In the late 2019 a new Coronavirus was identified as the cause of a group of atypical interstitial pneumonia cases in Wuhan, a city in the Chinese province of Hubei. In February 2020, the World Health Organization designated COVID-19 disease, which stands for Coronavirus 2019 disease. Following the progressive spread of the infection in other countries of the world, WHO declared the Pandemic on 11 March 2020. Italy was the first European country involved in the spread of the infection and among those with the highest number of victims. The Coronavirus responsible for COVID-19 has, as its main target organ, the respiratory system, being able to determine a serious acute respiratory syndrome similar to that of the cases found during the SARS epidemic of 2003: hence the name of the virus as SARS-CoV-2. Multisystem involvement and hyperinflammatory organ and systemic response are responsible for the patient's death. Overall, hospital mortality from COVID-19 is approximately 15% to 20%, but up to 40% among patients requiring ICU admission. The diagnosis of SARS-COV-2 infection is made by direct detection of viral RNA on different biological materials from patients with suspicious symptoms, and the first level diagnostic test is generally the nasopharyngeal swab; molecular investigations can also be carried out on samples from the distal respiratory tract (BronchoAlveolar Lavage, BAL). However, even if the specificity of the nasopharyngeal swab is high, its sensitivity can be affected by technical causes (sampling mode), as well as by intrinsic factors related to the method. The sensitivity data available are around 40-70%. The result is the possibility of a consistent series of false negatives (at least one third), represented by patients which have clinical characteristics compatible with SARS-COV-2 infection, but resulted, however, negative to the nasopharyngeal swab. These cases are framed as COVID-19-like cases and constitute a serious problem for the risk of not to recognize hospitalized patients suffering from COVID-19 infection.

The purpose of the study is to identify the clinical, laboratory and imaging characteristic which are similar or which can differentiate the hospitalized patients affected by COVID-19 pneumonia (with positive PCR on naso-pharyngeal swab) and patients with pneumonia with negative PCR for COVID-19. To do this, the investigators will compare the clinical, laboratory and imaging characteristics between interstitial pneumonia secondary to SARS-COV-2 infection, confirmed by molecular biology investigations (viral RNA research by PCR on nasopharyngeal swab) and cases of interstitial pneumonia negative to the nasopharyngeal swab. The cases studied will be represented by patients hospitalized for interstitial pneumonia in the participating hospital centers in the period between mid-March 2020 and end of May 2020. For these cases, all available data contained in the medical records of patients, enrolled within the time frame indicated above, will be retrospectively analyzed. Due to relative lack of sensitivity of COVID-19 PCR assay in nasopharyngeal swab, patients affected with COVID-19 infection can be mixed with other hospitalized patients and with unprotected health staff. On the basis of the comparison of the clinical, laboratory and imaging data the study would identify the characteristics which can differentiate patients with SARS-COV-2 pneumonia, confirmed by nasopharyngeal swab, and interstitial pneumonia and negative COVID-19 PCR on swab. The investigators aim to highlight any similarities or differences or the need for differentiated treatments.