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.

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.

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.