Estimation of the prevalence of fragile X syndrome in England from the Clinical Practice Research Datalink

Aims Accurate prevalence estimates are vital for the provision of healthcare services. We calculated the diagnosed UK prevalence of fragile X syndrome (FXS) and estimated the undiagnosed FXS population.Methods A retrospective study was conducted in the Clinical Practice Research Datalink. Patients with FXS were selected by relevant medical code. Point prevalence (1 January 2019) was calculated. Assuming no underlying change in birth incidence of FXS over time, the maximum observed age–sex-specific prevalence rates were applied to UK population data to determine a theoretical FXS population. Age and sex-specific mortality rates were inflated by an FXS:non-FXS hazard ratio applied annually to the theoretical FXS population.Results 1520 prevalent patients with FXS were selected; a prevalence of 1.15 per 10 000 population. The estimated prevalent FXS UK population was 7682. Males had a higher prevalence than females (1.49 vs 0.82 per 10 000). The highest prevalence for both males and females was observed in those aged 16–20 (4.10 and 1.46 per 10 000, respectively). Applying these prevalence rates to the UK population resulted in an estimated 18 300 with FXS, reducing to 16 663 after mortality adjustment; an extrapolated prevalence rate of 2.76 per 10 000.Conclusion The prevalence of diagnosed FXS in males and females was similar to that reported previously; however, this is likely to be a significant underestimate due to lack of confirmed diagnosis in older age groups. Our estimates of the prevalence rates are substantially higher and align more closely with previous population-screening estimates as well as imputed estimates based on allele frequencies.