Is there any basis for the results of the tuberculosis skin test after 72 hours?

Tuberculosis skin test generally refers to PPD skin test. The results of the PPD skin test are usually judged within 48-72 hours, and if they exceed 72 hours, they may have lost their relevance.

First, there is a basis

The PPD skin test is a method of diagnosing infection with Mycobacterium tuberculosis by injecting tuberculin and observing the reaction at the injection site. Normally, redness, swelling and induration will occur within 48-72 hours after the injection, and then gradually disappear. If these symptoms are not present during this time, the patient is not infected with Mycobacterium tuberculosis. However, if it exceeds 72 hours, it may result in inaccurate test results.

2. No basis

1. Positive: When a patient has blisters or necrosis at the injection site, it is usually a strong positive manifestation, which indicates the presence of active tuberculosis foci in the patient's body. Further tests are needed to determine whether tuberculosis is present.

2. Negative: If there is only slight redness at the injection site and there are no other abnormalities, it may be a sign of a negative result, but the possibility of a false negative cannot be ruled out. Patients are advised to re-test under the guidance of a doctor.

The PPD skin test is a common tuberculosis screening method, and the results should be read within 48-72 hours. If this time frame is exceeded, it may not be possible to draw the right conclusions. Because PPD skin testing can be inaccurate, it cannot be used as the sole diagnostic criterion. If necessary, a comprehensive analysis is required in conjunction with other relevant test results.