How Gout Is Diagnosed

The presence of gout on a joint can often be seen pretty clearly, but a healthcare provider will often want to perform tests to confirm the diagnosis and rule out other causes. As a disease characterized by the deposit of uric acid crystals in the joints, the healthcare provider may want to look for evidence of this by drawing out joint fluid with a needle to examine under the microscope. In some cases, a diagnosis can involve comparing symptoms with a series of lab and/or imaging tests.

Physical Exam

In a great many cases, a gout diagnosis can be made based on a review of your symptoms and medical history. In addition to a physical exam, your healthcare provider will want to have a description of the attack (including how it started and how long it lasted) and explore any ​risk factors that may have contributed to the attack.​

While this may be all that your healthcare provider needs to draw up a treatment plan, additional evidence may be required if this is your first attack or if recurrent symptoms have become severe.

Labs and Tests

The gold standard for making a gout diagnosis is by extracting synovial fluid from a joint and searching for evidence of uric acid crystals (called monosodium urate crystals) under a microscope. Synovial fluid is a thick, light-colored substance that lines the joint and lubricates the space between joints.

The procedure, known as a synovial fluid analysis, starts with an injection of a local anesthetic to numb the soft tissue over the joint. After a few minutes, the healthcare provider will insert a needle into the joint space to extract a sample of fluid, which would then be sent to the lab for analysis. Your healthcare provider may also examine the fluid herself under the microscope. 

In addition to searching for monosodium urate crystals, your healthcare provider will examine you for tophi, hardened lumps of uric acid found in later-stage disease.

Among the other lab tests that may be ordered:

• ​A uric acid blood test may be performed to check for acid levels above 6.8 milligrams per deciliter (although people with low levels can also have gout).
• Urea and creatinine blood tests may also be performed to see if reduced kidney function is contributing to gout or if hyperuricemia (excess uric acid) might be damaging your kidneys.
• ​A urinalysis may be used to check uric acid levels in your urine and assess your risk of kidney stones.

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Imaging Tests

To assist with the diagnosis, the healthcare provider may order imaging tests to evaluate the characteristics of a swollen joint or to check for tophi, crystal deposits, bone erosions, or cartilage loss. Imaging test options include X-ray, computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound.

Each test has its benefits and limitations:

• X-rays may reveal bone erosion and cartilage loss but may not be able to detect problems early on. 
• CT and MRI scans can detect damage to bone and cartilage, as well as larger tophi, but according to a study published in European Radiology, may still may not be able to detect early disease.
• Ultrasound is beneficial as it is portable, readily available, and doesn't use ionizing radiation. Ultrasound can also detect the earliest signs of gout, including crystal deposits, fluid accumulation, and the narrowing of the joint space that comes with cartilage loss. On the downside, they are unable to visualize deeper structures of a joint.

In practice, ultrasound is typically used if you have just begun to experience symptoms or recurrent attacks. Other imaging tests may be ordered based on the history of your symptoms or the severity of your condition.

Differential Diagnoses

While the symptoms of gout may seem definitive by appearance alone, there are two other conditions healthcare providers will look at that have remarkably similar features: pseudogout and septic arthritis.

Gout

Gout typically will have certain physical and diagnostic characteristics that separate it from other diseases, namely:

• Synovial fluid analysis: needle-shaped crystals
• White blood cell count: below 50,000
• Gram stain and culture: negative (excluding a bacterial infection)
• Location: typically base of big toe, mid-foot, or ankle

Pseudogout

Pseudogout is a condition where calcium crystals (not monosodium urate crystals) develop in the joint space. The disease can be differentiated from gout in the following ways:

• Synovial fluid analysis: rhomboid-shaped crystals
• White blood cell count: below 50,000
• Gram stain and culture: negative
• Location: typically knee or wrist

Septic Arthritis

Septic arthritis, also known as infectious arthritis, is usually caused by a bacterial infection and can be fatal if left untreated. It differs from gout in the following characteristic ways:

• Synovial fluid analysis: no crystals
• White blood cell count: typically above 50,000
• Grain stain and culture: positive (confirming a bacterial infection)
• Location: typically big joints (knee, hip, or shoulder)