A blood blister in the mouth: why does it happen, and when is a blood test needed?

A blood blister inside the mouth forms when blood leaking from a small vessel just under the mucosa lifts the thin layer above it. Its appearance is striking: a dark red, purple or nearly black, tense and shiny blister. But very different causes can lie behind the same appearance. We make the distinction by looking at how the blister started, how many there are and whether there is any sign of bleeding elsewhere in the body.

The most common cause is angina bullosa haemorrhagica. Although its name is alarming, it is a benign, self-limiting condition. In a systematic review bringing together 225 patients, the mean age was 55.4 and 66% of the blisters were on the palate; a third of the patients had no illness at all and the blood tests carried out were normal. In a study examining the 14-year records of four dental services in Brazil, too, the blister was on the soft palate in 65.2% of 23 patients, 73.9% had no pain, the most common trigger was trauma during chewing and no clotting disorder was found in any patient.

The typical story runs like this: while eating something hard or crunchy, swallowing a hot mouthful or during a procedure at the dentist, a sudden feeling of fullness on the palate, followed by a blood-filled blister seen in the mirror. In the Brazilian series the blisters burst within a few minutes or a few hours and left a superficial erosion in their place. DermNet also writes that the blisters generally burst by themselves and that the site heals without problems. In the review, recurrence was common (62%): it can appear again in the same person months or years apart.

Using a steroid inhaler for a long time is also associated with this picture. DermNet lists trauma, long-term use of steroids applied in the mouth or inhaled, diabetes and a family tendency among the possible factors; some of the reported cases are asthma patients using steroid inhalers. But the systematic review emphasises that no definite link with a general illness or a medicine could be established; the evidence rests on case reports. If you use an inhaler for asthma or COPD, do not stop the medicine yourself; the NHS writes that fitting a spacer to the inhaler can reduce the risk of steroid side effects in the mouth and throat; discuss this with your doctor.

The rarer cause, but one that must not be missed, is a fall in the platelets in the blood. In King's College Hospital's patient information on immune thrombocytopenia (ITP), the symptoms a low platelet count can cause are listed as bruising, pinpoint blood spots on the skin (petechiae), bleeding from the nose and gums, blood blisters inside the mouth that look dark purple or black, blood in the urine or stools, tiredness and heavy periods. The number of blisters alone does not make the distinction, because there can be more than one blister in angina bullosa haemorrhagica too. The real thing to look at is the accompanying picture: with a low platelet count, signs of bleeding are expected elsewhere in the body as well.

The most useful check you can do at home is to look at your skin. In good light, examine your lower legs, your ankles and the inner surface of your arms. MSD Manuals writes that in ITP small red or purple spots appear mostly on the lower legs and that minor knocks leave bruises. Also think about whether your gums have been bleeding more than usual when you brush in recent days, whether your nose has bled and whether you have any bruises you cannot explain. If none of these is present and the blister started with a mouthful of food, the picture is most probably angina bullosa haemorrhagica.

There are other pictures that get confused with a blood blister too. DermNet's list of differential diagnoses includes conditions that affect the skin and mucosa together, such as mucous membrane pemphigoid, bullous pemphigoid and oral lichen planus; in these the blisters appear again and again and in different places, take longer to heal and usually need a biopsy. A clear or bluish, fluid-filled blister on the inner surface of the lower lip, on the other hand, is not a blood blister but a mucocele arising from a salivary gland; we have explained that on a separate page.

There is also one rare but serious exception. A study examining seven new cases and 199 cases from the literature writes that extensive blisters can carry a risk of obstructing the airway and may require surgical intervention; a case has been reported in which a blood blister growing rapidly at the back of the throat blocked the airway and intubation followed by surgical tracheostomy was needed. That is why, if a blister growing deep in the throat is making it hard for you to breathe or swallow, you should call 112 without waiting.

The signs in the first list suggest angina bullosa haemorrhagica; those in the second suggest a picture that requires a blood test or an examination. The definite distinction is made by a doctor.

The order deliberately starts with the most dangerous situation. This page does not replace an examination.

1. 1

   First the airway and heavy bleeding

   If a blister growing in your throat is making it hard for you to breathe or swallow, call 112. The ITP information asks you to go to A&E without waiting in the following situations: a severe headache that comes with drowsiness, vomiting or loss of vision, a nosebleed lasting longer than 30 minutes, black or bloody stools, blood in the urine and other excessive bleeding or bruising. If one of these is present, go to the nearest A&E and call 112 if necessary.

2. 2

   Look for other signs on the skin and in the mouth

   Look to see whether there are pinpoint spots or unexplained bruises on your lower legs and your arms. Check in the mirror whether there is another blister or a bleeding point inside your mouth. Think about whether your gums and your nose have bled in recent days.

3. 3

   Do not try to burst the blister

   Blisters usually burst by themselves within a short time. Trying to open one with a needle or a fingernail opens the door to a new wound and to infection. For the superficial sore left behind after it bursts, rinsing with lukewarm salt water and soft, lukewarm food for a few days are enough; DermNet writes that a mild painkiller and a bland mouthwash are generally sufficient.

4. 4

   Take a photograph and make notes

   Take a photograph of the blister, and write down what you were doing when it appeared, how many there were and when it burst. By the time you see a doctor, the blister may be long gone; the diagnosis rests largely on this story and the image.

5. 5

   A blood count if needed

   DermNet writes that in a typical case of angina bullosa haemorrhagica tests are generally not needed, but are requested when other conditions need to be ruled out. If the blister appears spontaneously and in large numbers, or if there are other signs of bleeding, the doctor may ask for a full blood count and clotting tests. MSD writes that when ITP is suspected, blood tests measuring the platelet count and clotting are carried out, but that there is no established test that confirms ITP on its own.

6. 6

   Examination in a recurring or atypical picture

   If the blisters recur often, appear in different places or leave sores that last for weeks, a dentist or dermatologist examines you and asks for a biopsy if necessary. In the review, a biopsy had been carried out in 35% of patients.

The treatment of a blood blister differs greatly according to the cause behind it.

The following are the points where the wrong decision is most often made with this complaint.

The course below is the typical course of angina bullosa haemorrhagica. If yours does not fit it, see a doctor.

1. The first minutes and hours

   The blister swells quickly. In the Brazilian series most burst by themselves within a few minutes or a few hours.

2. After it bursts

   A superficial, slightly stinging erosion is left in its place. Soft, lukewarm foods make this period easier.

3. A few days

   In the Brazilian study the picture took a favourable course within a few days; DermNet also writes that the site heals without problems.

4. The following months

   Recurrence was common in the review. If a new blister appears, carry out the same checks: are there spots on the skin, bruises or other bleeding?

Because a typical blood blister goes without requiring any procedure, the cost is mostly about clarifying the cause. The items that determine the extent:

Whether a blood test is needed

A full blood count and clotting tests are carried out by a GP (family doctor) or in internal medicine.

Whether a biopsy is needed

With recurring or atypical blisters, the tissue sample and pathology are separate items.

The underlying condition

If a low platelet count or a skin and mucosal disease is found, its follow-up and treatment is a separate process.