Another population-based study from Quebec, Canada, presented data on the prevalence of AS between 1996 and 2006.18 The reported incidence sellectchem and prevalence rates were 11.5/100 000 person years and 140/100 000 population, respectively. The diagnosis was established on the basis of one physician claim based on the diagnostic code of AS (ICD-9 code: 720.0). The incidence and prevalence estimates of our study are higher than the Quebec study despite using more stringent criteria for diagnosis of AS. We defined AS based on two physician billing claims for the ICD-9 code 720 over 2 years, with at least one claim by

a rheumatologist or at least one hospitalisation record. Table 2 Comparison of various studies that assessed the incidence and prevalence of ankylosing spondylitis The population of Ontario is very diverse in terms of ethnicity and genetic background. Our extensive literature review reveals a clear need to conduct large population-based studies in Europe and Asia to obtain true estimates of the disease burden. Though there have been similar studies in the past, these attempts were based on cross-sectional studies or rates calculated

from hospital or clinic records and such estimates may be less accurate.13–17 19 There is growing concern about the financial and medical burden imposed on society by changing diagnostic and treatment paradigms of AxSpA. Of major concern is the potential overutilisation of MRI in the diagnosis of AxSpA and inappropriate use of TNFi for treating patients with mechanical back pain. There is immense interest in understanding the effect of these changing trends on the overall diagnosis and prevalence of AxSpA. It has been well understood for some time that all patients with AxSpA may not have classic X-ray changes of AS. Classification criteria for AxSpA that include the entity

non-radiographic axial SpA (nr-AxSpA) were established recently with the first TNFi trial in nr-AxSpA published in 2008.26 27 Classic sacroiliac joint changes seen in AS are not seen in patients with nr-AxSpA. It has AV-951 been argued that nr-AxSpA is a distinct entity and should not be considered early AS.28 Except for higher C-reactive protein (CRP) and slightly better responses to TNFi in AS, the burden of disease appears to be similar in AS and nr-AxSpA.29 A distinct difference between the two groups is the greater proportion of women in nr-AxSpA cohorts. We tracked the sex ratio of new patients diagnosed with AS over time to study the impact of these changes in overall prevalence and incidence trends of AS. Our study shows that the epidemiological trend with more females diagnosed with AS started between 2000 and 2005 when TNFi were introduced for the treatment of AS. Between 2005 and 2010, when we would have seen the greatest impact of MRI, there were steady trends in the sex ratio.