34 Chapter 2 indicator. The pressure on hospital capacity is an important aspect of surveillance, and is better reflected by contact and droplet precautions, as there is a direct relation between hospital capacity and the number of patients with contact and droplet precautions. In addition, in patients for whom a PCR test is considered too invasive, e.g. young children, contact and droplet precautions will still be in place. A disadvantage that all surveillance indicators in our study have in common is that they are not, by definition, the same as a SARI diagnosis. Testing for respiratory viral infections have been done in asymptomatic patients, and an ICD-10 diagnosis of pneumonia does not guarantee that the patient had a fever and cough. Although we lose some precision with the surveillance indicators we selected, there are large advantages when it comes to feasibility: data collection can be done automatically using routinely collected data, with no additional administrative burden for healthcare staff. Many (Dutch) hospitals have EMRs that allow for automated extraction of data. The main goal of SARI surveillance is to monitor trends rather than absolute numbers16. While having precise numbers, e.g. not using a proxy, but identifying patients who meet the SARI definition, could be advantageous for identifying risk groups or assessing vaccine effectiveness, this would add considerable complexity to the surveillance system as more manual administration would be needed. A possible explanation for the difference between the number of ICD-10 codes and the number of PCR tests and contact and droplet precaution labels is that the latter are done in patients with a suspected viral respiratory infection, while ICD-10 registrations are typically only used in confirmed infections. While positive PCR test results provide important additional information, solely using this indicator skews surveillance towards pathogens that are most commonly tested for, potentially missing out on emerging or less common respiratory pathogens. Moreover, contact and droplet precautions – and the decision to test a patient using PCR – are based on clinical presentation, which better reflects the definition of SARI and incorporates clinical judgement. Both PCR testing and contact and droplet precaution labels could contribute to the detection of newly emerging pathogens. A large increase of either of these surveillance indicators without an increase in positive test results could indicate a new pathogen or variant (assuming similar testing and registration behaviour). Our approach comes with several limitations. Firstly, as we do not have a gold standard for SARI in our dataset, it is not possible to indicate which surveillance indicator is most accurate. However, there is a strong correlation between the surveillance indicators, especially ICD-10 and contact and droplet precaution labels, and changes over time are still detectable. Comparing the proxies to the WHO case definition is an important future study objective, now that this proof-of-concept study demonstrated the potential use of these proxies for SARI surveillance. Secondly, data collection was performed retrospectively, while for a functioning, (near) real-time, surveillance system, data would be collected prospectively. For ICD-10 diagnostic
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