Diagnosis is made by an experienced doctor (rheumatologist) through a detailed history and physical examination for signs of joint and tendon inflammation, psoriasis and nail changes. Your doctor will also look for psoriasis nail lesions and skin lesions that might be in hidden places (under the hairline, arm pit, at the back, around the navel). Blood tests, X-rays, ultrasound or magnetic resonance imaging (MRI) of the joints are useful to confirm the diagnosis. If big joints (e.g. the knee joint) are swollen, your doctor may aspirate the fluid with a needle and send it for special examination. This will help the doctor to differentiate between infective, degenerative or inflammatory joint disease.