Synovial Fluid Analysis

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Synovial fluid analysis can be helpful in evaluating a febrile patient with an acute joint to rule out septic arthritis or acute monoarthritis. Synovial fluid should be analyzed for white blood cell (WBC) count differential, cultured, and tested with polarized light microscopy for crystals. Purulent synovial fluid with greater than 90% polymorphonuclear leukocytes (PMNs), low viscosity, and turbid clarity can be caused by infection or crystal arthropathy (gout or pseudogout). Urate crystals are needle-shaped and negatively birefringent; calcium pyrophosphate dihydrate crystals are rhomboidal and weakly positively birefringent. Noninflammatory fluids generally have a clear appearance, normal viscosity, fewer than 2000 WBCs/mm3, and less than 75% PMNs (Table 32-1).

Synovial fluid analysis should always be performed on freshly obtained fluid. A simple bedside test is to attempt to read newsprint through the synovial fluid; newsprint can be read through noninflammatory fluid (Fig. 32-5). Traditional tests on synovial fluid that are of limited or no value include measurement of glucose, lactate, and protein levels; subjective determination of viscosity; mucin clot test (examining the friability of the precipitate formed by mixing synovial fluid with dilute acetic acid); and immunologic tests. When looking for crystals and infection, direct examination, Gram stain, culture, and WBC count with differential are the only tests worth performing on synovial fluid. Inflammatory synovial fluid must be considered secondary to infection until proved otherwise by culture. The presence of crystals in the joint does not exclude the possibility of joint infection.

Synovial biopsy can facilitate a diagnosis in some settings. Arthroscopy has greatly simplified the acquisition of synovial tissue. This might be helpful in the diagnosis of granulo-matous disease or infiltrative processes such as lymphoma, metastatic disease, or amyloidosis (Klippel, 2001).

Joint Aspirate Fluid

Figure 32-5 Synovial effusions. A, Normal or edema fluid is clear, pale yellow, or colorless. Print is easily read through the tube. B, Fluid from noninflammatory joint disease is yellow and clear. C, An inflammatory effusion is cloudy and yellow. Print may be blurred or completely obliterated, depending on the number of leukocytes. The effusion is translucent. D, A purulent effusion from septic arthritis contains a dense clump that does not even allow light through the many leukocytes. E, Hemorrhagic fluid is red. The supernatant may be darker yellow-brown (xanthochromic). A traumatic tap is less uniform and often has blood streaks. (From Schumacher HR. Synovial fluid analysis and synovial biopsy. In Kelley WN, Harris ED, Ruddy S, et al [eds]. Textbook of Rheumatology, 5th ed, vol i. Philadelphia, Saunders, 1997, pp 609-625.)

Figure 32-5 Synovial effusions. A, Normal or edema fluid is clear, pale yellow, or colorless. Print is easily read through the tube. B, Fluid from noninflammatory joint disease is yellow and clear. C, An inflammatory effusion is cloudy and yellow. Print may be blurred or completely obliterated, depending on the number of leukocytes. The effusion is translucent. D, A purulent effusion from septic arthritis contains a dense clump that does not even allow light through the many leukocytes. E, Hemorrhagic fluid is red. The supernatant may be darker yellow-brown (xanthochromic). A traumatic tap is less uniform and often has blood streaks. (From Schumacher HR. Synovial fluid analysis and synovial biopsy. In Kelley WN, Harris ED, Ruddy S, et al [eds]. Textbook of Rheumatology, 5th ed, vol i. Philadelphia, Saunders, 1997, pp 609-625.)

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