The PPD test, also known as the tuberculin skin test, is a common clinical test used to screen for latent tuberculosis infection. But why do you need three days to see the results? The three-day rule is a very scientific rule in China. The first tuberculin skin test, the OT test, was gradually replaced by the PPD test, which has higher purity, higher specificity and sensitivity, and higher safety, because of its low specificity and sensitivity, and relatively more side effects. OT test, due to the lack of drug purity, has a non-specific reaction, redness, in addition to the specific hard nodal reaction after doing the skin test. Therefore, the result of the skin test is judged by the size of the hard nodules, not by the size of the red halo. Most of the non-specific reactions, on the other hand, disappear within one day, so using three days to see the results can maximize the elimination of the interference of non-specific reactions and make the skin test results more reliable. Although, the OT test was eliminated, the PPD test still continues this practice, with the hard nodes as the measurement, not the redness. So, when the medical staff is feeling around on your arm, don’t get me wrong, they are looking for the sides of the hard nodes. With the PPD test, most reactions start at 5-6 hours from the time the skin test is done, and peak at 48 to 96 hours. So theoretically, 48-96 hours is the time window in which you can come to measure the size of the hard nodes. We then set 72 hours as the time point to observe the results (taking the middle value).