Gout is a type of arthritis that happens when uric acid crystals form in a joint. These crystals can form when blood levels of uric acid are high. The underlying cause of gout is unknown, but it tends to run in families. Any joint may be involved, but the joint where the big toe meets the foot is the most common site. During gout attacks, which tend to occur periodically, the joints become swollen, red, and painful. To diagnose gout, a physician puts a needle into the affected joint and removes some joint fluid (synovial fluid), then uses a microscope to look for uric acid crystals in this fluid. The diagnosis is more difficult if a person is not in the middle of a gout attack.