When It’s Not Gout, It May Be Pseudogout
Gout is sometimes confused with other forms of arthritis because the symptoms – acute and episodic attacks of joint warmth, pain, swelling, and stiffness – can be similar. One form of arthritis often confused with gout is called pseudogout. The pain, swelling, and redness of pseudogout can also come on suddenly and may be severe, closely resembling the symptoms of gout. However, the crystals that irritate the joint are calcium phosphate crystals, not uric acid. Therefore, pseudogout is treated somewhat differently.
Source: National Institute of Arthritis and Musculoskeletal and Skin Disease – http://www.niams.nih.gov/Health_Info/Gout/default.asp#when