The PPD test usually begins to show a reaction after 48 hours, but it takes 72 hours to observe the results of the PPD test, mainly to see if there is erythema at the site of the injection in the body, and if the erythema is <5 mm, it is a negative result, indicating no TB infection or vaccination. If the erythema is 5-10 mm, the result is weakly positive, indicating that the vaccine was given or that there was a previous TB infection. If the erythema is 10-20 mm, a positive result indicates a possible previous TB infection. If the erythema is more than 2 cm, blistering, rupture, and redness, it is a strong positive result and should be considered as a tuberculosis infection now.