The major components of synovial fluid analysis are (1) assessing fluid clarity and color, (2) determining the cell count, (3) examining for crystals, and (4) obtaining cultures. When septic arthritis is suspected, a Gram stain should also be performed. Determinations of synovial fluid glucose and protein have little diagnostic value and should not be ordered. Although the viscosity of synovial fluid decreases with inflammation, evaluations of viscosity are not standardized and add little to the diagnostic value of synovial fluid analysis.
Examination of the synovial fluid begins with a visual determination of clarity and color. Although crystals, lipids, and even cellular debris may affect clarity, the major determinant of synovial fluid clarity and color is the cell count. Noninflammatory fluid, such as that associated with osteoarthritis, has a low cell count and is clear. Synovial fluid from moderately inflammatory forms of arthritis, such as systemic lupus erythematosus or mild rheumatoid arthritis, has higher cell counts and is translucent and yellow. Fluid from intensely inflammatory processes, such as septic joints or crystal-induced arthropathies, has very high cell counts and is opaque and white to yellow. Bleeding into a joint leads to a hemarthrosis with characteristic opaque, red synovial fluid.
Normal synovial fluid has <200 white cells/mcL, most of which are mononuclear. In pathologic effusions, the synovial fluid white cell count discriminates between noninflammatory forms of arthritis (<2000 white cells/mcL) and inflammatory arthritis (>2000 white cells/mcL with a neutrophil predominance). The synovial fluid white cell count can be an approximate guide to the cause of the underlying inflammatory arthritis (see below, “Classes of Synovial Fluid”).
Crystal analysis is best performed on a fresh wet preparation with a clean slide and cover slip. Synovial fluid analysis for crystals is performed under polarized light. The strength of birefringence and shape of the crystals are helpful in distinguishing among the different forms of microcrystalline disease.
- Monosodium urate crystals are needle-shaped and negatively birefringent (ie, the crystal is yellow when the long axis of the crystal is parallel to the slow axis of vibration of the red compensator used with polarized lenses to identify crystals under the microscope). Because of their strong birefringence, monosodium urate crystals are easily seen with a polarized light microscope. The sensitivity of an examination for urate crystals in acute gout is >90%.
- Calcium pyrophosphate dihydrate crystals are rhomboid-shaped and positively birefringent (ie, the crystal is blue when the long axis of the crystal is parallel to the slow axis of vibration of the red compensator used with polarized lenses to identify crystals under the microscope). Because they are weakly birefringent, calcium pyrophosphate dihydrate crystals are dim and difficult to detect even with a polarized light microscope.
- Calcium oxalate crystals can be seen in patients with primary oxalosis or in renal failure. These crystals are rod- or tetrahedron-shaped and positively birefringent.
- Cholesterol crystals are rectangular and tend to have notched corners. Lipids form spherules with birefringence in the shape of a Maltese cross. Because the arms of the cross that parallel the slow axis of vibration of the red compensator are blue, these spherules are positively birefringent.
- Hydroxyapatite crystals are not birefringent and form amorphous clumps that stain red with alizarin red S.
- Glucocorticoids from previous joint injections, talc from gloves, and even debris can form birefringent crystals and lead to mistaken diagnoses of microcrystalline disease.
The presence of intracellular crystals in synovial fluid inflammatory cells is diagnostic of a crystal-induced arthropathy. However, this diagnosis does not rule out infection, so it is always wise to culture the fluid from an acute monoarticular arthritis even when crystals are identified. In addition, a patient may have more than one crystal-induced arthropathy. Fifteen percent of patients with gout also have CPPD.
When aspirating a small joint, such as the first metatarsophalangeal joint, it is important to remember that monosodium urate crystals can be identified in interstitial fluid. Even when synovial fluid cannot be drawn into the syringe, enough interstitial fluid for crystal analysis can be pulled into the needle by maintaining negative pressure as the needle is withdrawn. The needle is then removed, the syringe is filled with air, the needle is replaced, and the air is used to express the contents of the needle onto a slide. The small amount of material obtained is often enough to allow detection of monosodium urate crystals.
Gram stain and culture should be performed on synovial fluid from any patient in whom infection is suspected. The sensitivity of synovial fluid cultures for nongonococcal septic arthritis is approximately 90%. Gram stain of synovial fluid has lower sensitivity (in the range of 50–75%) but high specificity. Microbiologic analysis usually is performed on fluid collected in a sterile tube. However, if the aspiration is difficult, material in the needle may be expressed onto a swab and sent for culture and sensitivity studies. Some significant pathogens are difficult to culture. Synovial fluid cultures are usually negative in the early phases of gonococcal arthritis; in cases of mycobacterial infection, cultures may require several weeks of incubation to isolate the causative agent.
Classes of Synovial Fluid
Four classes of synovial fluid have been defined and can serve as a guide to differential diagnosis.
Class I (noninflammatory) synovial fluid is defined by a synovial fluid white cell count of <2000/mcL. Class I fluid is transparent with a color ranging from clear to yellow. Osteoarthritis is the most common cause of class I synovial fluid. Other causes include post-trauma, chondromalacia patella, osteonecrosis, hypothyroidism (often with especially viscous fluid), Charcot arthropathy, amyloidosis, and sarcoidosis (which also can cause inflammatory synovial fluid).
Class II (inflammatory) synovial fluid has white cell counts from 2000/mcL to 75,000/mcL, occasionally up to 100,000/mcL. Polymorphonuclear leukocytes predominate. The appearance of class II synovial fluid ranges from translucent to opaque and is yellow or white; it is characteristic of noninfected, inflammatory forms of arthritis. In systemic lupus erythematosus, white cell counts are usually between 2000 cells/mcL and 30,000 cells/mcL. The cell counts in rheumatoid arthritis and the spondyloarthropathies are typically 5000–50,000 cells/mcL; however, the pseudoseptic presentations of these disorders can generate higher counts (but rarely ≥100,000 cells/mcL). In crystal-induced arthropathies, cell counts of 30,000–50,000 cells/mcL are typical, but ≥100,000 cells/mcL are sometimes observed. Other causes of class II fluid include systemic rheumatic diseases, such as dermatomyositis and mixed connective tissue disease; Still disease; relapsing polychondritis; postinfectious arthritis; and the systemic vasculitides.
Class III (septic) synovial fluid has white cell counts that are often >100,000/mcL, and the appearance is opaque and yellow (sometimes white). Class III synovial fluid is typical of septic arthritis caused by infection with Staphylococcus aureus, streptococci, and gram-negative organisms. Although these infections classically cause very inflammatory fluid (≥100,000 cells/mcL), synovial fluid cell counts can be considerably lower early in the course of the infection, in partially treated infection, or in cases of overwhelming sepsis. Counts <50,000 cells/mcL are common in gonococcal arthritis and in chronic infections, such as those caused by mycobacteria or fungi.
Class IV (hemorrhagic) fluid is red and opaque. In contrast to bloody returns due to a traumatic aspiration, class IV fluid is “defibrinated” and does not clot ex vivo. Class IV synovial fluid is typical of trauma, tuberculosis, pigmented villonodular synovitis, neoplasia, coagulopathies, and Charcot arthropathy.