Joint-Fluid Crystal

Gout and pseudogout are the 2 most common crystalline arthropathies and are caused by deposition of monosodium urate (MSU) and calcium pyrophosphate dihydrate (CPPD) crystals, respectively. This causes inflammation, pain, and destruction of the joint. These 2 pathologies are often diagnosed clinically but can only be done so with certainty by microscopic analysis of synovial fluid.

A normal joint-fluid aspirate is negative for any crystals.

Some other parameters for joint-fluid reference ranges are as follows ^[1] :

• Synovial appearance - Clear
• Synovial color - Pale yellow
• Synovial red blood cell (RBC) count - 0
• Synovial white blood cell (WBC) count - 0-150/mm ³
• Synovial neutrophils - 7%
• Synovial lymphocytes - 24%
• Synovial monocytes - 48%
• Synovial macrophages - 10%
• Glucose - Same as fasting blood glucose
• Protein - 1-3 dL
• Lactic acid dehydrogenase - < 25 mg/dL
• Uric acid - 6-8 mg/dL
• Gram stain - Negative

Note that the exact ranges can vary per lab.

Diagnosis of crystalline arthropathy like gout and pseudogout is often clinical. However, they can be confirmed with the aid of joint-fluid analysis.

Gout

See the list below:

• As seen under the microscope, monosodium urate (MSU) crystals are long, thin and pointed, and are negatively birefringent. These may be within the cell, outside the cell, or within tophi.

• White blood cell counts in the synovial fluid are usually within the inflammatory range of 10,000-20,000/µL.

Pseudogout

See the list below:

• Calcium pyrophosphate dihydrate (CPPD) crystals are shorter and less sharp than MSU crystals. In contrast, they are positively birefringent under polarized light. Crystals can be within or outside the cell.

• White blood cell counts in the synovial fluid are usually within the inflammatory range of 10,000-20,000/µL.

Infection in the joint should always be in the differential when considering gout and pseudogout, so the fluid should be sampled sterilely and sent for gram stain and culture. ^[2]

See the list below:

• Specimen type - Synovial fluid from joint

• Container - Any sterile container such as that used for urinalysis. (Joint fluid needs no additive in the container.)

• Collection method - Sterile needle aspiration (Sterile collection also allows the fluid to be sent for culture as well, depending on clinical suspicion for infection. Lastly, it minimizes risk of introducing bacteria into the joint.)

• Specimen volume - At least 1 mL

Related tests are as follows:

• Gram stain

• Culture and sensitivity

• Cell count

• Cell differentiation

• Color and character

Gout and pseudogout are the two most common crystalline arthropathies and are caused by deposition of monosodium urate (MSU) and calcium pyrophosphate dihydrate (CPPD) crystals, respectively. Such deposition causes inflammation, pain, and destruction of the joint. These two pathologies are often diagnosed clinically. Some have advocated for ultrasonography to play a role in the screening process. ^[3] However, the criterion standard of diagnosis is still microscopic analysis of synovial fluid. Although these crystals have been found in other inflammatory arthritides such as rheumatoid arthritis, they still have an unknown role in these disorders. ^[4]