A joint that suddenly becomes hot, red, severely painful, and swollen, particularly if you also have a fever or feel unwell, may be infected. Septic arthritis is a medical emergency because infection can destroy a joint within days and can spread through the body. This is especially concerning in people with artificial joints, recent joint procedures, weakened immune systems, diabetes, or injecting drug use.
Gout and other forms of inflammatory arthritis can look similar, which is exactly why a hot, swollen joint should be assessed urgently rather than assumed to be a flare of a known condition. A doctor may need to take a sample of fluid from the joint to identify the cause.
Do not wait to see if it improves. Seek urgent care or call your local emergency number if a joint becomes acutely hot, swollen, and painful, particularly with fever, chills, or rapidly worsening symptoms. Prompt antibiotics or drainage may be needed if infection is present.