Objective This study was aimed at investigating the frequencies of non-radiographic axial spondyloarthritis (nr-axSpA) and ankylosing spondylitis (AS) diagnoses and their ratios in relation to symptom duration in patients referred because of chronic back pain and suspicion of axial SpA.

Methods In this monocentre study, orthopaedists and primary care physicians were requested to refer patients with chronic low back pain (duration >3 months) and onset of back pain before 45 years of age to a SpA-specialised rheumatology outpatient clinic for further diagnostic investigation, if proposed screening parameters were present. The ratio of nr-axSpA to AS was analysed in relation to the duration of symptoms.

Results A diagnosis of definite axial SpA was made in 43.7% of the referred patients (n=522). Axial SpA was diagnosed in a similar percentage of about 50% if back pain duration was <9 years but decreased to 36% if symptom duration was >9 years. Nr-axSpA represented the majority of patients (67.3%) only if duration of back pain was 1 year and less at the time of referral. Between 1 and 6 years of back pain duration the probability of nr-axSpA and AS was nearly equal (1–3 years: 52.5% and 47.5%, respectively; 3–6 years: 53.7% and 46.3%, respectively). In patients with back pain duration of 6–9 years, AS was more likely (61.1%) to be diagnosed than nr-axSpA (38.9%), and this increased further over time.

Conclusions Non-radiographic axial SpA represents an important differential diagnosis of back pain, especially in patients with recent symptom onset.