Can the nucleic acid on the 16th be used on the 19th in three days?

no. The substance of nucleic acid detection is the nucleic acid of virus. Nucleic acid detection is to find out whether there is nucleic acid of foreign invading virus in respiratory tract samples, blood or feces of patients, so as to determine whether they are infected by Covid-19. Because the nucleic acid test report is divided into 24 hours and 42 hours, the nucleic acid of No.16 has exceeded the validity period, so it can't be used any more. Once this method is positive for nucleic acid, it can prove that there is virus in the patient.