Chapter 12 - Synovial Fluid

Physiology

• It is a viscous liquid found in the cavities of the movable joints (diarthroses) or synovial joints.

  • The bones in the synovial joints are lined with smooth articular cartilage and separated by a cavity containing the synovial fluid.
  • The joint is enclosed in a fibrous joint capsule lined by the synovial membrane.
  • The synovial membrane contains specialized cells called synoviocytes that secrete a mucopolysaccharide containing hyaluronic acid and a small amount of protein (approximately one fourth of the plasma concentration) into the fluid.
    • The polymerization of hyaluronic acid contributes to the noticeable viscosity to the synovial fluid.

  

• It is formed as a nonselective ultrafiltrate (excluding high molecular weight proteins) of plasma across the synovial membrane.

  • The majority of the chemical constituents have concentrations similar to plasma values.

• It reduces friction and lubricates in the joints, provides nutrients to the vascular-deficient articular cartilage and lessens the shock of joint compression.

Specimen Collection and Handling

• It is collected by needle aspiration called arthrocentesis.
• Normal synovial fluid does not clot; however, fluid from a diseased joint may contain fibrinogen and will clot.
  • To prevent clotting, syringes are moistened with heparin anticoagulants.
  • Powdered anticoagulants should not be used because they may produce artifacts that interfere with crystal analysis.
• Specimens are distributed into the following:
  • a sterile heparinized tube for Gram stain and culture
  • a heparin or ethylenediaminetetraacetic acid (EDTA) tube for cell counts
  • a nonanticoagulated tube for other tests
  • The nonanticoagulated tube for other tests must be centrifuged and separated to prevent cellular elements from interfering with chemical and serologic analyses.
  • a sodium fluoride tube for glucose analysis
• All testing should be done as soon as possible to prevent cellular lysis and possible changes in crystals.

Color and Clarity

Viscosity

• The simplest method to measure viscosity is to observe the ability of the fluid to form a string from the tip of a syringe.
  • A string that measures 4 to 6 cm is considered normal.
  • If it is too viscous, it may need to be pretreated by adding a pinch of hyaluronidase to 0.5 mL of fluid or one drop of 0.05% hyaluronidase in phosphate buffer per milliliter of fluid and incubating at 37°C for 5 minutes to be able to perform other tests such as cell count.
• To measure the amount of hyaluronate polymerization, a Ropes or mucin clot test is performed.
  • When added to a solution of 2% to 5% acetic acid, normal synovial fluid forms a solid clot surrounded by clear fluid.
  • As the ability of the hyaluronate to polymerize decreases, the clot becomes less firm, and the surrounding fluid increases in turbidity.
  • The mucin clot test is reported in terms of good (solid clot), fair (soft clot), low (friable clot), and poor (no clot).
  • The mucin clot test is not routinely performed as little diagnostic information is obtained; however, it can be used to identify questionable specimens.

Cell Counts

• Cells are counted using the Neubauer counting chamber without any dilution unless the specimen is turbid or bloody.
  • Traditional WBC diluting fluid cannot be used because the acetic acid forms mucin clots, so hypotonic saline (0.3%) or saline that contains saponin is a suitable diluent.
  • Normal saline can be used, sometimes with methylene blue added to differentiate RBCs and WBCs.
• WBC counts less than 200 cells/L are considered normal; however, variations depend on the type of disease, the pathogenicity of the agent and antibiotic administered.

Differential Count

• Differential counts should be performed on cytocentrifuged preparations or on thinly smeared slides.
• Specimens should be incubated with hyaluronidase prior to slide preparation.

Cell/Inclusion	Description	Clinical Significance	Picture
neutrophils	polymorphonuclear leukocyte	bacterial sepsis, crystal-induced inflammation
lymphocyte	mononuclear leukocyte	nonseptic inflammation
macrophage (monocyte)	large mononuclear leukocyte, may be vacuolated	normal, viral infection
synovial lining cell	similar to macrophage but may be multinucleated, resembles a mesothelial cell	normal
lupus erythematosus (LE) cell	contains ingested round body	lupus erythematosus
Reiter cell	vacuolated macrophage with ingested neutrophils	Reiter syndrome, nonspecific inflammation
rheumatoid arthritis cell (ragocyte)	neutrophil with dark cytoplasmic granules containing immune complexes	rheumatoid arthritis, immunologic inflammation
cartilage cells	large, multinucleated cells	osteoarthritis
rice bodies	macroscopically resemble polished rice, microscopically show collagen and fibrin	tuberculosis, septic and rheumatoid arthritis
fat droplets	refractile intracellular and extracellular globules, stained with Sudan dyes	traumatic injury, chronic inflammation
hemosiderin	inclusions within clusters of synovial cells	pigmented villonodular synovitis

Crystal Identification

• Examination should be performed soon after collection to ensure that crystals are not affected by changes in temperature and pH.
• Fluid must be examined prior to WBC disintegration as some crystals are found intracellularly.
• Specimens may be first examined under low and high power using a regular light microscope with Wright stains, however this should not replace the use of polarized microscopy.

Crystal	Shape	Significance	Picture
monosodium urate	needles, found extracellularly or within the cytoplasm of neutrophils	gout (impaired metabolism or consumption of purines, chemotherapy, impaired excretion of uric acid)
calcium pyrophsphate	rhombic square or rodded, found within vacuoles of neutrophils	pseudogout (cartilage calcification from degenerative arthritis, endocrine disorders that increase serum calcium)
cholesterol	notched, rhombic plates	chronic inflammation
corticosteroid	flat plates of varied shapes	injections
calcium oxalate	envelopes	renal dialysis
hydroxyapatite (calcium phosphate)	macroscopic red splotches, does not polarize	calcified cartilage degeneration, osteoarthritis
	microscopic thick jagged rods (in electron microscopy)

Chemistry Test

• Synovial fluid is chemically an ultrafiltrate of plasma, chemistry test values are approximately the same as serum values.
  • Simultaneous blood and synovial fluid samples should be obtained, preferably after the patient has fasted for 8 hours to allow equilibration between the two fluids.
• The most frequently requested test is the glucose determination, as markedly decreased values are indicative of inflammatory (group 2) or septic (group 3) disorders.
  • Under these conditions, normal synovial fluid glucose should not be more than 10 mg/dL lower than the blood value.
  • To prevent falsely decreased values caused by glycolysis, specimens should be analyzed within 1 hour or preserved with sodium fluoride.
• Because the large protein molecules are not filtered through the synovial membranes, normal synovial fluid contains less than 3 g/dL of protein (approximately one third of the serum value).
  • Increased levels are found in inflammatory and hemorrhagic disorders; however, measurement of synovial fluid protein does not contribute greatly to the classification of these disorders.
• Elevated synovial fluid uric acid level may be used to confirm the diagnosis when the presence of crystals cannot be demonstrated in the fluid.
  • Measurement of serum uric acid is often performed as a first evaluaton of suspected cases of gout.
  • Fluid analysis for crystals is frequently still required.

Microbiologic Tests

• An infection may occur as a secondary complication of inflammation caused by trauma or through dissemination of a systemic infection.
  • Bacterial infections are most frequently seen; however, fungal, tubercular, and viral infections also can occur.
• Both Gram stains and cell cultures are as organisms are often missed on Gram stain.
  • Patient history and other symptoms can aid in requests for additional testing.
  • Routine bacterial cultures should include an enrichment medium, such as chocolate agar.
  • In addition to Staphylococcus and Streptococcus, common organisms seen are the fastidious Haemophilus species and N. gonorrhoeae.

Serologic Tests

• Serologic testing plays an important role in confirming joint disorders supplementary to serum tests.
• Rheumatoid arthritis and lupus erythematosus are diagnosed in the serology laboratory by demonstrating the presence of their particular autoantibodies in the patient’s serum and synovial fluid.
• Arthritis is a frequent complication of Lyme disease, so demonstration of antibodies to the causative agent Borrelia burgdorferi confirms it.
  • The extent of inflammation can be determined through measurement of the concentration of acute phase reactants such as fibrinogen and C-reactive protein.