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Background Infection is the leading cause of death in RA. Increasing age and use of corticosteroids (CS) are known risk factors (RF) for serious infection (SI). To what extent defects in the innate immune system (IIS) contribute to this risk is unknown. MBL, a component of the IIS is a liver protein with diverse functions including opsonisation and complement-mediated killing of various micro-organisms. Objectives The aim of this study was to determine whether MBL deficiency is a RF for multiple SIs in RA. Methods From 01/01/2007 to 30/01/2013, 207 RA patients derived from the clinics of a single Rheumatologist (private consulting rooms, rural and city hospital clinics) were evaluated to identify RFs for SI (infections requiring admission to hospital or IV antibiotics). MBL concentrations were mostly determined at the time of commencement of synthetic or biologic DMARD therapy. Results Patients with an MBL equal to or less than 56 ng/mL were found to be 4.2 times more likely to have at least one SI (OR =4.2, 95% CI 1.10 – 16.20, p = 0.036). Higher rates of SI were observed in recipients of biologic DMARDs (26.4%) compared to synthetic DMARDs (11.8%). Among recipients of biologic agents, SI rates (at least 1) were 3.7, 5.4 and 11.5 per 100 patient-years of treatment for those with normal MBL concentrations (>1300 ng/ml), partial deficiency (<1300 but >56 ng / ml) and severe deficiency (< 56 ng / ml) respectively. Multiple SIs were observed in 6 patients, 5 of whom had MBL concentrations <56 ng / mL and all of whom had MBL concentrations <400 ng / mL (OR = 51.8, 95% CI 5.6 – 474.9, P=0.0005). Conclusions MBL deficiency is a RF for SI in RA and especially multiple SIs. The risk for at least one SI is comparable to that of maintenance therapy with CS (dose = 5 mg or greater each day). Risk profiling, the development of serious infection risk calculators and the use of customised Action Plans for suspected infection may help to minimise the morbidity and mortality associated with sepsis in RA patients. Disclosure of Interest None Declared

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