Purpose of tuberculin skin test

The tuberculin test is a common method for diagnosing tuberculosis. It is also one of the two main skin tests and can basically replace a variety of puncture tests. Its main purpose is to provide important diagnostic basis for BCG vaccination. If the tuberculin test is positive, it means that the patient has been infected with tuberculosis bacteria, and there is no need for BCG vaccination at this time.

Purpose of the test

(1) To provide a basis for BCG vaccination. For example, if the tuberculin test is positive, it indicates that the body has been infected with tuberculosis and there is no need for BCG vaccination. Those who are negative are candidates for BCG vaccination.

(2) To provide a basis for determining the immune effect: Generally, a tuberculin test should be performed 3 months after BCG vaccination to understand whether the body has developed immunity to BCG. If the tuberculin test result is positive, it means that the BCG vaccination is successful. Otherwise, the BCG vaccination needs to be repeated.

(3) Used for diagnosis and differential diagnosis: The tuberculin test plays an important role in the diagnosis and differentiation of tuberculosis in adolescents, children and the elderly, and is a commonly used auxiliary examination method.

principle

Tuberculin skin reaction is a delayed cellular hypersensitivity reaction. It is when antigens (tuberculosis bacteria or BCG) enter the body and sensitize the body's immune T lymphocytes, causing them to differentiate and proliferate in large numbers. When the sensitized body is invaded by antigens again, the sensitized lymphocytes will bind to them and cause allergic inflammation.

It manifests as the formation of nodules or even blisters and necrosis at the site of tuberculin injection. A positive tuberculin test indicates that the body has been infected with tuberculosis or vaccinated with BCG, and also indicates that the body has a certain immunity to tuberculosis. However, a small number of people with weakened immune systems (about 5%) test negative or have false negative results due to technical reasons. Usually after BCG vaccination, if the PPD skin test is negative, it means that the vaccination has failed.

Judgment method

my country stipulates that the observation reaction time is 72 hours, and the reaction can be measured within 48-96 hours. The recording method is to express the measured transverse diameter of the nodule in millimeters × the longitudinal diameter in millimeters. If blisters, nodules, necrosis and lymphadenitis are present, records should be made.

1. Negative reaction

No nodules or the average diameter of nodules is <5mm.

2. Positive reaction

The average diameter of the nodules is 5mm or above and is positive; 5-9mm is generally positive; 10-19mm is moderately positive; and those larger than 20mm with local blisters, bleeding, necrosis and lymphangitis are strongly positive.