Because many reagents are too sensitive now, in other words, I would rather kill 1000 by mistake, so there will be a certain false positive probability. Therefore, after the preliminary test results come out, if there is any doubt, the tester will be advised to go to the CDC for a follow-up visit. The method of follow-up visit has low sensitivity, but it takes more than two months to find out the infection. One more thing, the hospital has no right to diagnose, so the result is not ideal.