In a new study, University of Colorado Boulder researchers found that if only one at-home test is available, the most accurate results for COVID start two days after the start of symptoms. For the flu and respiratory syncytial virus, however, it’s recommended to take a rapid test immediately after symptoms arise. The study used a mathematical model to answer questions about at-home rapid test timing and accuracy, not only for COVID but also for emerging tests for RSV, the flu and other infectious diseases. “You cannot use the same test in the same way and expect to get the same results for different pathogens, or even different variants,” said Casey Middleton, first author of the study and a doctoral student in the CU Boulder department of Computer Science and the IQ Bio program. For COVID testing, the reason it’s better to wait a couple days before testing lies in how the rapid tests detect disease, and their surprisingly high false negative rate when used too soon. “Especially really early on for COVID, when your symptoms first start, we find that if you take that one test right away, there is a 95% chance that a negative result is a false negative,” said Middleton. Waiting two days after symptoms brings that rate down to 70%.