Scarlet fever glass test: what to know

To perform the check, press the side of a clear glass firmly against a patch of rash and look through it. A rash that fades under pressure is described as blanching. Spots that do not fade can be a warning sign, but the result is not a diagnosis and the test can be difficult on dark skin or with certain rash types. Scarlet fever usually causes a fine, rough rash alongside symptoms such as sore throat and fever. A clinician may need to examine the skin and throat, because the glass test cannot identify the germ causing an illness.

If a new rash appears, note when it started and whether fever, sore throat or swollen glands are present. Do not press hard over broken, blistered or painful skin. Take a photograph in good light if the rash changes before an appointment, but do not delay care to repeat the test. Arrange clinical advice for a rough rash with fever or throat symptoms; testing and antibiotic treatment may be considered. Read about Strep Throat, Fever and Antibiotic Treatment.

Seek emergency help if spots do not fade with firm pressure, especially when the person is feverish or looks very unwell. Also seek urgent care for breathing difficulty, confusion, severe headache, a stiff neck, fainting, rapidly worsening illness, purple or blistering skin, or marked drowsiness. Same-day medical advice is appropriate for a spreading rough rash, persistent fever, worsening sore throat, or difficulty drinking. Do not rely on a negative glass test when symptoms are concerning.

Ask whether the rash needs immediate examination, whether a throat test is useful, how to monitor the skin at home, and which changes mean emergency care. Explain what the glass test showed, where the rash began, whether it fades elsewhere, and any medicines or allergies.