Consistency between the National Patient Register and the Swedish Cancer Register

Purpose The Swedish National Patient Register (NPR) is widely used as a data source in epidemiological studies, but the consistency of all cancer diagnoses compared to the Swedish Cancer Register (SCR) remains unclear. Using NPR supplementary for detecting safety signals is beneficial due to shorter data extraction delays compared to using SCR alone. This study aims to evaluate the consistency of NPR for cancer diagnoses compared to SCR and its potential use in pharmacoepidemiology. Methods Patients with a cancer diagnosis recorded in SCR during 2018–2020 were included. To measure the consistency of NPR diagnoses with SCR as the gold standard, positive predictive value (PPV), and sensitivity were calculated. As an empirical example showing differences in identification of cancer diagnoses in NPR and SCR, two nested case–control studies for the association between antidiabetic medications and pancreatic cancer were repeated using the two registers. Conditional logistic regression was performed and the 95% confidence intervals (CIs) for the odds ratios (ORs) were checked for overlaps. Results For breast, male genital organs, and oral cancers consistency was high (PPV: 87.5%–97.4%, sensitivity: 82.2%–91.0%), while for female genital organs, thyroid, and ill‐defined, secondary, and unspecified sites cancers it was low (PPV: 8.8%–90.0%, sensitivity: 19.9%–32.3%). All the CIs for the ORs from the nested case–control studies overlapped when pancreatic cancer was identified in NPR or SCR. Conclusion Consistency of cancer diagnoses in NPR when compared to SCR depends on cancer type with higher consistency for some cancers and lower for others. Differences in diagnostic processes for different cancer types and coding of cancer in the two registers may explain part of the inconsistent results.