Background Confirmed COVID-19 cases have been reported in 213 countries and regions and as of 12 July 2020, over 12 million cases, with 561617 deaths MESHD have been reported worldwide. The number of cases changes quickly and varies depending upon which source you use to track, so in the current epidemiological context, the early recognition is critical for the rapid identification of suspected cases (with SARS-CoV-2 infection MESHD-like symptoms and signs MESHD) to be immediately subjected to quarantine measures. Although surveys are widely used for identifying COVID-19 cases, outcomes and associated risks, no validated epidemiological tool exists for surveying SARS-CoV-2 infection MESHD in the population so far. Methods Our study is the phase II of the EPICOVID19 national survey, launched in April 2020 including a national convenience sample of 201121 adults TRANS, who voluntarily filled the EPICOVID19 questionnaire. The phase II questionnaire was mailed to all subjects who underwent tests for COVID-19 by nasopharyngeal swab (NPS) and who accepted to be involved in the second phase of the study, focused on the results reported for NPS and/or serological IgG/IgM tests. We evaluated the capability of the self-reported symptoms collected through the EPICOVID19 questionnaire to discriminate the COVID-19 among symptomatic subjects, in order to identify possible cases to undergo instrumental measurements and clinical examinations. We defined a method for the identification of a total score and validated it with reference to the serological and molecular clinical diagnosis, using four standard steps: identification of critical factors, confirmation of presence of latent variable, development of optimal scoring algorithm and validation of the scoring algorithm. Findings 2703 subjects [66% response rate] completed the Phase II questionnaire. Of 2703 individuals, 694 (25.7%) were NPS(+) and of these 84 (12.1% of the 694 NPS(+)) were asymptomatic TRANS. In the individuals who performed serological testing SERO, of the 472 who did IgG(+) and 421 who did IgM(+), 22.9% and 11.6% tested positive, respectively. Among IgG(+) 1 of 108 subjects was asymptomatic TRANS (0.9%) while 5/49 subjects among IgM(+) were asymptomatic TRANS (10.2%). Compared with NPS(-), among NPS(+) subjects there was a higher rate for Fever MESHD Fever HP (421 [60.7%] vs 391[19.5% ]; p<0.0001), Loss of Taste and/or Smell (365 [52.6%] vs 239 [11.9% ]; p<0.0001) and Cough MESHD Cough HP (352 [50.7%] vs 580 [28.9% ]; p<0.0001). Also for other symptoms the frequencies were significantly higher in NPS(+) subjects than in NPS(-) ones (p<0.001). Among groups with serological tests SERO, the symptoms with higher percentages in the subjects IgG(+) were Fever MESHD Fever HP (65 [60.2%] vs 43[11.8% ]; p<0.0001) and Pain MESHD Pain HP in muscles, bones, joints (73 [67.6%] vs 71 [19.5% ]; p<0.0001). For the COVID-19 self-reported symptoms items, exploratory (proportion variance explained [89.9%]) and confirmatory factor analysis results (SMSR 0.072; RMSEA 0.052) highlights the presence of one latent variable (factor) underlying the symptoms. We define the one-factor solution as EPICOVID19 diagnostic scale and optimal score for each items was identified: Respiratory problems (1.03), Chest pain MESHD Chest pain HP (1.07), Loss of Taste and/or Smell (0.97) and Tachycardia MESHD Tachycardia HP ( palpitations HP) (1.05) were the most important symptoms. The cut-off score was 2.56 ( Sensitivity SERO 76.56%; Specificity 68.24%) in NPS(+) and 2.59 (Se 80.37; Sp 80.17) in IgG(+) subjects.