A definitive diagnosis of gout is based upon the identification of monosodium urate (MSU) crystals in joint fluid or a tophus. All synovial fluid samples obtained from undiagnosed, inflamed joints should be examined for these crystals. Under polarized light microscopy, the crystals have a needle-like shape and strong negative birefringence. This test is difficult to perform, and often requires a trained technician. The fluid must also be examined relatively quickly after aspiration, as temperature and pH affect their solubility.