PPD nodule and redness can be understood as the nodule is contained within the redness, which is equivalent to the word “swelling” in redness. After the PPD test, through 48-72 hours of observation, you need to measure the diameter of the nodule, and do not need to measure the red halo around the nodule. As a simple example, it is equivalent to an omelette, the data to be known is the diameter of the yolk, for the diameter of the egg white around the yolk, does not need to be measured. After the skin test is performed, a judgment is made by measuring the diameter of the nodule, and when the diameter is >5mm, it is considered positive, and according to the size of the diameter of the nodule, it can be categorized as mildly positive, moderately positive, and strongly positive. Strong positivity also occurs when the skin breaks down or there are blisters after the skin test is performed.