30 Chapter 2 When showing the incidence proportion, a similar pattern can be seen (figure 2). The RT-PCR test incidence proportion noticeably diverges from the other two surveillance indicators after the first COVID-19 cases were reported in the Netherlands in week 9 202013. During this study period, 11,959 PCR tests were done, and 4,683 contact and droplet isolation labels and 3,908 ICD-10 codes were registered. The total number of registrations for the three surveillance indicators combined is 20,550 for 117,404 hospital admissions. Note that the y-axis ends at 0.3 instead of 1.0 to enhance visibility of differences between the different surveillance indicators. Virological test results Figure 3 shows the positive virological test results per week. Since the onset of the COVID-19 pandemic in 2020, there was an increase in the number of overall RTPCR tests performed, accompanied by a lower proportion of positive test results. The number of RT-PCR tests that had an unknown test result, e.g. because the test was lost or the analysis was stopped, was stable over time, with roughly one unknown test result each week (data not shown). We therefore only show the positive test results over time, as a proportion of the total number of tests done. 2023 2021 2022 2019 2020 2017 2018 0 5 101520253035404550 0 5 101520253035404550 0.0 0.1 0.2 0.3 0.0 0.1 0.2 0.3 0.0 0.1 0.2 0.3 0.0 0.1 0.2 0.3 Weeknumber Incidence proportion Contact and droplet precautions ICD−10 RT−PCR Figure 2. Weekly incidence proportion for the three surveillance indicators over time, Leiden University Medical Center, Leiden, the Netherlands, 1 January 2017–30 April 2023 (n = 117,404 hospital admissions).
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