152 Chapter 6 at around 40%, of antimicrobial overprescribing for RTIs (18, 19). This confirmed our assumption that the large number of antimicrobial prescriptions included in our combined dataset had diluted any potential registration bias and allows us to interpret our findings accordingly. Moreover, an additional multivariable regression analysis (Supplement 5) including patients without determinants in the SN data showed similar outcomes, from which we concluded that there is a low risk of bias due to missing SN data. These two specific registries (ELAN/SN) have been successfully combined in earlier studies, focussing on cardiovascular risk (11, 20), but this is the first time that the approach has been used for research into AMR. Those earlier studies had methodological issues similar to our study, but nevertheless produced reliable and valid data. Studies of patterns of antimicrobial prescription have been previously conducted using large healthcare registries, but without including socioeconomic data (21, 22). Antimicrobial prescribing The number of antimicrobial prescriptions per year was relatively stable except for the year 2020. This significant drop in antimicrobial prescriptions was largely due to the COVID-19 pandemic, which resulted in relatively fewer bacterial and viral infections and allowed physicians to test their patients before treating them with antimicrobial medication for any presumed bacterial infection (23). With fewer other RTIs registered, there was a corresponding decrease in GP visits and consequently less prescribing of antimicrobials (24). A report on the total prescription of antimicrobials in The Netherlands showed a comparable decline in antimicrobial prescribing in 2020 (4). RTIs and UTIs were the most common reasons with similar prescription rates for an antimicrobial prescription in our study. Cross-sectional/longitudinal observational studies performed in the United Kingdom (UK) also reported RTI and UTI as the most common reason (21, 25), only with relatively fewer prescriptions for UTIs compared to RTIs. Our study showed relatively more antimicrobial prescription for an UTI. Other studies in this domain differ in details that might explain for differences in the results reported. The study by Pouwels et al. only included patients with an UTI who were older than 14 years (21), while UTI’s at a young age are quite common. The study by Dolk et al. also included ear nose throat infections as a RTI (25). In both absolute and relative numbers, RTIs in our study accounted for the vast majority of all antimicrobial overprescribing (81.5%) and within prescriptions for RTIs (39.6%).
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