26 Chapter 2 SARI surveillance. Using data between 2017 and 2023 from one hospital in the Netherlands, we compare RT-PCR, contact and droplet precaution labels, and ICD10 codes to assess SARI counts and incidence proportions and to evaluate their suitability as surveillance tool. Methods Study design and population We conducted a retrospective observational study at the Leiden University Medical Center (LUMC, Leiden, the Netherlands), a tertiary university hospital in one of the larger metropolitan areas of the Netherlands. Almost 21,000 patients are admitted to the LUMC every year. Patients of all ages hospitalised for at least 24 h between 1 January 2017 and 30 April 2023 were included. Patients who were hospitalised for less than 24 h at the LUMC but were transferred to another hospital were included in our study. A patient could be included multiple times, if more than one hospitalisation occurred within the study period, with the exception of readmissions within 10 days of the previous hospitalisation. For all included hospitalisations, we collected data on the presence of the three different surveillance indicators detailed below. The study period was divided into three timeframes. The first period (pre-COVID-19) consists of data from week 1 2017 to week 8 2020. The second period starts in week 9 2020, when the first COVID-19 case was reported in the Netherlands and includes data up to week 53 2020. As the registration policy for contact and droplet precaution labels was changed at the end of 2020 (see below), we included a third time period, in which these changes were fully implemented. The third period starts in week 1 2021 and ends at the end of our study period (week 18 2023). Surveillance indicators ICD-10 diagnostic codes ICD-10 diagnostic codes8 indicative of conditions seen in SARI patients were selected. These codes included: J00–J22 (upper and lower respiratory tract infections), U07.1 and U07.2 (COVID-19 infections). For children, J40 (bronchitis), J45.9 (asthma, unspecified) and J98.8 (other respiratory disorders) are frequently used for SARI in our hospital and were therefore included. In order to be included, ICD-10 codes had to be registered between hospital admission and 7 days after hospital discharge and be registered by the treating physician in the EMR. To avoid the inclusion of patients with chronic disease, mainly asthma, we only included ICD10 codes if the same ICD-10 code was not registered in the previous year.
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