Introduction: Whether data on International Classification of Diseases (ICD)-codes from the Swedish National Patient Register (NPR) correctly correspond to subtypes of inflammatory bowel disease (IBD) and phenotypes of the Montreal classification scheme among patients with prevalent disease is unknown.

Materials and methods: We obtained information on IBD subtypes and phenotypes from the medical records of 1403 patients with known IBD who underwent biological treatment at ten Swedish hospitals and retrieved information on their IBD-associated diagnostic codes from the NPR. We used previously described algorithms to define IBD subtypes and phenotypes. Finally, we compared these register-generated subtypes and phenotypes with the corresponding information from the medical records and calculated positive predictive values (PPV) with 95% confidence intervals.

Results: Among patients with clinically confirmed disease and diagnostic listings of IBD in the NPR (N = 1401), the PPV was 97 (96-99)% for Crohn's disease, 98 (97-100)% for ulcerative colitis, and 8 (4-11)% for IBD-unclassified. The overall accuracy for age at diagnosis was 95% (when defined as A1, A2, or A3). Examining the validity of codes representing disease phenotype, the PPV was 36 (32-40)% for colonic Crohn's disease (L2), 61 (56-65)% for non-stricturing/non-penetrating Crohn's disease behaviour (B1) and 83 (78-87)% for perianal disease. Correspondingly, the PPV was 80 (71-89)% for proctitis (E1)/left-sided colitis (E2) in ulcerative colitis.

Conclusions: Among people with known IBD, the NPR is a reliable source of data to classify most subtypes of prevalent IBD, even though misclassification commonly occurred in Crohn's disease location and behaviour and also among IBD-unclassified patients.

BACKGROUND AND AIMS: Inflammatory bowel disease (IBD) has been associated with spondyloarthritis (SpA), but population-based estimates are scarce. Here we compare the occurrence of SpA before and after a diagnosis of IBD to the general population, overall and by IBD subtype and age.

METHODS: We used a nationwide register-based cohort study of 39,203 patients diagnosed with IBD during 2006-2016, identified from Swedish registers and gastrointestinal biopsy data, and 390,490 matched reference individuals from the general population. Conditional logistic regression models were used to estimate odds ratios (ORs) for a prior (prevalent) SpA diagnosis and conditional Cox regression to calculate hazard ratios (HRs) for a subsequent (incident) SpA diagnosis in IBD patients.

RESULTS: IBD patients were more likely to have prevalent SpA at IBD diagnosis (2.5%) compared to reference individuals (0.7%) with an OR of 3.48 (95%CI:3.23-3.75). They also more often received an incident diagnosis of SpA; during 23,341,934 person-years of follow-up in IBD patients, there were 1,030 SpA events (5.0/1,000 person-years) compared to 1,524 SpA events in the reference group (0.72/1,000 person-years), corresponding to an HR of 7.15 (95%CI:6.60-7.75). In subgroup analyses, associations were most pronounced among patients with Crohn's disease [(OR=5.20; 95%CI:4.59-5.89), and (HR=10.55; 95%CI:9.16-12.15)] and paediatric onset IBD [(OR=3.63; 95%CI:2.35-5.59) and (HR=15.03; 95%CI:11.01-20.53)].

CONCLUSION: IBD patients more frequently experience SpA both before and after the diagnosis of IBD compared to the general population, supporting evidence of a shared pathophysiology. The variation in SpA comorbidity across IBD subtypes and age-groups, calls for targeted approaches to facilitate timely diagnosis and intervention.

BACKGROUND AND AIMS: Register-based research suggests a shared pathophysiology between inflammatory bowel disease [IBD] and spondyloarthritis [SpA], but the role of familial [genetic and environmental] factors in this shared susceptibility is largely unknown. We compared the risk of SpA in first-degree relatives [FDRs] and spouses of IBD patients with FDRs and spouses of matched population-based reference individuals.

METHODS: We identified 147,080 FDRs and 25,945 spouses of patients with incident IBD [N=39,203] during 2006-2016 and 1,453,429 FDRs and 258,098 spouses of matched reference individuals [N=390,490], by linking nationwide Swedish registers and gastrointestinal biopsy data. Study participants were followed 1987-2017. Cox regression was used to estimate hazard ratios [HRs] of SpA.

RESULTS: During follow-up, 2,430 FDRs of IBD patients [6.5/10,000 person-years] and 17,761 FDRs of reference individuals [4.8/10,000 person-years] were diagnosed with SpA, corresponding to an HR of 1.35 [95%CI:1.29,1.41]. In subgroup analyses, the increased risk of SpA was most pronounced in FDRs of Crohn's disease patients [HR=1.44; 95%CI:1.34,1.56] and of IBD patients aged <18 years at diagnosis [HR=1.46; 95%CI: 1.27,1.68]. IBD patient's spouses also had a higher SpA rate than reference individuals' spouses, but the difference was less pronounced [4.3 vs. 3.5/10,000 person-years; HR=1.22; 95%CI:1.09,1.37]. No subgroup-specific risk pattern was identified among spouses.

CONCLUSIONS: The observed shared familial risks between IBD and SpA support shared genetic factors in their pathogenesis. However, spouses of IBD patients were also at increased risk for SpA, reflecting the influence of environmental exposures or similarities in health-seeking patterns.