Erythema
Erythema means redness of the skin caused by increased blood flow in the small vessels of the dermis. It is a description of appearance rather than a diagnosis, and it is one of the most frequently used words in dermatology.
The word means redness and nothing more. It says that small vessels in the dermis are carrying more blood than usual, which is why the skin above them looks the way it does. What caused that is a separate question entirely.
Clinicians qualify it constantly, and the qualifiers carry the information. Transient erythema comes and goes; persistent erythema stays. Blanching erythema pales when pressed, because the blood can be pushed out of the vessels, while non-blanching erythema does not, which suggests blood has left the vessels altogether. That simple test is used at the bedside more than almost any other.
There is a significant limitation to the term, and it is one dermatology has been openly correcting. Increased blood flow does not read as red in every skin. In deeper skin tones it commonly appears purple, grey, or as a darker shade of brown, and clinical descriptions written around lighter skin therefore fail when applied elsewhere.
Conditions including eczema, rosacea and psoriasis have been under-recognised or judged less severe than they were for exactly this reason. Where colour is unreliable, texture, scale, thickening and warmth carry more of the assessment.
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