Getting Fluid Out of a Joint

knee joint injection
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Arthrocentesis is a medical procedure in which a needle is inserted into a joint in order to remove fluid. The purpose of performing arthrocentesis is usually twofold: first, the removal of excess fluid will usually decrease symptoms of pain and improve joint mobility; second, the fluid removed can be used to diagnose the problem that is causing your symptoms.

Tests on Joint Fluid

A number of different tests can be performed on a sample of joint fluid to look for signs of infection, inflammation, or other specific causes of pain.

  The most common test is to analyze the cell count of joint fluid.  Cells found within joint fluid will give your doctor an indication of whether or not the body is simply making excess normal fluid, or if there is a problem of infection or inflammation.

Specifically, the white blood cell count (WBC) is the most helpful cell type.  Normal joint fluid will have very few white blood cells.  Joints with inflammation may have hundreds to thousands of white blood cells.  Joints with infection (a septic joint), often has tens of thousands of white blood cells.  Because of concerns with infection in the setting of a replaced joint, even mildly increased white blood cell counts raise the possibility for joint replacement infection.

A fluid sample can also be microscopically analyzed for infection by looking for the infection under a microscope (gram stain) or trying to grow the bacteria (culture).

  These tests are helpful to determine if infection is causing the joint fluid accumulation, and can also help to direct appropriate treatment.

Finally, your doctor is likely to check for crystals in the fluid.  Crystals are commonly found in inflammatory conditions such as gout and pseudogout.  These crystals can often be seen in these conditions when the joint fluid is assessed under the microscope.

There are other tests that can be performed, but these are the most commonly performed tests to evaluate a sample of joint fluid.  The results of these tests are not instantaneous, and may take several days.  Specifically, trying to grow bacteria often takes several days to detect infection.

Risks of Joint Fluid Removal

When performing arthrocentesis it is of utmost importance to observe strictly sterile conditions as joint fluid is prone to becoming infected if bacteria are introduced. For this reason the skin is cleaned well and sterile instruments are used to aspirate the joint fluid.

One of the major concerns about placing a needle inside any joint is the possibility of introducing infection into that joint.  While this is not a common complication, if it does occur it can have serious consequences, and even require surgery to clean out the infection.

Also Known As: joint fluid aspiration, joint tap, synovial fluid aspiration

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