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Aspiration of Joints-Tests That Orthopedic Surgeons Should Think About

When you inject steroids into a joint, it is better to aspirate the joint first to see the color of the fluid. You do not want to inject steroids when the patient has infection. Aspiration and analysis of joint fluid is the best method for diagnosis of possible infection. Aspirate the joint for:

Cell count with differential
Glucose level
Gram stain
Crystals
Culture and sensitivity
Cell Count with Differential

Synovial cell count greater than 50,000 usually points to infection.
Lower count may be present in infected cases.
If you have more than 100,000 cells, this means that the join is infected (join is septic).
In infection, the synovial fluid leukocytic count is rarely below 20,000 cells.
In 50% of patients with septic arthritis, the WBC count is greater than 50,000. The normal synovial fluid is only 2 mL, and it has less than 200 leukocytes. With degenerative arthritis, the fluid is considered noninflammatory and the cells are less than 2000. With inflammatory synovitis or inflammatory arthritis such as rheumatoid arthritis, gout, pseudogout, or Reiter’s syndrome, the cells will be greater than 2000 leukocytes. Suspect infection if the leukocyte number increases.


Trauma

Less than 5,000 cells.
Usually the patient has a large hematoma and swelling of the knee after trauma.
With aspiration of the knee you will find blood

The chance of ACL tear is about 70%.
If it is nontraumatic recurrent hemorrhagic effusion, then suspect Pigmented Villonodular Synovitis (PVNS). Differential that is 90% PMN, this can indicate infection or crystal induced synovitis, even if the leukocytic count is low. In general, if the differential is more than 50% PMN cells, this usually occurs in inflammatory and greater than 90% usually occurs in infection. In infection, the synovial glucose level is less than half of the serum (bacteria consumed it). Gram staining identifies the affecting organism in about 1/3 of the times, so it really gives a low yield. Gram staining is not a very good test. Gram-negative stain does not mean that there is no infection. When you aspirate the fluid, look for crystals under the microscope. You need to analyze the synovial fluid for crystals and the presence of crystals does not rule out the presence of infection. Infection can occur in the presence of gout and pseudogout in about 5% of the cases. Clinically, it is difficult to differentiate infection from crystal induced synovitis. The clinical presentation is almost similar. Sometimes the color of aspirated fluid is similar and you cannot tell if it is gout or an infection. In aspirate that is cloudy or purulent, look for crystals. Examine the fluid for crystals. Gout will give you needles.  It is negative birefringent crystals (urate crystals). Pseudogout is a metabolic disease where calcium pyrophosphate dehydrate crystals (CPPD) are formed within the joint space. Pseudogout crystals are rhomboid shaped and positively birefringent. Cultures should be performed especially if there is a concern about infection, despite the presence of crystals. The color of the fluid may help. Fluid with a light, straw color is usually noninflammatory. If the fluid is yellowish or greenish in color, it is inflammatory. If the fluid is creamy, yellow, or opaque in color, this is usually infection. When you do a culture, do aerobic culture and anaerobic culture. Consider slower growing organisms like Propionibacterium acnes (P acne) that may take a few weeks to grow up on a culture. With these septic joints, in older patients, you may want to consider an echocardiogram for endocarditis. You may also want to do some labs for immunosuppressive patients, HIV, or Hepatitis B.
 

Author
Nabil Ebraheim, MD

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