The dramatic clinical response to TNFi could have resulted in inc

The dramatic clinical response to TNFi could have resulted in increased awareness and changing perceptions of physicians towards AS. Our data thus do not support the notion that MRI use has resulted in changing gender ratios in the diagnosis of AS. A sharp drop in incidence of AS in males above 65 years kinase inhibitor Lenalidomide was noted from 1995 to 2002, with no significant decrease in female AS incidence in the same age group (see online supplementary figure S1). In the early part of the study some prevalent cases could have been identified wrongly as incident

cases. However, a 36-month look-back period was included in the design to reduce this possibility. Earlier diagnosis of AS in males could have resulted in this shift in age group of incident cases but this was not reflected in an increase in the proportion of patients with AS diagnosed in the younger age groups (see online supplementary figures S1 and S2). Earlier diagnosis and longer survival could have resulted in the increase in prevalence of AS despite stable incidence rates. It is well known that diagnostic delays are higher in female patients with AS.30 A higher proportion of male patients in our study were diagnosed in the 15–45 year age group and this remained stable throughout the follow-up period (see online supplementary figure S2). The increase in female patients with AS seen from early 2000 onwards was reflected in an increasing proportion of female patients

with AS being diagnosed in the 45–65 year age group. Our study is not designed to answer whether this reflects later onset of disease or delay in diagnosis of female patients with AS.

Increased awareness of SpA in early 2000 could have led to the diagnosis of female patients with AS who were symptomatic for several years. This is a health database-based study, one limitation is that the data can only provide information on patients who had access to healthcare providers. We could not study the effect of HLA-B27 on the incidence and prevalence of AS due to the lack of availability of these data from the ICES databases. In addition, some patients with AS could have been misdiagnosed as chronic back pain and wrongly classified. The diagnosis of AS was not based on the modified New York criteria but on a diagnostic algorithm including physicians’ billing codes. Physicians might have used the same code to identify patients with other forms of SpA including nr-axSpA. Diagnostic algorithms utilising health administrative AV-951 data and the ICD-9 code-based definition have been18 25 31 validated.25 But the ICD-9 code 720 for AS has been validated only in the Veterans Affairs healthcare system in the USA.25 The use of one ICD-9 code of 720 has high sensitivity (91%) and specificity (99%) for identifying individuals with AS.25 Compared with the Quebec study that used this algorithm, we used a much stricter algorithm with two billing codes that has 100% specificity for a diagnosis of AS.

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