33 Surveillance of Severe Acute Respiratory Infections 2 in which case the test was not repeated. Moreover, using age-stratified analysis provides important additional information otherwise missed by the aggregated analysis, as was seen by RSV peak in young children in summer 2021. Using a proxy for SARI surveillance, most commonly ICD-10 codes, is not new5-7. Germany has established a SARI surveillance system using ICD-10 diagnostic codes (J09–J22)5. Portugal6 and Scotland7 also used ICD-10 codes but include additional diagnostic codes to the list Germany used. Portugal for example, also included diagnostic codes for cardiovascular diseases (I20–I25, acute myocardial infarction; I50 and I51, heart failure), ILI symptoms (R05, cough; R51, headache; M79.1, myalgia; among others) and respiratory diagnosis or infection (I40.9, myocarditis; A49.9, bacterial infection; J45, asthma; and more). Scotland also included U07.1, U07.2 (COVID-19) and J04 (acute laryngitis and tracheitis). However, the use of the number of RT-PCR tests and contact and droplet precaution labels for surveillance is new and has specific advantages and disadvantages. Firstly, RT-PCR tests for respiratory pathogens are widely used, especially in adults, and are likely to reflect that a patient presented with symptoms of a viral respiratory infection. A potential disadvantage of this indicator is that the number of RT-PCR tests is likely to be influenced by changing protocols. During the COVID-19 pandemic, the threshold for testing was low, and patients were frequently tested for the presence of respiratory viruses if they presented with fever, but no respiratory symptoms. In contrast, the low numbers year-round in the pre-pandemic years suggest that RTPCR tests were only performed in specific patient groups. As protocols and public awareness could change again in the future, this can significantly influence the number of RT-PCR tests that are performed. Moreover, RT-PCR tests may not be performed if patients already tested at home, in another hospital or at their general practitioner. The large number of patients who only had a RT-PCR test, but no contact and droplet precaution labels or ICD-10 registration probably reflects a group of patients that had a negative RT-PCR test in the emergency department, where contact and droplet precautions were in place while waiting for the test result but not registered in the EMR. Secondly, like RT-PCR tests, contact and droplet precautions are taken based on clinical presentation of a (viral) respiratory infection. The adherence to these precautions is likely to have improved since the start of the pandemic. A downside of using labels as a surveillance indicator is that they are also indicated in infections unrelated to SARI. These are mumps, pertussis, diphtheria, encephalitis, epiglottitis, parvovirus B19, meningitis, acute subglottic laryngitis, rubella and scarlet fever. However, compared with the numbers of patients with SARI, these numbers are extremely low. A second limitation of contact and droplet precaution labels is that registration may not always be correct in EMRs. This is less likely to happen with PCR testing as the request for testing is done within the EMR. However, this lack of registration is likely to be randomly distributed over time. There are several potential advantages of contact and droplet precaution labels over PCR as a surveillance
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