Indications
Arthrocentesis is primarily used to establish the cause of the joint effusion. Removing joint fluid has two purposes, a therapeutic as well as a diagnostic purpose. The removal of joint fluid may also relieve pain and pressure on the joint. As a side benefit of the procedure, after joint fluid is withdrawn, a corticosteroid can be injected into the joint using the same injection site that was used to perform the arthrocentesis. It’s efficient and essentially “kills two birds with one stone.”
Testing
In the laboratory, joint fluid is analyzed for:
Appearance: The fluid is observed by the human eye for color and clarity. Normal joint fluid is viscous (sticky) and appears clear to light yellow. Cloudy joint fluid is abnormal and suggestive of inflammation or an infection. Bloody joint fluid is also abnormal and may be caused by trauma to the joint. Microscopic examination: The joint fluid is examined under a microscope for the presence of blood cells, crystals, and bacteria. Normal joint fluid has no or few blood cells. Large numbers of red blood cells indicate bleeding in the joint. Large numbers of white blood cells can occur with infection, inflammatory arthritis, gout, or pseudogout. If red blood cells or white blood cells are observed, a cell count can be performed. Crystals are abnormal in joint fluid. Uric acid crystals indicate gout; CPPD crystals occur with pseudogout. Bacteria in joint fluid is also abnormal and indicative of infection. Chemical analysis: The joint fluid is tested for glucose, protein, and lactic dehydrogenase (LDH). Abnormal joint fluid results may indicate inflammation or infection.
Conditions
Arthrocentesis identifies the cause of joint effusion and swelling and based on the results of joint fluid analysis, the following conditions may be suspected:
Septic arthritis, especially non-gonococcal bacterial arthritis Other infectious arthritis sources such as gonococcal infections, tuberculosis, fungal infections, Lyme disease Crystal arthropathies, gout, and pseudogout Rheumatic disorders Osteoarthritis Trauma Hemarthrosis (bleeding into joint space)
Osteoarthritis
In the joint fluid analysis, the cell count and differential count distinguish between non-inflammatory and inflammatory effusions. Non-inflammatory effusions are suggestive of osteoarthritis or trauma to the joint. Inflammatory effusions could also be septic arthritis or crystal-induced arthritis.
A joint fluid cell count and differential count that is indicative of osteoarthritis looks like this:
Appearance: Clear fluid, high viscosity, and good mucinCrystals: Basic calcium phosphate (BCP) crystals, Apatite crystalsWhite Blood Cell Count (WBC): Non-inflammatory fluid: 200-2000 WBC/mm3; WBC count usually less than 500 cells (mostly mononuclear, a certain type of white blood cell)
Since the knee joint is the largest synovial cavity (joint space) in the body, it is the likely site of significant joint effusion and a common site for arthrocentesis. From the joint fluid that is drained and analyzed, important diagnostic information can be gathered.
