The tuberculin (tuberculin) skin test is an important tool to screen for TB infection in the unvaccinated BCG population and is a powerful tool for clinical diagnosis of TB disease. While the old tuberculin (OT) was mostly used in the past, the more accurate and stable pure protein derivative of Mycobacterium tuberculosis (PPD) is now used. The occurrence of 5U of OT or PPD 72h after intradermal injection of 5U of OT or PPD with a red, hard nodule of 5mm or more is considered a positive reaction. Negative test results indicate: 1, no infection: generally OT 1mm negative can exclude tuberculosis, but false negatives should be noted; 1, biological healing of the lesion, which is actually rare. Positive test is seen in: 1, after BCG vaccination (mostly weakly positive reaction; 2, has been naturally infected but not yet developed; 3, the onset of tuberculosis; 4, has been healed of tuberculosis. According to whether the child is at high risk of tuberculosis or has a history of BCG vaccination, different criteria for positive nodules have been established, such as a history of close tuberculosis contact with immunocompromised children is a high-risk group, the red hard diameter ≥ 5mm is positive; in areas with high incidence of tuberculosis, extremely poor people in any country and people with poor seedlings, diabetes mellitus chronic patients whose positive criteria are red hard diameter ≥ 10mm; general population (not In the general population (not the above two types of people) who have been vaccinated with BCG, the red hard diameter ≥15mm is considered a positive reaction. Since OT and PPD both contain various antigens of Mycobacterium (including tuberculosis or non-tuberculosis) and common antigens of BCG, cross-reactivity can occur, so non-tuberculosis Mycobacterium and positive reaction after BCG vaccination should be excluded when determining whether it is tuberculosis infection. Since BCG inoculation is common in China, the most important thing at present is how to distinguish between natural infection and positive reaction of nodules after BCG inoculation. Generally speaking, the former positive reaction is stronger, showing hard nodes with deeper red color, harder texture, thicker, clear edges, range or area trap (more than 15mm in diameter), hard nodes do not disappear after 72-96h, and pigmentation remains after fading. If the result of tuberculin test cannot be distinguished from natural infection or reaction after BCG inoculation, the test should be repeated after six months to one year, and the positive reaction of BCG tends to gradually decrease, while the positive reaction of natural infection does not change. If the junction factor test reaction has increased from less than 10 mm to more than 10 mm in diameter within the last 2 years, and the increase is more than 6 mm, the nature of the junction factor reaction after BCG vaccination is different from that after natural infection. In contrast, the nodulin reaction after natural infection is a Koch-type reaction, i.e., a strong metamorphic reaction caused by the release of tumor necrosis factor from macrophages after the contact between the sensitized T-lymphocytes and the antigen again, so the positive reaction after natural infection is stronger.