From the Guidelines
Synovial fluid analysis is a crucial diagnostic procedure for identifying the cause of joint inflammation or pain, and it should be performed when clinical judgment indicates that diagnostic testing is necessary in patients with possible acute gout, as recommended by the American College of Physicians 1. The test involves collecting fluid from the affected joint through arthrocentesis and analyzing it for appearance, viscosity, cell count, crystal presence, and culture.
- Normal synovial fluid is clear to pale yellow, highly viscous, has fewer than 200 white blood cells per cubic millimeter, and contains no crystals or bacteria.
- In inflammatory conditions like rheumatoid arthritis, the fluid appears cloudy with increased white blood cells.
- Infectious arthritis shows purulent fluid with very high white cell counts and possible bacterial growth on culture.
- Osteoarthritis typically shows clear to slightly cloudy fluid with mildly elevated cell counts.
- Crystal-induced arthropathies like gout or pseudogout can be diagnosed by identifying specific crystals under polarized light microscopy, as supported by the European League Against Rheumatism evidence-based recommendations 1. This test is particularly valuable when the diagnosis is unclear, infection is suspected, or to differentiate between inflammatory and non-inflammatory joint conditions.
- The results guide appropriate treatment, whether antibiotics for infection, anti-inflammatory medications for inflammatory arthritis, or specific therapies for crystal-induced conditions.
- The 2018 updated European League Against Rheumatism evidence-based recommendations for the diagnosis of gout suggest that searching for crystals in synovial fluid or tophus aspirates is recommended in every person with suspected gout, because demonstration of MSU crystals allows a definitive diagnosis of gout 1.
- Additionally, the Infectious Diseases Society of America clinical practice guidelines recommend that a diagnostic arthrocentesis should be performed in all patients with suspected acute prosthetic joint infection, unless the diagnosis is evident clinically and surgery is planned and antimicrobials can be safely withheld prior to surgery 1.
From the Research
Analysis of Synovial Fluid
The analysis of synovial fluid is a valuable tool in the diagnosis and treatment of joint disease, providing useful diagnostic information in terms of the degree of joint inflammation and presence of haemarthrosis 2, 3, 4, 5, 6.
- The gross appearance of synovial fluid can provide useful diagnostic information, with characteristics such as color, volume, and viscosity being important to describe 5.
- Microscopic analysis of synovial fluid confirms the presence of inflammatory or infectious processes and allows for the detection and identification of crystals 2, 3, 5.
- The use of polarized light microscopy is fundamental for the analysis of synovial fluid and the identification of crystals, with the detection of monosodium urate and calcium pyrophosphate dihydrate crystals allowing for a precise diagnosis of gout and calcium pyrophosphate crystal-related arthritis 2, 3, 5.
- Microbiological studies of synovial fluid, including gram stain and culture, are key to the confirmation of infectious conditions 2, 3, 4, 6.
- The analysis of synovial fluid can be used to establish a diagnosis of septic arthritis, with a threshold level of 50,000-100,000 leukocytes per microliter and a percentage of polymorphonuclear cells of 90% being indicative of septic arthritis 6.
- The detection of procalcitonin and lactate in synovial fluid may be useful in establishing a septic genesis of arthritis 6.
Components of Synovial Fluid Analysis
The complete study of synovial fluid includes:
- Macroscopic analysis: color, volume, and viscosity 5
- Microscopic analysis: detection of crystals, inflammatory cells, and infectious agents 2, 3, 5
- Specific stains: identification of non-birefringent crystals such as calcium hydroxypatite 5
- Microbiological studies: gram stain, culture, and detection of procalcitonin and lactate 2, 3, 4, 6