Synovial fluid, sometimes called joint fluid, may be drawn from a symptomatic joint or bursa during a joint aspiration (arthrocentesis). Examination of the this fluid can help rule out or support certain diagnoses, such as gout, pseudogout, rheumatoid arthritis, or an infection.
There are many ways to analyze synovial fluid, including physical and visual examinations, looking at the joint fluid under a microscope, assessing the fluid’s chemical make-up, and testing the fluid for bacteria, fungi, or other abnormal contents. A doctor will decide what types of analyses should be performed based on the patient’s symptoms and health history.
Physical and Visual Examination of Synovial Fluid
The synovial fluid sample will be checked for characteristics such as:
- Volume. Healthy joints contain just 0.15 to 4.0 mL of synovial fluid, depending on the size of the joint. Too much fluid indicates inflammation.1,2
- Viscosity. Normal synovial fluid is viscous, and less viscous fluid may indicate inflammation.
- Color and clarity. Normal synovial fluid is clear and colorless or straw colored. Abnormal fluid may look cloudy, opaque, and/or colored (e.g. pink or red, indicating blood cells).
Variations in the fluid’s volume, viscosity, color, or clarity may indicate the need for further lab testing.
Microscopic Analysis of Synovial Fluid
A specially trained lab technician will prepare a synovial fluid sample and examine it under a microscope for crystals, white blood cell count, microorganisms, and possible unusual traits.
- Crystals are not seen in normal synovial fluid. The presence of uric acid crystals in the synovial fluid indicates gout. The presence of calcium pyrophosphate crystals in synovial fluid indicates pseudogout. (A trained lab technician can determine the crystal type by its size, shape, and how it refracts light.)
- A white blood cell count that is higher than normal may indicate infectious arthritis, gout, or rheumatoid arthritis. Also important can be the types of white blood cells found in the fluid sample, as this differentiation can provide additional clues for an accurate diagnosis. For example, an extremely high number of white blood cells called neutrophils can indicate a bacterial infection.
- Microorganisms (e.g. bacteria) indicate infection and can sometimes be detected under a microscope, and typically this can be verified with a culture study.
- Red blood cell counts may be high after a traumatic injury and slightly elevated in patients with osteoarthritis. They may also be present if the patient has a bleeding disorder, such as hemophilia.
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Chemical Analysis of Synovial Fluid
- Uric acid content may be higher than normal for people with gout.
- Glucose levels that are lower than normal may indicate infection. Slightly low glucose levels are also sometimes seen in rheumatoid arthritis.
- Lactate dehydrogenase (LDH), an enzyme normally found in synovial fluid, might be at elevated levels in people who have rheumatoid arthritis, infectious arthritis, or gout.
- Protein levels that are higher than normal may indicate an infection.
Assessment of the chemical make-up of the synovial fluid is often done by a lab technician and can be helpful for an arthritis diagnosis.
Cultures, Gram Stains and Other Tests
Bacteria and fungi are not normally seen in healthy synovial fluid and their presence indicates an infection.
Microbiologic studies such as cultures and Gram stains are performed on synovial fluid samples and can help detect and identify microorganisms.
A doctor may order other lab tests not listed above to further ascertain the fluid’s composition and help make or rule out certain diagnoses.
Making a Diagnosis with Lab Results
Lab tests are not infallible and doctors do not use lab test results alone to make a diagnosis. Rather, these results are used in conjunction with information gathered from a physical examination, patient interview, and other sources such as X-rays.
Results are usually available to patients within 1 to 3 days. In an emergency, lab tests might be expedited and results will be available sooner.