How is the tuberculosis skin test performed

We must have all been to the hospital and seen doctors helping patients diagnose their illnesses. One diagnostic method is to use skin tests to determine whether a person is infected with a certain pathogen. The most common one is the tuberculosis skin test. However, many people do not know much about this skin test method. Some people even think that this skin test requires cutting a piece of skin. Let’s learn about tuberculosis skin test.

Tuberculosis skin test

Indications

The tuberculin skin test is indicated for:

1. Auxiliary diagnosis of tuberculosis.

2. Three months after BCG vaccination, understand the body's cellular immune response to BCG.

3. Determine the prognosis of children with allergic diathesis (atopy).

Prepare

1. Commonly used preparations

(1) OT (old Tuberculin) - crude preparation of tuberculosis metabolites (including tuberculin);

(2) PPD (Purified protein derivative, PPD-RT23) - Purified tuberculin protein derivative

2. Commonly used concentration of OT (5IU, 0.1ml) PPD 1:10000 skin test

method

1. Inject 0.1 ml (containing 1 U, 0.0001 mg) of pure tuberculin protein derivative (PPD) intradermally on the forearm surface.

2. Measure the diameter of the injection site nodule 48 to 72 hours after PPD intradermal injection. Use the tip of a ballpoint pen to slide along the skin surface from one distal end of the nodule toward the nodule. When the tip of the ballpoint pen reaches the edge of the nodule and feels resistance, stop sliding, leaving a ballpoint pen line on the skin. Slide the ballpoint pen in the same manner from the other far end of the nodule (180° angle) to leave another line mark. The distance between the two lines is the diameter of the nodule. The transverse diameter of the nodule was measured in the same way, and the average value of the transverse diameter and diameter was calculated.

Precautions

1. Use PPD to assist in the diagnosis of tuberculosis

For those with a negative 1U (0.00002mg) tuberculin skin test, a 5U (0.0001mg) skin test can be performed, and a 250U (0.005mg) skin test can be performed if necessary.

2. When assisting in the diagnosis of tuberculosis, the limitations of the tuberculin skin test should be fully recognized. There are many factors that affect this trial, including failure of intradermal injection, failure of PPD, low cellular immune function in children, and severe tuberculosis infection.

3. The tuberculin skin test cannot distinguish between the immune response caused by BCG vaccination and natural infection with Mycobacterium tuberculosis.

4. The tuberculin skin test cannot distinguish between nontuberculous mycobacterial infection and tuberculosis infection.

5. When judging the prognosis of children with allergic constitution, a positive tuberculin skin test caused by BCG vaccination, Mycobacterium tuberculosis or natural infection with nontuberculous mycobacteria indicates that the patient's Th1 cell function is good, which helps to alleviate clinical symptoms.

6. Negative tuberculin reaction may occur in the following situations:

(1) Infected with tuberculosis for less than four weeks.

(2) Severe tuberculosis or severe malnutrition.

(3) Use of immunosuppressants (hormones).

(4) Immunodeficiency (lymphocytic leukemia, HIV, tuberculosis, etc.).

If you want to do this kind of skin test, you must find a regular hospital to do it. After all, this skin test is related to the diagnosis results. If there is a slight deviation leading to misdiagnosis, it may be fatal for the patient. If it is confirmed that you are infected with tuberculosis, you should cooperate with the doctor and actively receive treatment. After all, medicine is so advanced that there is still great hope for a cure.