Synovial Fluid Overview

Synovial Fluid

• 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.

Synovial Fluid Formation

• 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.

Synovial Fluid Functions

• Supplies nutrients to articular cartilage.
• Lubricates joints, reducing friction between bones during movement.

Reasons for Analysis

• Infection
• Hemorrhage
• Degenerative disorders (arthritis)
• Inflammatory disease (e.g., SLE)

Synovial Fluid Collection

• 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.

Synovial Fluid Examination

• 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.

Classification of Synovial Fluid

• 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.

Synovial Fluid Color and Clarity

• 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).

Synovial Fluid Viscosity

• 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).

Synovial Fluid (Hematology)

• 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%).

Microscopic Exam for Crystals

• Use regular and polarized light.
• Crystals: Intra- or extra-cellular.
  • Monosodium urate (gout arthritis)
  • Calcium pyrophosphate (pseudo gout)
  • Cholesterol (chronic inflammatory)
  • Hydroxyapatite (calcified cartilage degeneration)
  • Corticosteroid (drug injections)
  • Calcium oxalate, Apatite

Synovial Fluid Chemistries

• 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.

Microbiology

• 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