Synovial Fluid Analysis

General Description

Synovial fluid analysis consists of a set of tests that evaluate possible changes in the fluid that lubricates the joints in order to identify the causes of pain or inflammation.

This study determines the underlying process in each case, as joint diseases may be infectious, hemorrhagic, inflammatory, or degenerative.

When is it indicated?

When a patient presents symptoms consistent with a joint disorder, synovial fluid analysis is performed. This test is typically indicated in the presence of:

• Pain
• Redness
• Sensation of warmth
• Swelling
• Accumulation of synovial fluid
• Fever
• Difficulty bending the joint

This study helps diagnose septic arthritis, rheumatoid arthritis, osteoarthritis, gout, among other conditions.

In some cases, synovial fluid aspiration also helps relieve pressure in the joint caused by fluid accumulation.

How is it performed?

Synovial fluid analysis is divided into two main stages. First, a sample is obtained:

• The skin over the joint to be sampled is disinfected.
• In some cases, especially when a large needle is required, local anesthesia is administered to reduce discomfort associated with the puncture.
• The needle is inserted into the joint space, and fluid is aspirated using a syringe until a sufficient amount is obtained for analysis. Typically, two full tubes are collected, one with anticoagulant and one without.
• The skin is disinfected again after needle removal, and a dressing is applied to the puncture site.

Once in the laboratory, the synovial fluid is analyzed through various studies:

• Appearance and physical characteristics: healthy synovial fluid is straw-colored, transparent, and slightly viscous. Any variation may indicate pathology.
• Biochemistry: changes in substances normally present in the fluid are assessed (glucose, uric acid, or protein content).
• Microscopic examination: a drop of fluid, usually after centrifugation, is placed on a slide to determine whether cell counts are normal or altered.
• Culture or Gram stain: performed to detect the presence of infectious agents.

Risks

Synovial fluid sampling may cause pain for a few days, although this typically resolves with reduced physical activity or application of ice.

Although rare, needle insertion to obtain the sample may cause bleeding or infection.

What to expect from synovial fluid analysis

The patient is only present during sample collection. This is an outpatient procedure after which normal activities can be resumed, although with some movement limitations. The patient does not usually experience pain, as anesthesia is administered if the procedure is expected to be painful. On the day of the puncture, informed consent must be signed.

The patient is positioned with the joint exposed in the most appropriate posture to facilitate the specialist’s work. During the procedure, which lasts only a few minutes, the patient must remain as still as possible.

Results are usually available within one to two days, except in cases where culture is performed. Results are considered abnormal in the following situations:

• Appearance: color and texture of synovial fluid may show variations such as:
• Decreased viscosity: inflammatory process
• Turbidity: presence of crystals, erythrocytes, leukocytes, or microorganisms
• Reddish color: presence of blood
• Biochemistry: pathology is present when concentrations of its components vary:
• Low glucose levels: infection or inflammation
• High protein levels: bacterial infection
• High uric acid levels: gout
• Microscopy: synovial fluid contains leukocytes and erythrocytes. Any change in this composition indicates disease:
• Increased erythrocytes: infection, gout, or rheumatoid arthritis
• Changes in the proportion of different leukocyte types: bacterial infections are associated with increased neutrophils, while elevated eosinophils may indicate parasitic or tuberculous arthritis or Lyme disease
• Monosodium urate or calcium pyrophosphate crystals are formed in cases of gout

Specialties in which synovial fluid analysis is requested

Synovial fluid analysis is performed by pathologists at the request of Rheumatology or Pediatric rheumatology.