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We read with great interest the study by Goodacre et al. , where post-exertion oxygen saturation adds modest prognostic information to clinical assessment of suspected COVID-19 in the ED.
This is partially in contrast with the findings of our study , performed within the Fenice Network (Italian group for clinical research in Emergency Medicine). In consecutive ED patients with SpO2 ≥ 95%, the most promising version of a standardised quick walk test (QWT) yielded a sensitivity of 83.3% (95% confidence interval [CI] = 35.9% to 99.6%), and specificity of 93.4% (95% CI = 91.5% to 95.0%). The positive and negative predictive values (PPVs and NPVs) were 8.6% (95% CI = 2.9% to 19.0%) and 99.9% (95% CI = 99.3% to 100.0%).
This difference is probably due to 3 factors:
1. The test standardisation. In our case series, the test was standardised for all centres and consists of a 30-40 metres walk at the maximun possible speed for each patient. Conversely, in the study by Goodacre et al., exertion SpO2 was either recorded after different, not standardised, intentional tests or could have made opportunistically after a spontaneous patient’s effort. The latter condition is particularly worrisome for the purpose of a thorough assessment of the prognostic value of the post-exertion oxygen saturation.
2. The outcome definition. Goodacre et al. considered patients who died or required respiratory, cardiovascular or renal support within 30 days after initial p...
2. The outcome definition. Goodacre et al. considered patients who died or required respiratory, cardiovascular or renal support within 30 days after initial presentation as having an adverse outcome. We think that such a definition has two problems. On the one hand, the post-exertion oxygen saturation depends just on pulmonary dysfunction, which is not necessarily related to the dysfunction of other organs or to death. Using a combined outcome may introduce a bias. On the other hand, waiting as many as 30 days to assess the occurrence of the outcome may lead to consider events that are not related to the condition of the lung at the time of initial presentation. For these reasons, we considered the need for invasive mechanical ventilation within 15 days from the initial presentation as the outcome to be predicted by the QWT.
3. The population studied. Goodacre et al. studied a more heterogeneous and probably sicker than ours, since we enrolled only patients with a rest SpO2 ≥ 95% on room air. This is indeed an important point, as the main target of the QWT is to identify patients with a minor pulmonary dysfunction that yields a normal saturation at rest but a decompensation after mild exertion. Of note, in the study of Goodacre et al., about 50% of the patients who experienced the adverse outcome had a baseline saturation <95% and would have been excluded from our study. Indeed, when the authors performed a secondary analysis limited to more appropriate cases, improved sensitivity, specificity and discriminant value were found.
We believe that, albeit further prospective studies are essential to confirm our findings , a standardised QWT is promising, because it can be performed rapidly, without specialized equipment and by nonmedical staff, and may have the potential to reliably identify patients who can be safely discharged home or hospitalized in low‐intensive regimens, after the ED visit.
1. Goodacre, S., Thomas, B., Lee, E., Sutton, L., Loban, A., Waterhouse, S., Simmonds, R., Biggs, K., Marincowitz, C., Schutter, J., Connelly, S., Sheldon, E., Hall, J., Young, E., Bentley, A., Challen, K., Fitzsimmons, C., Harris, T., Lecky, F., Lee, A., Maconochie, I., & Walter, D. (2020). Post-exertion oxygen saturation as a prognostic factor for adverse outcome in patients attending the emergency department with suspected COVID-19: a substudy of the PRIEST observational cohort study. Emergency Medicine Journal, Dec 3: emermed-2020-210528. doi: 10.1136/emermed-2020-210528.
2. Paglia, S., Nattino, G., Occhipinti, F., Sala, L., Targetti, E., Cortellaro, F., Cosentini, R., Costantino, G., Fichtner, F., Mancarella, M., Marinaro, C., Sorlini, C., Bertolini, G., & Fenice Network (Italian group for clinical research in Emergency Medicine) (2020). The Quick Walk Test: A Noninvasive Test to Assess the Risk of Mechanical Ventilation During COVID-19 Outbreaks. Academic Emergency Medicine, Advance online publication: 10.1111/acem.14180. doi: https://doi.org/10.1111/acem.14180
3. Kalin, A., Javid, B., Knight, M., Inada-Kim, M., & Greenhalgh, T. What is the Efficacy and Safety of Rapid Exercise Tests for Exertional Desaturation in Covid-19: A Rapid Systematic Review, 17 November 2020, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-105883/v1].