Abstract
Background : The diagnosis of Covid 19 is made by the detection of viral RNA by PCR on nasopharyngeal swabs. In some patients the test is falsely negative, while other biological samples are positive. The aim of the study is to identify characteristics and prognostic factors for swab negativity in COVID-19 patients with BA-confirmed disease. Materials and methods : Multicentric retrospective case-control study of patients admitted for COVID-19 between March and November 2020 in two internal medicine units of the AOU Careggi and in the Internal Medicine of the Hospital of Varese. Were enrolled patients aged ≥18 years hospitalized for COVID-19 with viral RNA isolation on biological specimen, considering as cases the patients negative to swab but positive to BA. For each case, four controls with positive swab at admission were enrolled Results : the study included 95 patients, 19 cases and 76 controls. The mean time between symptoms onset and swab was 2.65±1.9 days in cases, with a statistically significant (p= 0.003) difference compared to controls (5.53±3.0 days). Patients with negative swab had a longer mean length of stay and more frequent adverse outcome than controls. Conclusions : Swabbing within a short time of symptoms onset is a predictor for false negative. Patients with repeated negative swabs have a worse clinical picture with longer hospital stay, greater need for non-invasive ventilation and higher frequency of adverse outcome.