Background: Although chronic low back pain (≥3 months) before the age of 45 and inflammatory back pain (IBP) are regarded as early presenting and key features of axial spondyloarthritis (axSpA), and Magnetic Resonance Imaging (MRI) can be used to demonstrate sacroiliitis, the substantial delay in the diagnosis of axSpA has not improved.(1) Additionally, knowledge on the prevalence of chronic low back pain before the age of 45 and IBP in combination with the axSpA-related genetic risk factor Human Leukocyte Antigen-B27 (HLA-B27) in the general population is scarce. Objectives: To estimate the prevalence of chronic low back pain before the age of 45 and IBP in combination with the presence of HLA-B27 in a large Dutch population based cohort. Methods: Participants of the Lifelines cohort, a large population-based cohort of the northern region of the Netherlands, filled out a questionnaire on chronic low back pain and IBP. Chronic low back pain was defined as an affirmative answer to the question ‘Did you suffer from low back pain for ≥3 months?’. IBP was questioned based on the validated European Spondyloarthropathy Study Group (ESSG) IBP criteria and was confirmed if at least 4 out of the following 5 criteria were present: (a) onset before age 45, (b) insidious onset, (c) improvement with exercise, (d) associated with morning stiffness, (e) at least 3 months duration. Participants reporting to have been diagnosed with axSpA were identified using variations of the search terms “Bechterew”, ”spondyloarthritis” and “ankylosing spondylitis”. The Illumina global screening array (GSA) beadchip-24 v1.0 was used to genotype genome-wide SNPs in a subset of Lifelines participants. HLA-B haplotypes were imputed using neighboring SNPs with HIBAG, which is an R-package, using published parameter estimates.(2) The predicted HLA-B haplotype was considered valid if the posterior probability was >80%. Results: In total 94,277 Lifelines participants answered the chronic low back pain question, of which 93,665 (99.4%) completed ...