The use of antigen tests for the diagnosis of COVID-19 in Italy has risen sharply in autumn 2020. Italian regions like Alto Adige, Veneto, Toscana, Lazio, Piemonte and Marche did a large use of these tests for screening and surveillance purposes or for implementing diagnosis protocols in addition to molecular tests. As a consequence of this situation the basic test positivity rate (TPR) definition (i.e. number of new positive cases divided by the number of molecular tests) used at the beginning

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... of the outbreak has lost in accuracy. In this paper, we emphasize the enormous potential of TPR index as a tool for monitoring the COVID-19 pandemic, and we show that the peaks of this index are related to the peaks of patients admitted in hospitals and intensive care units, which occurred on average from 16 and 12 days later approximately, considering all the Italian regions in the first phase of the pandemic. To conduct the study we have defined a novel version of the TPR index which takes into account the number of tests done with respect to the opulation, the number of infected individuals, and the number of patients healed, integrating it with antigen tests. Unfortunately, this index given the lack of data concerning antigen tests, so far cannot be computed for most Italian regions. However, using a limited set of data on antigen tests which were made available in November 2020, we have shown that the TPR forecasting properties are preserved in the second phase. Without any shadow of doubt, we argue that the defined TPR index could be successfully used as a forecasting tool to monitor the impact of COVID-19 in the healthcare system, and that antigen tests data should be standardized soon. We conclude the paper, defining an extended version of the TPR index to be used if more omplete data on antigen tests will be made available, also including vaccinate people.