28 Chapter 2 Data collection CTcue (IQVIA) is a clinical data collection tool that can be used to identify patients and extract data from their EMRs. We collected (structured) data for the following variables: age, date of hospital admission and date of hospital discharge, ICU admission during hospitalisation and information on our three surveillance indicators (ICD-10, RT-PCR and contact and droplet precautions), as described above. The clinical data collection tool was previously validated using Dutch EMR data and showed high accuracy11,12. In order to validate the accuracy of our data collection, we selected 2 random weeks for each surveillance indicator, and checked the results with regular quality control data in our hospital. This was done to ensure that the data collection tool did not miss any admissions or relevant variables. Statistical analyses For each surveillance indicator, we plotted the absolute count per week during the study period and visually compared trends. Next, we plotted the incidence as a proportion of the total number of hospitalised patients for a specific week (incidence proportion). For the number of PCR tests, for example, we plotted the number of unique patients who were tested for at least one respiratory virus using an RT-PCR test, as the proportion of all newly hospitalised patients, for each week. A subanalysis including only patients that were admitted to the intensive care unit (ICU) at any point during their hospital admission was performed. In a second subanalysis, the results for RT-PCR tests and contact and droplet precautions were split by age group. We estimated the Pearson correlation coefficient between the different surveillance indicators over several time periods. Finally, we plotted the number of positive RTPCR tests for each week in the study period. R software (version 4.3.1, R Foundation) was used to analyse the data and create the graphs. Results A total of 417,119 hospitalisations were registered at Leiden University Medical Center between 1 January 2017 and 30 April 2023. Of these, 299,715 re-admissions and admissions with a duration of less than 24 h were excluded. A total of 117,404 admissions were included in our analysis. The flowchart of inclusion and exclusion can be found in Supplementary Figure 1. Information on data validation can be found in the Supplement. In our study period, 11,959 RT-PCR tests for respiratory viruses were registered, 4,683 contact and droplet precautions were registered, and 3,908 ICD-10 diagnostic codes of interest were registered. The overlap between the presence of the different surveillance indicators in the three different time periods can be seen in Supplementary Figure 2, 3 and 4. There were no missing data for any of the collected variables in our analysis.
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