What is the difference between redness and induration in a PPD test?

PPD refers to the PPD skin test. Generally, differences between redness and induration caused by the PPD skin test may lie in duration, appearance, texture, size, and pain sensation. If any discomfort occurs, prompt medical attention is recommended. The specific analysis is as follows:

1. Duration

Redness from a PPD skin test typically appears as a red lesion within 24–48 hours and usually lasts for a short period, gradually fading within 1–2 days. Induration, on the other hand, develops as a nodular elevation within 48–72 hours and may take a longer time to resolve.

2. Appearance

Redness usually presents as a localized red patch on the skin, possibly accompanied by mild swelling and itching. Induration, however, appears as a firm lump beneath the skin, with a hard texture that is not easily movable and typically lacks obvious redness.

3. Texture

Redness feels soft to the touch, with noticeable elasticity and pliability. In contrast, induration is characterized by a hard, solid nodule that is difficult to compress.

4. Size

Redness tends to be more widespread, covering a relatively large red area at the test site (such as the back or forearm). In comparison, induration is a localized nodule, often with a smaller diameter.

5. Pain Sensation

Redness is usually associated with mild itching but generally does not cause significant pain. Induration, however, may cause local pain or discomfort when touched or pressed.

During the PPD skin test, patients should actively cooperate with their healthcare provider. After the test, results should be observed and interpreted by a qualified physician.