A weak positive PPD usually indicates a previous infection with Mycobacterium tuberculosis. The PPD test, also known as the tuberculosis bacillus test, involves injecting tuberculosis toxin into the skin and observing skin changes 48-72 hours later. It is considered negative if only redness appears on the skin, or negative if the nodules are very small, less than 5 mm in diameter. When negative, it means that the organism has not been infected with Mycobacterium tuberculosis and BCG vaccination can be given to prevent tuberculosis. If the nodule is larger than 5 mm and smaller than 2 cm in diameter, it is considered weakly positive. Weak positives mostly indicate that the body has been infected with Mycobacterium tuberculosis and does not require vaccination. If the nodule is larger than 2 cm in diameter or has blisters in the periphery, it is strongly positive. A strong positive nodule usually indicates active tuberculosis and requires clinical symptoms and imaging to confirm the diagnosis, followed by anti-tuberculosis treatment.