Your doctor can diagnose a boil or carbuncle by examining your skin. If you get several boils within a short period of time, your doctor may do blood tests to check for diabetes or other medical conditions that can increase your risk of repeated infections.
In many otherwise healthy people, a small boil will form a white tip (come to a head) and drain within five to seven days. However, carbuncles or very large boils can last longer and may not drain on their own. These may need to be drained by a physician, and you may have to take antibiotics.