Bruising: What Causes Bruises and When They Matter

What a bruise is

When a blow damages small blood vessels (capillaries) under the skin, blood seeps into the surrounding tissue. Because the skin stays intact, the blood is trapped and shows through as a dark mark. Doctors call a bruise a contusion or, when it is flat and larger than about 1 cm, an ecchymosis. Smaller bleeding spots have different names:

Petechiae and purpura do not fade when you press a glass over them, unlike most rashes.

Bruise color stages

The color shows roughly how old a bruise is, though the timing varies widely between people and body sites:

1. Red or pink: within hours, as fresh blood collects.
2. Blue, purple or black: day 1 to 5, as the blood loses oxygen.
3. Green: around day 5 to 7, as hemoglobin is broken down into biliverdin.
4. Yellow or brown: day 7 to 14, from bilirubin and hemosiderin.
5. Fades completely: usually within 2 to 3 weeks.

On darker skin, bruises may look dark purple, brown or black rather than blue, and color changes can be harder to see.

Common causes of bruising

• Minor injuries: bumping into furniture, sports and falls. Shins, knees, thighs and forearms are the usual sites.
• Age: a common reason for easy bruising in later life. Skin thins and blood vessels become fragile, especially on the backs of the hands and forearms after years of sun exposure (actinic or senile purpura). These bruises are harmless.
• Medicines: blood thinners (warfarin, apixaban, rivaroxaban, clopidogrel), aspirin, NSAIDs, steroid tablets and creams, and SSRI antidepressants.
• Supplements: high-dose fish oil, ginkgo, garlic and vitamin E can increase bruising.
• Intense exercise: heavy lifting can cause small bruises from muscle strain.

Medical causes of bruising

• Low platelets (thrombocytopenia): from immune thrombocytopenic purpura, viral infections, medicines, liver disease or bone marrow problems. It causes petechiae, bruising, nosebleeds and bleeding gums.
• Inherited bleeding disorders: von Willebrand disease, the most common one, causes easy bruising, nosebleeds and heavy periods. Hemophilia causes bleeding into joints and muscles.
• Leukemia and other bone marrow cancers: bruising with tiredness, pale skin, infections, fever and bone pain.
• Liver disease: the liver makes clotting factors, so cirrhosis leads to bruising and bleeding.
• Vitamin K deficiency: from poor diet, antibiotics or malabsorption.
• Vitamin C deficiency (scurvy): bruising, bleeding gums and bleeding around hair follicles.
• Cushing syndrome and long-term steroid use: thin, fragile skin.
• Connective tissue disorders: such as Ehlers-Danlos syndrome.

Bruising on the trunk

Bruises on the chest, back, belly and sides are less common from everyday knocks, because the trunk is less exposed. Bruising here without a clear cause is worth a check, as it is more likely to reflect a bleeding tendency, low platelets or a medicine effect. Bruising on the side of the belly or around the belly button can, rarely, be a sign of internal bleeding, such as from pancreatitis. In children, unexplained bruises on the trunk, ears, neck or buttocks raise concern for non-accidental injury and should be assessed.

How doctors investigate bruising

A doctor will ask about injuries, medicines and supplements, other bleeding, family history and symptoms such as fever or weight loss, and examine the pattern of bruises. Tests may include:

• full blood count, including platelet count
• clotting tests (PT/INR and aPTT)
• von Willebrand factor and factor levels
• liver and kidney function
• a blood film, and occasionally bone marrow tests

Treatment and home care

• Cold: apply an ice pack wrapped in cloth for 15 to 20 minutes, several times in the first 24 to 48 hours.
• Elevate the bruised area to reduce swelling.
• Warmth after 48 hours may help the blood clear.
• Pain relief: acetaminophen (paracetamol) is preferred over aspirin or NSAIDs, which can increase bleeding. Follow the label dose.
• Protection: pad furniture corners and use good lighting to prevent knocks in older adults.

Medical causes are treated directly, for example by adjusting a blood thinner, treating ITP or replacing a deficient vitamin.

Frequently asked questions

Why am I bruising without injury?

Many "unexplained" bruises come from bumps you did not notice, especially on the legs. Medicines such as aspirin and blood thinners, aging skin, and supplements are common causes. Frequent unexplained bruises, or bruising with other bleeding, need blood tests to check platelets and clotting.

What does bruising on the trunk mean?

Bruises on the chest, back or belly are less often caused by minor bumps, so unexplained trunk bruising deserves a check for a bleeding disorder, low platelets or medicine effects. In a child, unexplained trunk bruising should always be assessed by a doctor.

When should I worry about a bruise?

See a doctor if bruises appear without a cause, are very large or painful, last more than 2 to 3 weeks, or come with nosebleeds, bleeding gums, heavy periods, tiny red spots, fever or weight loss. Get emergency care for bleeding that will not stop or a head injury on a blood thinner.

How long does a bruise take to heal?

Most bruises fade in 1 to 3 weeks, passing through purple, green and yellow stages. Bruises on the legs often heal more slowly than those on the face or arms. A large hematoma can take a month or longer.

Can low iron cause bruising?

Iron deficiency itself does not usually cause bruising. However, heavy bleeding from a bleeding disorder can cause both bruising and iron deficiency. If you have both, ask your doctor to check your platelet count and clotting.