Why do schools need to conduct PPD screening? A popular science article explaining the tuberculin test

1. What is PPD screening?

PPD (Purified Protein Derivative) screening is a method of detecting whether the human body is infected with Mycobacterium tuberculosis through skin testing, commonly known as the "tuberculin test."

- Principle: Inject specific proteins of tuberculosis bacteria into the skin to observe the immune response. If there are immune cells sensitized to tuberculosis bacteria in the body, redness, swelling and hardening will form at the injection site.

- Purpose: It is not to diagnose pulmonary tuberculosis directly, but to screen for latent tuberculosis infection (LTBI), that is, people who carry tuberculosis bacteria in their bodies but have not yet developed the disease.

2. Why should schools organize PPD screening?

1. Schools are “high-risk areas” for tuberculosis transmission

- Crowded places: Classrooms and dormitories are not well ventilated, and the risk of droplet transmission is high.

- Differences in students’ immunity: Adolescents are in the growth and development period, and some of them have weak immunity, making it easy for latent infection to develop into active tuberculosis.

2. The “time bomb” nature of latently infected people

- Latent infection ≠ illness: About 5%-10% of latently infected people will develop active tuberculosis in their lifetime, especially when their immunity is weakened (such as staying up late, high stress, malnutrition).

- Early detection and early intervention: PPD screening can identify high-risk groups and reduce the incidence rate by 60%-90% through preventive treatment (such as taking isoniazid for 3-6 months).

3. Legal requirements for public health prevention and control

- my country's "Standards for School Tuberculosis Prevention and Control" clearly stipulates:

- New students are required to undergo tuberculosis screening (PPD or chest X-ray).

- When a case of tuberculosis occurs in a school, close contacts need to be screened for PPD.

3. How to perform PPD screening? How to read the results?

1. Operation steps

- Injection: Inject 0.1 ml of PPD reagent intradermally on the inner forearm to form a skin mound of 6-10 mm in diameter.

- Observation: Check injection site reaction 48-72 hours later and measure diameter of induration (not redness).

2. Result judgment

Image from Internal Medicine Volume 1, 3rd Edition, People's Medical Publishing House

3. Pay attention to special circumstances

BCG influence: People who have been vaccinated with BCG may show weak positive results (usually nodules <10mm), but it is impossible to completely distinguish between natural infection and vaccine reaction.

False negative: People with AIDS, severe malnutrition, or those taking immunosuppressants may not be able to produce a normal immune response.

4. What should I do if PPD is positive? Do I need treatment?

1. Positive ≠ sick

A positive PPD test only indicates that you have been infected with tuberculosis, not that you are currently developing the disease. A comprehensive assessment should be made based on the following tests:

- Chest X-ray or CT: Check whether there are active lesions in the lungs (such as infiltration shadows, cavities).

- Sputum test: Detection of the presence of tuberculosis bacteria in sputum (the gold standard for confirming active TB).

2. Treatment of two groups of people

- Latently infected people (normal chest X-ray, no symptoms): After evaluation by the doctor, preventive medication (such as isoniazid + vitamin B6, 3-6 months of treatment) may be recommended. Regular follow-up to monitor for symptoms such as cough and fever.

- Patients with active tuberculosis (abnormal chest X-ray, positive sputum test): Immediately isolate and treat, and take anti-tuberculosis drugs as required (at least 6 months).

5. Is PPD screening safe? Who should not do it?

1. Security

- Non-infectious: PPD reagents do not contain live bacteria and will not cause infection.

- Mild reactions: Itching, redness and swelling at the injection site may occur. A very small number of people may develop a low-grade fever, which usually disappears on its own within 1-2 days.

2. People who should not use this product

- Fever period of acute infectious diseases (such as influenza, chickenpox).

- During a flare-up of a severe skin disease (e.g. eczema, psoriasis).

- People with immunodeficiency diseases (such as AIDS) or taking immunosuppressants.

6. Frequently asked questions from parents and students

1. "My child has received BCG vaccination, why does he need to do PPD?"

- The protective effect of BCG decreases with age and does not completely prevent infection. PPD screening can help identify high-risk individuals.

2. “Will PPD-positive students be discriminated against by their classmates?”

- Latently infected people are not contagious, and normal study and life will not be affected, so there is no need to panic.

3. “Do preventive treatments have serious side effects?”

- The incidence of isoniazid hepatotoxicity is low (about 1%) and can be effectively monitored by regularly checking liver function while taking the drug.

Summary: The “three-level meaning” of PPD screening

1. Protect individuals: detect latent infection early and reduce the risk of disease.

2. Protect the collective: avoid school-based epidemics and maintain the health of teachers and students.

3. Public health: Control the spread of tuberculosis at the source and reduce the social burden.

Remember: PPD screening is the "first line of defense" for tuberculosis prevention and control in schools. Cooperating with the inspection is not only responsible for yourself, but also a respect for the health of others!