It is difficult to clinically differentiate between gout and pseudogout. Because gout and pseudogout crystals are composed differently, it must be established which crystal type is causing the joint inflammation. Crystal deposits associated with pseudogout are made primarily of calcium, unlike gout crystals, which are made of uric acid. Furthermore, attacks of gout typically occur in joints of the toes and feet, whereas pseudogout typically occurs in the knees.
Awareness of these patterns alone is insufficient in making a distinction between gout and pseudogout; further testing is required. Usually, the only difference is the type of crystal in the joint. Pseudogout crystals extracted in an arthrocentsis are positively birefringent (able to split a ray of light in two) under a polarized light microscope, but gout crystals are negatively birefringent. Trauma, surgery or illness may cause attacks of gout, pseudogout or a combination of the two.