Strongly positive PPD results are more likely to be caused by Mycobacterium tuberculosis infection, which is usually not diagnosed by blood sampling, but requires tests such as sputum smear, sputum culture and chest X-ray. PPD is also known as bacillus-conjugated bacillus test. Positive PPD result is usually caused by two reasons: Mycobacterium tuberculosis infection and BCG vaccination. However, BCG vaccination is mostly positive, and a strong positive result usually suggests that Mycobacterium tuberculosis infection is the cause. Currently, sputum smear or sputum culture to find Mycobacterium tuberculosis is the main method to confirm the diagnosis. Chest X-rays are also routinely preferred to aid in the diagnosis of TB. In addition, blood collection for antibody detection, gamma interferon release test and T-SPOT test are also widely used as auxiliary diagnosis. When PPD results are strongly positive, a combination of other tests under the guidance of a doctor is needed to further confirm the diagnosis.