Synovial Fluid Overview

Study Notes

Referred to as “joint fluid,” a viscous, mucinous liquid found in the cavities of movable joints (diarthroses) or synovial joints.
Lubricates most joints.
Joints are encased in a fibrous capsule lined by a synovial membrane.
The synovial membrane contains specialized cells called synoviocytes.

An ultrafiltrate of plasma across the synovial membrane.
Synoviocytes secrete a mucopolysaccharide containing hyaluronic acid and a small amount of protein into the fluid, making it noticeably viscous.
Contents include hyaluronic acid, mucopolysaccharides, a limited amount of plasma protein (about one-fourth the plasma concentration), and glucose and uric acid levels equivalent to plasma.

Arthrocentesis is the process of collection.
Fluid volume varies with joint size; normal adult knee fluid is less than 3.5 mL, but can increase to greater than 25 mL with inflammation.
Normal synovial fluid does not clot; diseased joint fluid may contain fibrinogen and clot.
Collection requires specific tubes:
- Sterile heparinized tube for Gram stain and culture
- Ethylenediaminetetraacetic acid (EDTA) tube for cell counts
- Non-anticoagulated tube for crystal evaluation and chemistries/immunological tests
- Sodium fluoride tube for glucose analysis.

Macroscopic: Volume, color & clarity, inclusions, viscosity, clotting, and mucin clot.
Microscopic: Cell counts, differential, crystals, and cytology.
Chemical: Protein, glucose, lactic acid, and uric acid.
Other: Aerobic and anaerobic cultures.

Normal: Colorless to pale yellow (straw) and clear.
Non-inflammatory: Includes degenerative joint diseases.
Inflammatory: Includes immunologic disorders (e.g., lupus, rheumatoid arthritis, gout crystals).
Septic: Includes microbial infections.
Hemorrhagic: Includes traumatic injury, tumors, hemophilia, and anticoagulant overdose.

Normal: Colorless to pale yellow (straw) and clear; resembles egg white.
Abnormal: Bloody (hemarthrosis, traumatic tap), white/opaque with turbidity (indicating pus cells or debris).

Measured by string test and Rope's test (mucin clot).
String test: evaluates the ability of the fluid to form a string. Normal = 5cm; 4-6cm is normal.
Rope's test: add 2-5% acetic acid to synovial fluid; measures degree of hyaluronate polymerization. Results reported as: Good/normal (solid clot), Fair (soft clot), Poor (no clot).

Cell Counts:
- 0 RBCs/uL
- <200 WBC/uL
- Allow a longer time for cells to settle before counting.
- Use a saline diluent; acetic acid diluent will clot the sample.
Differential Count:
- Performed on cytocentrifuged preparations or thinly smeared slides.
- Incubate with hyaluronidase prior to preparation, stain with Wright’s stain.
- Normal includes monocytes (48%), macrophages (10%), lymphocytes (<24%), neutrophils (<7%), and synovial lining cells (4%).

Total protein: Normal value (NV) = 1.07–2.13 g/dL. Increasing values suggest inflammatory conditions or joint hemorrhage.
Glucose: Similar to current blood levels. Decreased in inflammation or sepsis.
Lactate: Assists in differentiating septic and inflammatory arthritis.
Uric acid: Increased in gouty arthritis; if gout suspected but no crystals, need uric acid level.

Gram stain, acid-fast stains, and cultures.
Certain organisms associated with age groups.
- Children (H. influenzae)
- Adults (16-50 years): Staph., Strep. pneumoniae, Strep. pyogenes, Neisseria gonorrhoeae
- Adults (>50): Staph. aureus
Serology (serum results more reliable; not done often for diagnosing RA or LE): Autoantibodies