, Crohn's disease, good clinical response to NSAIDs, family history of Human Leukocyte Antigen HLA B27 and inflammatory syndrome.~Plan Investigation:~Patients in the study will be seen first in consultation to gather the necessary information: history taking (including venous and arterial embolic events and obstetric history), the usual treatment, ASAS criteria of therapeutic AS, current and previous, comprehensive physical examination.~APLA will be measured at baseline and at least 3 months if initially positive.~OUTCOME:~The primary endpoint is the presence or absence of APLA: anti-cardiolipin antibodies (CLA) immunoglobulin M (IgM) or immunoglobulin G (IgG), circulating anticoagulant lupus (CCA), anti-beta2 glycoprotéine1 antibodies (β2GP1A) IgM or IgG.~The secondary endpoints are:~The presence of thrombosis (past or present) or obstetrical accident (anterior)~Relationship between the presence of APLA and a particular form of AS (axial or peripheral)~Relationship between the presence of APLA and anti-TNF alpha therapy~Calendar:~The expected study duration is 16 months.~Statistical Analysis Plan A descriptive analysis will be performed. The variables are described by their number and percentage; quantitative variables are described by their mean and standard deviation. A calculation of the prevalence of APLA will be made. Univariate analysis will be performed for the secondary objectives by the Chi 2 test or Fisher exact test according to the application conditions.
The main objective is to evaluate the prevalence of anti-phospholipid antibodies (APLA) in patients with ankylosing spondylitis (AS).~The secondary objectives are: (1) To determine whether the presence of these antibodies is symptomatic (thrombosis or not); (2) Identify a possible relationship between the presence of APLA and a particular form of SA (axial or peripheral); (3) To determine whether the presence of APLA is more frequent in patients receiving anti-tumor necrosis factor (TNF) alpha therapy.