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Background:Both the Swedish National Patient Register (NPR) and the Swedish Quality Register for inflammatory bowel disease (IBD, SWIBREG) are important sources of research data and information. However, the validity of a diagnosis of IBD in these registers is unknown. Methods:Medical charts of 129 randomly selected patients from the NPR and 165 patients registered both in SWIBREG and the NPR were reviewed. Patients were classified according to standardized criteria for ulcerative colitis (UC), Crohn’s disease (CD), or IBD unclassified (IBD-U). Positive predictive values (PPVs) for UC, CD, IBD-U (only SWIBREG), or having any form of IBD were then calculated. Results:For cases with ≥2 diagnoses of IBD in the NPR (hospitalizations or non-primary care outpatient visits), the PPV was 93% (95% CI: 87–97) for any IBD, 79% (66–88) for UC and 72% (60–82) for CD. In UC patients with ≥2 UC diagnoses but never a CD diagnosis, the PPV increased to 90% (77–97). The PPV for CD in patients with ≥2 CD diagnoses but never a UC diagnosis was 81% (67–91)). Combining data from SWIBREG (≥1 record) and the NPR (≥1 record), the PPV was 99% for any IBD (97–100), 96% (89–99) for UC, and 90% (82–96) for CD. Conclusion:The validity of the UC, CD, and IBD diagnoses is high in the NPR but even higher when cases were identified both in SWIBREG and the NPR. These results underline the need for a well-functioning Swedish Quality Register for IBD as a complement to the NPR. [ABSTRACT FROM PUBLISHER]