<unk>  may be diagnosed and treated without further investigations in someone with hyperuricemia and the classic acute arthritis of the base of the great toe (known as podagra). Synovial fluid analysis should be done if the diagnosis is in doubt. Plain X-rays are usually normal and are not useful for confirming a diagnosis of early<unk> . They may show signs of chronic<unk>  such as bone erosion. Synovial fluid A definitive diagnosis of<unk>  is based upon the identification of monosodium urate crystals in synovial fluid or a tophus. All synovial fluid samples obtained from undiagnosed inflamed joints by arthrocentesis should be examined for these crystals. Under polarized light microscopy, they have a needle-like morphology and strong negative birefringence. This test is difficult to perform and requires a trained observer. The fluid must be examined relatively soon after aspiration, as temperature and pH affect solubility. Blood tests Hyperuricemia is a classic feature of<unk> , but nearly half of the time<unk>  occurs without hyperuricemia and most people with raised uric acid levels never develop<unk> . Thus, the diagnostic utility of measuring uric acid levels is limited. Hyperuricemia is defined as a plasma urate level greater than 420 μmol/l (7.0 mg/dl) in males and 360 μmol/l (6.0 mg/dl) in females. Other blood tests commonly performed are white blood cell count, electrolytes, kidney function and erythrocyte sedimentation rate (ESR). However, both the white blood cells and ESR may be elevated due to<unk>  in the absence of infection. A white blood cell count as high as 40.0×109/l (40,000/mm3) has been documented. Differential diagnosis The most important differential diagnosis in<unk>  is septic arthritis. This should be considered in those with signs of infection or those who do not improve with treatment. To help with diagnosis, a synovial fluid Gram stain and culture may be performed. Other conditions that can look similar include CPPD (pseudogout), rheumatoid arthritis, psoriatic arthritis, palindromic rheumatism, and reactive arthritis.<unk> y tophi, in particular when not located in a joint, can be mistaken for basal cell carcinoma or other neoplasms. File:Light microscopy of a touch preparation of a<unk>  tophus, showing urate crystals.Light microscopy of a touch preparation of a<unk>  tophus, showing needle-shaped crystals. File:Birefringence microscopy of<unk> , annotated.Uric acid crystals in polarized light, showing negative birefringence, with yellow color when aligned parallel to the axis of the red compensator, and blue when aligned perpendicularly to it. File:Birefringence microscopy of pseudogout, annotated.In contrast, CPPD (pseudogout) displays rhombus-shaped crystals with positive birefringence. File:Gichtfuss im Roentgenbild 002.Gout on X-rays of a left foot in the metatarsal-phalangeal joint of the big toe. Note also the soft tissue swelling at the lateral border of the foot.
Gout
Septic arthritis
Greyout
Rheumatoid arthritis
Blackout 
Calcium pyrophosphate dihydrate crystal deposition disease
Barefoot
Alkaptonuria