The Glass Test and Non-Blanching Rashes

When to get help

A rash that does not fade under pressure is an emergency. Call 999 in the UK, 112 or 999 in Ireland and the EU, 911 in the US and Canada, 000 in Australia, or 111 in New Zealand. Do not wait to see whether it spreads, and do not wait for a fever.

NICE guideline NG240 goes further and names three findings as red flags for meningococcal disease: a haemorrhagic, non-blanching rash with lesions larger than 2 mm, known as purpura; a rapidly progressive or spreading non-blanching petechial or purpuric rash; and any symptoms or signs of bacterial meningitis combined with a non-blanching rash. NICE also instructs, in the same guideline, not to rule out meningococcal disease just because a person does not have a rash.

How to do the test

The Meningitis Research Foundation describes it in one sentence: press a clear glass tumbler firmly against the rash, and if you can see the marks clearly through the glass, get urgent medical help immediately. Use a glass rather than plastic so you can see through it, and press hard enough to blanch the surrounding skin.

• Marks fade or disappear under the glass. That rash is blanching. Most baby rashes are, including viral rashes, heat rash and hives.
• Marks stay visible under the glass. That rash is non-blanching. It means small amounts of blood have leaked under the skin. Treat it as an emergency.

Undress your baby to check properly. NG240 says to check all over the body including the nappy area, and to look for petechiae in the conjunctivae — the whites of the eyes.

Brown, black and tanned skin

This is where the test most often fails, and every source we have used says so. NICE notes that rashes may be difficult to see on brown, black or tanned skin, and directs clinicians to look for petechiae in the conjunctiva instead. The NHS advises checking paler areas — the palms of the hands, soles of the feet, the roof of the mouth, the whites of the eyes and inside the eyelids. The Meningitis Research Foundation makes the same point: the darker the skin, the harder a septicaemic rash is to see.

Use a bright light. Phone torches are fine. Compare one patch of skin with another.

What the two possible results actually mean

A non-blanching rash is a red sign in the NICE traffic light table for fever in the under-5s, and NG143 tells clinicians to consider meningococcal disease in any child with fever and a non-blanching rash, "particularly if any of the following features are present: an ill-looking child; lesions larger than 2 mm in diameter (purpura); a capillary refill time of 3 seconds or longer; neck stiffness".

A blanching rash is not a clean bill of health. It tells you about the rash in front of you at that moment, and nothing else. NG240 contains an instruction written for precisely this problem: tell families to look out for any changes in the rash, because it can change from blanching to non-blanching. A rash you checked an hour ago is not the rash you have now.

The mistake worth naming

The commonest error is waiting for a rash. The Meningitis Research Foundation is blunt: if you are seriously worried about someone who is ill, do not wait for a rash to appear — get medical help. The first symptoms are usually fever, vomiting and feeling unwell, and in babies, limb pain, pale skin and cold hands and feet often appear earlier than the rash. Not everyone with meningitis gets a rash at all, and a very ill baby needs help even if there are only a few spots, or none.

The second commonest error is testing once. A rash of small red pinpricks that spreads quickly and turns into red or purple blotches is the classic pattern the NHS describes; the pinprick stage can look trivial. If your baby is unwell, recheck.

Rashes that are not this

Most rashes in babies are blanching and self-limiting: viral rashes that arrive with a cold, heat rash in a hot room, hives, the fine spotty rash of roseola after the fever breaks, and the sandpaper rash of scarlet fever. Each has its own page on this site. The glass test does not diagnose any of them — it only separates the group that needs an ambulance from the group that does not.

Where the test came from, and its limits

The tumbler test is a public-health teaching tool rather than a diagnostic instrument, and the guidance treats it that way. NICE never tells clinicians to press a glass on anything; it tells them to identify a non-blanching rash, and the glass is simply the easiest way for a parent to do that at home. That distinction explains why every source pairs the test with a warning. It has no false positives worth arguing about — a rash that stays visible under firm pressure needs seeing — but it has plenty of false negatives, because rashes arrive late, change character, and are harder to see on some skin than others.

It is also worth knowing what the test cannot tell you about a rash that does not fade. Small broken blood vessels have other causes, including the pinpoint marks that can follow forceful coughing or vomiting, usually above the nipple line. Those are recognised and usually harmless. But that is a distinction for a clinician to make after seeing your baby, not one to make yourself at 2am with a torch.

What to do while you wait

Photograph the rash with something for scale, such as a coin, and note the time. It helps whoever sees you next to know how fast it is moving. Keep your baby with you and keep them cool rather than wrapped. Do not give anything by mouth if they are drowsy. If they are already under the care of a clinician for a fever and the rash is new, that is a reason to go back, not a reason to assume it has already been accounted for.