When to test for COVID-19 using real-time reverse transcriptase polymerase chain reaction: a systematic review

•The best time to perform real-time reverse transcriptase polymerase chain reaction is between days one and seven after symptom onset.•The three most frequent symptoms of COVID-19 are fever, cough, and dyspnea.•Evidence supports clinical practice.•The symptomatology of new variants of COVID-19 must be considered. The aim of this study was to evaluate the time in days between symptom onset and first positive real-time reverse transcriptase polymerase chain reaction (RT-PCR) result for COVID-19. This systematic review was conducted in the MEDLINE (PubMed), Embase, and Scopus databases using the following descriptors: "COVID-19”, “SARS-CoV-2”, “coronavirus”, “RT-PCR”, “real time PCR”, and “diagnosis”. The included studies were conducted in 31 different countries and reported on a total of 6831 patients. The median age of the participants was 49.95 years. The three most common symptoms were fever, cough, and dyspnea, which affected 4012 (58.68%), 3192 (46.69%), and 2009 patients (29.38%), respectively. Among the 90 included studies, 13 were prospective cohorts, 15 were retrospective cohorts, 36 were case reports, 20 were case series, and six were cross-sectional studies. The overall mean time between symptom onset and positive test result was 6.72 days. Fourteen articles were analyzed separately for the temporal profile of RT-PCR test results; the best performance was on days 22-24, when 98% of test results were positive. These findings corroborate the RT-PCR COVID-19 testing practices of some health units. In addition, the most frequently described symptoms of these patients can be considered the initial symptoms of infection and used in decision-making about RT-PCR testing.