Last night you banged your knee on the nightstand. Now you have a large red mark that really hurts. Most likely, you have a contusion (bruise). This is when a blow doesn't break the skin but does break blood vessels beneath the skin. Blood leaking from the broken vessels causes redness and swelling. As it heals, your bruise is likely to turn colors like purple, green, and yellow. This is normal. The bruise should fade in 2 or 3 weeks.
Factors That Make You More Likely to Bruise
Almost everyone bruises now and then. Certain people do bruise more easily than others. You're more prone to bruising as you get older. That's because blood vessels become more fragile with age. You're also more likely to bruise if you have a clotting disorder such as hemophilia or take medications that reduce clotting, including aspirin.
When to Go to the Emergency Room (ER)
Bruises almost always heal on their own without special treatment. But for some people, a bad bruise can be serious. Seek medical care if you:
Have a clotting disorder such as hemophilia.
Have cirrhosis or other serious liver disease.
Take blood-thinning medications such as warfarin (Coumadin).
Apply an ice pack or bag of frozen peas to a bruise (keep a thin cloth between the cold source and your skin). This can help reduce redness and swelling.
What to Expect in the ER
A doctor will examine your bruise and ask about any health conditions you have. In some cases, you may have a test to check how well your blood clots. Other treatment will depend on your needs.
Sometimes a bruise gets worse instead of better. It may become larger and more swollen. This can occur when your body walls off a small pool of blood under the skin (hematoma). In that case, your doctor may need to drain excess blood from the area.