Completeness and generalizability of the Swedish MS register.

The Swedish Multiple Sclerosis (MS) Register is a nationwide clinical tool and research database. Its generalizability to the broader MS population has not been previously assessed. We aimed to quantify the register's completeness and potential disparities between those who were included and those who were not. Using linked national registers, we identified persons with MS through December 2020 as those with ≥ 3 MS diagnoses in the National Patient Register. We assessed the MS register's completeness and compared characteristics between included and non-included persons, using overlap weighting to account for age differences. Among 21,320 prevalent (2020) and 7,355 incident (2011-2020) MS cases, the completeness of the MS register was 85% and 87%, respectively, but varied by county (67-94% in 2020). Prevalent cases not included in the register were older (mean 62.4 vs. 52.3 years) and more likely to have no salaried income (55.3% vs. 30.5%); had fewer years of education (20.8% vs. 12.0% with ≤ 9 years); more often been hospitalized (41.5% vs. 32.2%); and collected more unique prescribed drugs (median 7 vs. 5 drugs per year). Incident cases were also older at first MS diagnosis (mean 50.3 vs. 39.2 years). All differences were attenuated by adjusting for age, although disparities remained. The Swedish MS Register demonstrates high completeness, but may not fully represent older persons with MS, those diagnosed later in life, and those of lower socioeconomic status. These limitations should be considered when interpreting findings based on the register's data.