The glass test is an informal but widely recognized emergency triage method used to quickly assess a specific type of rash. It is designed for the public to check for signs of potential internal bleeding beneath the skin, which can signal a severe underlying illness. This simple procedure is not a medical diagnostic tool; instead, it provides a fast warning sign that immediate professional medical attention may be required. The test determines if a rash is non-blanching, indicating that blood has leaked out of the circulatory system.
How to Perform the Glass Test
The test requires only a clear, colorless drinking glass with a flat rim for accurate observation. First, locate the rash and select a spot with clearly visible red or purple markings. The glass should be clean and dry before beginning the procedure.
Place the side of the glass firmly against the skin where the rash is present. Apply enough pressure to cause the skin immediately surrounding the rash to turn pale or white, temporarily pushing the blood out of the capillaries.
While maintaining this firm pressure, look through the bottom or side of the glass directly at the rash spots. Observe whether the rash itself disappears or fades under the pressure, similar to the surrounding skin. If the spots remain clearly visible and do not lighten in color despite the pressure, the result is considered positive for a non-blanching rash.
Understanding Blanching and Non-Blanching Rashes
The glass test helps differentiate between a common blanching rash and a non-blanching rash. A blanching rash appears red because the small blood vessels, or capillaries, near the skin’s surface are dilated with blood. When pressure is applied, this blood is temporarily squeezed out of the capillaries, causing the redness to disappear until the pressure is released.
A non-blanching rash, however, does not fade because the discoloration is caused by blood that has leaked out of the capillaries and into the surrounding tissue. These spots are tiny areas of hemorrhage beneath the skin’s surface, and this leaked blood remains visible even when direct pressure is applied.
These hemorrhagic spots are termed petechiae if they are small, typically less than three millimeters in diameter, resembling pinpricks. Larger spots, ranging from three millimeters to one centimeter, are called purpura. The persistence of these red or purple spots under pressure indicates a breakdown in the integrity of the blood vessel walls.
Immediate Action Following a Non-Blanching Result
A non-blanching result from the glass test points toward internal bleeding and requires immediate emergency medical intervention, particularly if the individual is also feeling unwell. This finding can be a symptom of meningococcal septicemia, a severe bloodstream infection that can progress rapidly.
Immediately contact your local emergency services, such as calling 911 or 999, to seek professional medical help. Do not wait for other symptoms to develop or for the rash to spread before seeking assistance, as the rapid progression of associated serious conditions means time is a significant factor in treatment and recovery.
It is important to remember that a blanching rash does not automatically mean an illness is harmless, as many serious infections begin without a rash or with a blanching one. This test is most informative when a person is already experiencing other concerning symptoms like fever, lethargy, or confusion. Never delay seeking medical advice if someone is seriously unwell, regardless of the glass test result.