After receiving a positive test result for COVID-19, you may continue to test positive for antigens for a few weeks. The National Academy of Sciences, Engineering, and Medicine (NASEM) states that you can test positive for NAAT up to 90 days after the initial positive result. This can make it difficult to determine if a positive test is due to a new infection or a reinfection. If you have any questions or concerns about your individual circumstances, it is best to consult with a healthcare provider.
Dr. Stephen Kissler, a postdoctoral fellow at Harvard's T. H. Chan School of Public Health in the Department of Immunology and Infectious Diseases, told TODAY that people are likely to test positive for COVID-19 at home for six to 10 days.
The New York Times reports that home antigen tests can test positive for up to 14 days. However, it is difficult to determine how contagious someone is based on rapid tests since most people who become infected still don't test positive for antigens 10 days after symptoms appear.