When synovial fluid is aspirated, 1 ml or more should be placed into a sterile tube for possible culture and a similar quantity into a heparinized tube for cell count. The remainder can be used for other procedures. A cell count is performed on anticoagulated fluid using 0.9% saline as the WBC pipette diluent (the usual diluent, Turk’s solution, contains acetic acid, which coagulates synovial fluid mucin and produces clots that interfere with accuracy of the WBC count).

Synovial fluid tests

Mucin clot. The Ropes test for mucin clot is performed by adding a few drops of aspirate to 10-20 ml of 5% acetic acid in a small beaker. After 1 minute, the beaker is shaken. A well-formed clot remains compact; a poor clot is friable and shreds apart easily. In general, noninflammatory arthritides form a good clot. In noninfectious inflammation, the clot may be good to poor, whereas the clot of acute bacterial infection is poor.

Viscosity (string sign). Aspirate is allowed to drip slowly from the end of a needle. The length of the strand formed by each drop before it separates is the end point. In normal fluid and in noninflammatory arthritides the strands string out more than 3 cm. In acute inflammatory conditions fluids drip with little, if any, stringing.

Synovial fluid glucose. Synovial fluid glucose value is usually within 10 mg/100 ml (0.5 mmol/L) of the serum glucose value and is always within 20 mg/100 ml. A blood specimen for glucose should be obtained as close as possible to the time of joint aspiration. The patient should have fasted at least 6-8 hours to achieve baseline values and to compensate for delay in glucose level equilibration between blood and synovial fluid. In degenerative arthritis, synovial fluid glucose levels are usually normal. In acute inflammatory noninfectious arthritis (SLE, RA, gout, etc.) there may be a mild or even a moderate decrease (up to 40 mg/100 ml below serum levels). In acute bacterial (septic) arthritis there typically is a marked decrease of more than 40 mg/100 ml (2.22 mmol/L) below serum glucose levels, but a decrease of this extent is said to occur in only about 50% of cases.

Microbiologic studies of synovial fluid. Gram stain is reported to demonstrate organisms in 40%-75% of patients with septic arthritis (literature range, 30%-95%), more often with gram-positive than with gram-negative bacterial infection. However, Gram stain is not always easy to interpret, especially when only a small number of organisms are present, due to debris from the joint that may take up some of the stain. Culture of synovial fluid is the mainstay of diagnosis in septic arthritis. Since gonococci are a major cause of joint infection, provision should be made for the special conditions necessary to culture these organisms. It has been reported that blood cultures may be positive and synovial fluid cultures negative in 15% of patients with septic arthritis. Previous antibiotic therapy considerably decreases the chance of diagnosis by culture.

Synovial fluid white blood cell count. There is considerable overlap between WBC counts of noninflammatory conditions and noninfectious inflammatory diseases in the area of 500-2,000 WBCs/ mm3 (0.5-2.0 x 109/L) and between infectious disease and noninfectious inflammatory conditions in the area of 10,000-100,000 WBCs/mm3 (10-100 x 109/L). Thus, only very high or very low WBC counts are of great help without other information. A very high percentage of neutrophils (>75%, usually >90%) is said to occur in most cases of untreated acute bacterial infectious arthritis. A neutrophil percentage of more than 90% may therefore be a more reliable indicator of bacterial infection than cell count, synovial glucose level, or Gram stain (unless the cell count is >100,000, the synovial glucose level is >40 mg/ 100 ml below the fasting, simultaneously drawn serum glucose level, or the Gram stain discloses unequivocal organisms).

Other examinations. Other examinations include a serologic test for RA. The RA tests on synovial fluid occasionally show positive results before serum tests. In SLE, LE cells sometimes form in synovial fluid and can be demonstrated on the same Wright’s-stained smear used for differential cell count. Synovial fluid total complement (CH50, Chapter 22), is said to be decreased in SLE and active RA (more often if the patient with RA has a positive latex tube agglutination test result). Total complement is reported to be increased in Reiter’s syndrome. Most other noninflammatory and inflammatory arthritic diseases are associated with normal synovial fluid complement levels. However, there are sufficient exceptions to these general observations that synovial fluid complement assay has not become popular. For example, in one study, 15% of SLE patients had normal synovial fluid total complement values and only about 40% of patients with Reiter’s syndrome had elevated levels.

Synovial fluid specimen collection

An anticoagulated tube is necessary for cell count, WBC differential, and examination for crystals. Heparin is preferred, but liquid ethylenediamine tetraacetic acid can be used. Without anticoagulation, spontaneous small or large clots may form that trap WBCs and alter the cell count. A sterile tube should be used for fluid that is to be cultured; heparin anticoagulant is preferred. This must be sent to the laboratory immediately. A tube without anticoagulant is needed for the mucin test, complement assay, and glucose assay. Some prefer a tube with fluoride anticoagulant for glucose to help preserve the specimen. Fluid for complement assay should be frozen immediately if the assay is not performed within 1-2 hours.