Hailed as the most valuable test in rheumatology, synovial fluid analysis provides essential diagnostic information for the appropriate management and treatment of urgent and emergent arthritic conditions.4,30 It has been established as a fundamental component to the complete and appropriate work-up of arthritic diseases. With the possibility of potential joint destruction and chronic disability, the role of synovial fluid analysis in the expedient diagnosis and treatment of acute joint disease cannot be overemphasized.
Controversy exists concerning what constitutes the appropriate guidelines for a “routine” synovial fluid analysis.4,19–21,23,24 This controversy arises from multiple issues that include the clinical scenario, physician competency with appropriate arthrocentesis techniques, the sensitivity and specificity of individual tests, the availability and proficiency of laboratories, and cost. Classification schemes have been established based upon gross, microscopic, biochemical, and microbiological analyses. The most traditional and cited classification for synovial fluid is normal, noninflammatory, inflammatory, septic, or hemorrhagic (Table 77-4).4,19,20,24 Despite the controversy that exists with guidelines and classification schemes, it is critical to differentiate between an inflammatory and noninflammatory process, with the intent of expediting the diagnosis and treatment of a possible infectious etiology.4
A detailed discussion of synovial fluid analysis is beyond the scope of this book. A brief discussion of the most essential components that can be performed in the ED will be presented.
Pathophysiology of Synovial Fluid
Synovium refers to the 1 to 3 cell thick structure that lines the joint space and terminates at the articular cartilage margin.19 This structure overlies a highly vascularized subsynovium, both of which are supported by the dense fibrous joint capsule.19 The synovium produces synovia, an ultrafiltrate of plasma that includes hyaluronate. The synovia serves to lubricate, nourish, and clear the metabolic waste of the avascular articular cartilage.20 Synovial fluid has been demonstrated to possess potent bactericidal activity against the most common gram-positive organisms responsible for septic arthritis.28
The synovium has been described as a double barrier in which molecules must pass through the endothelial microvasculature as well as the synovium and its matrix.20 This double barrier is responsible for the retention of plasma protein. In the presence of an inflammatory process, the barrier is disrupted and protein can leak through the synovium. Difficulty arises in the diagnostic interpretation of protein and smaller molecules (i.e., sodium, chloride, urea, urate, and lactate) found in synovial fluid secondary to the effects of damaged endothelial permeability and variable lymphatic drainage.20,23,24
Gross Analysis of Synovial Fluid
The color of synovial fluid varies depending on the amount of protein, blood, and breakdown products of hemoglobin. Normal synovial fluid usually appears clear to a straw or yellow color. Inflammatory and purulent synovial fluid may appear xanthochromic to white. Hemorrhagic synovial fluid is red and must be distinguished from a traumatic arthrocentesis. A traumatic aspiration usually clots and is more than often nonhomogenous.19,20 A hematocrit may be sent on a bloody aspirate to distinguish between a traumatic tap and hemorrhagic fluid. A vein was pierced by the needle (i.e., traumatic tap) if the synovial fluid hematocrit is equal to the serum hematocrit.20
The following synovial fluid properties observed during bedside gross analysis were found to better predict a potentially septic joint when compared to synovial fluid cell count alone.30
The clarity of synovial fluid refers to the amount and type of particles within the fluid. Normal synovial fluid is usually transparent and newspaper print can be easily read through a glass tube containing this fluid.19 Inflammatory and purulent synovial fluid is translucent to opaque secondary to the presence of leukocytes. Opaque fluid can also represent crystals and other particulate matter. Infected synovial fluid cannot be differentiated from noninfected synovial fluid based on gross appearance alone.4
Synovial fluid viscosity is determined by the intactness and concentration of hyaluronate. Viscosity can grossly be assessed by observing a drop of fluid fall from the tip of the needle. The “string” formed will normally be 5 to 10 cm in length.22 In inflammatory and septic synovial fluid, the hyaluronidase is depolymerized and degraded.20 The string formed in these conditions is shorter, or not formed at all, and the fluid falls as a drop. Processes resulting in a significant effusion without inflammation may also dilute hyaluronate without degrading it and result in decreased viscosity. Clotted white blood cells may enhance viscosity in the presence of inflammation.20 EPs must be cautious with their interpretations of viscosity because even quantitative measures often fail to distinguish between inflammatory and noninflammatory states.4
The mucin clot test evaluates the degree of polymerization of hyaluronate.4,19 The mucin clot test is performed by one of two methods. The first method involves mixing the supernatant of a centrifuged specimen with a few drops of glacial acetic acid. The second method involves mixing 1 mL of synovial fluid to 4 mL of 2% acetic acid. A “good” clot consists of a dense white precipitant that indicates a high degree of polymerization and a high viscosity.19 A “poor” clot consists of little to no precipitate and suggests an inflammatory process that has depolymerized the hyaluronate. Controversy exists concerning the subjectiveness of the clot's endpoint.4
Microscopic Analysis of Synovial Fluid
The total leukocyte count, more than any other test, aids in distinguishing between an inflammatory, noninflammatory, and septic process.20 Although a significant overlap may exist, the total leukocyte count can be used to identify synovial fluid as normal, noninflammatory, inflammatory, or septic (Table 77-4). Using this classification scheme, a total leukocyte count of less than 3000 cells/L is considered noninflammatory. A count between 3000 and 20,000 cells/L is considered inflammatory. The range of 20,000 to 50,000 cells/L may be inflammatory or septic. A count greater than 50,000 cells/L is considered septic until proven otherwise. The overlap between categories is considerable and clinicians must be cautious of basing a diagnosis solely on the total leukocyte count. Depending on the acuteness of the inflammation, several arthritic conditions such as gout, pseudogout, and rheumatoid arthritis may yield a significantly elevated total leukocyte count approaching 100,000 cells/L.21 Immunocompromised hosts and some infectious diseases, such as tuberculosis and Neisseria gonorrhoeae, may have lower absolute counts than expected.4
The differential leukocyte count may further aid in distinguishing between inflammatory, noninflammatory, and septic synovial fluid. The cell count and differential (WBC) of the joint fluid is the best diagnostic test for septic arthritis, while the serum ESR and WBC perform poorer in the identification of infection.37 However, using the traditional cut-off value of 50,000 WBC/mm3 lacks sensitivity enough to safely rule-out a septic arthritis.39 Inflammatory processes generally have greater than 70% neutrophils while septic synovial fluid has greater than 90% neutrophils.19 Again, the overlap can be significant depending on the arthritic process and its acuteness. Crystal-induced processes may present with high neutrophil counts, while immunocompromised hosts, fungal infections, and tuberculosis may present with lower neutrophil counts.4,21 In general, a synovial fluid containing greater than 90% neutrophils in the presence of an elevated total leukocyte count should be highly suspicious of a septic process. High eosinophil counts may suggest parasitic infection, allergic reactions, tumor, or Lyme disease.20,23 High monocyte counts may suggest viral infection (e.g., rubella and hepatitis B) or serum sickness.22
Crystal identification is an essential component of synovial fluid analysis. Crystal identification requires the use of a polarized light microscope with higher-powered lenses and oil immersion capabilities. Crystals can be identified based on their shape, size, and birefringence. Birefringence is defined as the crystals' ability to bend the light passing through it into two distinct directions, negative or positive. The light's ability to bend negatively or positively is transformed into a specific color (yellow or blue) under the polarized microscope.19,20 Caution must be taken as artifact and tissue debris can often imitate birefringent material.
Crystal analysis requires an experienced technician and is rarely if ever performed in the ED. Monosodium urate crystals are commonly seen in gout. These crystals are needle-shaped, 2 to 25 μm in length, and have strong negative birefringence. They may clump together in sheets.22,25 Local anesthetic solutions have the ability to dissolve monosodium urate crystals. Therefore, the joint cavity should not be penetrated with the needle when anesthetizing the skin and subcutaneous tissue.21 Calcium pyrophosphate dihydrate crystals are seen in pseudogout. These crystals are rhomboidal or rectangular, 2 to 10 μm in length, and have weak positive birefringence.22,25 These crystals may be more difficult to detect than monosodium urate crystals because of the weaker birefringence. Cholesterol crystals may present in multiple forms and sizes. They are typically flat rhomboidal plates, 5 to 50 μm in length, and have both negative and positive birefringence.22 Artifact “crystals” can be produced by a variety of substances. Corticosteroids can be detected weeks after injection and have variable shapes but no regular geometric form. Maltese crosses are strong birefringent particles that are secondary to multiple compounds such as talc powder, lipids, calcium oxalate, and dust.22
Biochemical Analysis of Synovial Fluid
As discussed previously under the pathophysiology section, difficulty arises with the interpretation of total protein secondary to the effects of damaged endothelial permeability and variable lymphatic drainage.20 In theory, the damage caused by an inflammatory process should increase the permeability of proteins into the synovial fluid. Multiple studies have shown that protein samples were only able to classify synovial fluid into an inflammatory or noninflammatory process in approximately 50% of the cases.23 Furthermore, the total protein count was unable to differentiate among various groups of arthritides, including rheumatoid arthritis and osteoarthritis.4
Theoretically, inflammatory and infectious processes consume glucose and thus lower the level present in synovial fluid. One study has shown that glucose levels were able to classify synovial fluid into an inflammatory or noninflammatory process in less than 50% of cases.23 In 50% of the septic joints analyzed, the glucose level was not significantly decreased. Another study reports glucose analysis having a sensitivity of 20% and specificity of 84% in detecting inflammatory joint disease.24 Synovial fluid glucose levels can vary from serum glucose levels when taken less than 6 hours after oral intake.24 These studies are just a few of many that confirm synovial fluid glucose levels are not reliable to diagnose or rule out a septic joint.
Other biochemical markers have been studied to elicit a marker to differentiate a septic from a nonseptic joint. These include lactate, lactic dehydrogenase, and numerous immunologic and inflammatory mediators. These biochemical markers, at present, are not sensitive or specific to rule out a septic joint. They are not recommended for routine synovial fluid analysis. Bacterial polymerase chain reaction (PCR) techniques with aspirated joint fluid are currently being studied and are theoretically superior tests, able to identify difficult to culture organisms. However, many drawbacks including high false positives due to contamination limit usage until further studies are done.38
Microbiological Analysis of Synovial Fluid
The Gram's stain is an easily performed test that yields rapid results and can lead to the expedient diagnosis and treatment of a septic joint. The Gram's stain has a sensitivity of 50% to 70% for nongonococcal infections and less than 10% for gonococcal joint infections.24 Although the sensitivity of this test is low, the specificity of the Gram's stain approaches 100%. This makes it an essential component of routine synovial analysis. N. gonorrhoeae is identified as a gram-negative intracellular diplococci. Staphylococcus aureus and Streptococcus are responsible for approximately 70% of nongonococcal septic arthritis and can be identified as gram-positive cocci in clusters and gram-positive cocci in chains, respectively.27 Recent studies have identified an increased prevalence of MRSA-associated osteoarticular pathology such as septic arthritis and subperiosteal abscesses.36 These diagnoses should be considered in patients with and without risk factors for community-acquired MRSA infection.36
Bacterial identification is essential when confronted with the possibility of a septic joint. Synovial fluid cultures have a sensitivity of 75% to 95% for nongonococcal bacteria and 10% to 50% for gonococcal bacteria in the absence of previous antibiotic treatment. Difficulty arises from the low sensitivity of cultures for some organisms, culture methods, specimen preparation, and the length of time for some bacteria to grow.24 Recent advances with the use of polymerase chain reaction techniques have shown increased sensitivity and specificity for detecting N. gonorrhoeae.19,26