Validating the portal population of the United Kingdom Multiple Sclerosis Register

•Validation of a self declared MS Population with a clinically diagnosed one.•We analysed the UK MS Register for a number of key characteristics. The internet (n = 11,021) and clinical (n = 3,003) populations were studied for key shared epidemiology. We found them to be closely matched for mean age at diagnosis (clinical = 37.39, portal = 39.28) and gender ratio (female %, portal = 73.1, clinical = 75.2)•There is a representative population of people with MS in the online element of the UK MS Register – The MS Register can be utilised as a valid cohort for Clinical and PRO research•Kernel Density of the populations for age can be seen in the graphs: The UK Multiple Sclerosis Register (UKMSR) is a large cohort study designed to capture ‘real world’ information about living with multiple sclerosis (MS) in the UK from diverse sources. The primary source of data is directly from people with Multiple Sclerosis (pwMS) captured by longitudinal questionnaires via an internet portal. This population's diagnosis of MS is self-reported and therefore unverified. The second data source is clinical data which is captured from MS Specialist Treatment centres across the UK. This includes a clinically confirmed diagnosis of MS (by Macdonald criteria) for consented patients. A proportion of the internet population have also been consented at their hospital making comparisons possible. This dataset is called the ‘linked dataset’. The purpose of this paper is to examine the characteristics of the three datasets: the self-reported portal data, clinical data and linked data, in order to assess the validity of the self-reported portal data. The internet (n = 11,021) and clinical (n = 3,003) populations were studied for key shared characteristics. We found them to be closely matched for mean age at diagnosis (clinical = 37.39, portal = 39.28) and gender ratio (female %, portal = 73.1, clinical = 75.2). The Two Sample Kolmogorov-Smirnov test was for the continuous variables to examine is they were drawn from the same distribution. The null hypothesis was rejected only for age at diagnosis (D = 0.078, p