• Caterina Giannitto, Federica Mrakic Sposta, Alessandro Repici, Giulia Vatteroni, Elena Casiraghi, Erminia Casari, Giorgio Maria Ferraroli, Alessandro Fugazza, Maria Teresa Sandri, Arturo Chiti, and Balzarini Luca.
• Radiology Division, Humanitas Clinical and Research Center, IRCCS, Via Alessandro Manzoni, 56, 20089, Rozzano, Milan, Italy. caterina.giannitto@gmail.com.
• Radiol Med. 2020 Dec 1; 125 (12): 1260-1270.

ObjectivesWe aimed to assess the diagnostic performance of CT in patients with a negative first RT-PCR testing and to identify typical features of COVID-19 pneumonia that can guide diagnosis in this case.MethodsPatients suspected of COVID-19 with a negative first RT-PCR testing were retrospectively revalued after undergoing CT. CT was reviewed by two radiologists and classified as suspected COVID-19 pneumonia, non-COVID-19 pneumonia or negative. The performance of both first RT-PCR result and CT was evaluated by using sensitivity (SE), specificity (SP), positive predictive value (PPV), negative predictive value (NPV) and area under the curve (AUC) and by using the second RT-PCR test as the reference standard. CT findings for confirmed COVID-19 positive or negative were compared by using the Pearson chi-squared test (P values < 0.05) RESULTS: Totally, 337 patients suspected of COVID-19 underwent CT and nasopharyngeal swabs in March 2020. Eighty-seven out of 337 patients had a negative first RT-PCR result; of these, 68 repeated RT-PCR testing and were included in the study. The first RT-PCR test showed SE 0, SP = 100%, PPV = NaN, NPV = 70%, AUC = 50%, and CT showed SE = 70% SP = 79%, PPV = 86%, NPV = 76%, AUC = 75%. The most relevant CT variables were ground glass opacity more than 50% and peripheral and/or perihilar distribution.DiscussionNegative RT-PCR test but positive CT features should be highly suggestive of COVID-19 in a cluster or community transmission scenarios, and the second RT-PCR test should be promptly requested to confirm the final diagnosis.

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