The difference between blush and induration in the tuberculosis skin test is mainly reflected in the nature of the reaction, clinical significance and measurement standards. Blush is a reddened area of the skin , and induration is a firm nodule that can be touched under the skin. The two reflect different stages of immune response.
1. Reaction properties:
Flushing is a vasodilation reaction in the surface layer of the skin, which manifests as a reddish plaque with unclear boundaries around the injection site, usually appearing 6-12 hours after injection. Induration is a localized tissue hyperplasia formed by the immune response of cells in the dermis, a subcutaneous mass with clear boundaries when palpated, and usually reaches its peak within 24-72 hours.
2. Clinical significance:
The redness is mostly caused by inflammatory mediators such as histamine, and only indicates a nonspecific allergic reaction . The induration reflects the body’s delayed hypersensitivity reaction to tuberculin. A diameter of ≥5 mm indicates the possibility of Mycobacterium tuberculosis infection. A strong positive of ≥15 mm requires further investigation of active tuberculosis in combination with clinical symptoms.
3. Measurement standards:
The measurement of blush is based on the maximum horizontal diameter of the red area visible to the naked eye, but it is not used as a basis for judgment in international standards. The vertical diameter of the nodule should be measured after the boundary is determined by palpation. It is the only diagnostic indicator recommended by the WHO, and the blush area should be avoided during measurement.
4. Duration:
The erythema usually subsides within 48 hours, while the induration can last for 5-7 days. Pigmentation or desquamation may occur in the later stages of the induration, while the erythema rarely leaves any skin changes. The difference in duration between the two helps to distinguish the type of immune response.
5. Interference factors:
Flushes are easily affected by physical factors such as skin friction and local hot compresses. Induration is greatly affected by recent vaccinations and the use of immunosuppressants, and false negatives may occur in people with low immune function such as AIDS .
After the tuberculin test, the injection site should be kept dry and clean, and scratching or applying ointment should be avoided. It is recommended that the nodule measurement be performed by professional medical staff within 48-72 hours. Pressing by yourself may affect the result. Pay attention to supplementing high-quality protein and vitamin A/C in your daily diet, and exercise moderately to enhance immunity, but avoid strenuous exercise that may cause sweating and infection at the skin test site. If symptoms such as blisters, ulcers, or fever occur, seek medical attention immediately.
