Differences in the prevalence of ankylosing spondylitis in primary and secondary care: only one-third of patients are managed in rheumatology.

Objective: Knowing not only the prevalence of SpA, but also the proportion managed in rheumatology, has implications for health care planning. The aims of this study were to determine the prevalence of SpA and the proportion managed in rheumatology and to examine differences in group characteristics.

Methods: For the primary care population, we used the Primary Care Clinical Informatics Unit Research (PCCIUR) electronic primary care database, covering one-third of the Scottish population. Patients with AS, and various extraspinal manifestations were identified using Read Codes. For secondary care, we used the Scotland Registry for Ankylosing Spondylitis, which collected data on clinically diagnosed AS patients >15 years of age seen in rheumatology clinics between 2010 and 2013. Prevalence estimate denominators were computed using the adult PCCIUR and Scottish 2011 mid-year population estimates, respectively. Differences in the characteristics of both patient groups were examined using simple descriptive statistics.

Results: The prevalence of AS in primary care was 13.4/10 000 (95% CI 12.8, 14.0) and 4.7/10 000 in rheumatology (95% CI 4.5, 4.9). Rheumatology patients were younger overall and at diagnosis (mean age 51 vs 62 years and 35 vs 38, respectively; both P < 0.001) and more likely to have a history of uveitis (34 vs 22%), IBD (12 vs 6%) and psoriasis (14 vs 6%) (all P < 0.001).

Conclusions: This is the first study to estimate the prevalence of clinically diagnosed AS in primary and secondary care simultaneously, indicating that only one-third of patients are managed in rheumatology. This has important ramifications for health care planning and indicates a large 'silent' proportion of patients who may have serious pathology and would benefit from additional assessment in a specialist clinic.