What Is an Arthrocentesis?
When Is an Arthrocentesis Indicated?
Arthrocentesis is primarily used to establish the cause of the joint effusion. Removing joint fluid really has two purposes, though -- 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 As a side benefit of the procedure, after joint fluid is withdrawn, a corticosteroid can be injected into the joint using the same injection site that was used to perform the arthrocentesis.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."
Joint Fluid Analysis Is Comprised of What Tests?
In the laboratory, joint fluid is analyzed for:
- Appearance - observed by 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 - 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 - tested for glucose, protein, and lactic dehydrogenase (LDH). Abnormal joint fluid results which may indicate inflammation or infection are: Glucose - less than 40 mg/dl; Protein - greater than or equal to 3 g/dl; LDH - greater than 333 IU/L
- Other analyses - Joint fluid is observed in a test tube after one hour for formation of a fibrin clot (fibrin is the protein formed during normal blood clotting). Any clot indicates there is a problem with the synovial membrane (a layer of tissue that lines the joint). Another test known as the mucin clot test (acetic acid is added to synovial fluid) estimates the production of hyaluronate (a component of cartilage).
What Arthritic Conditions Can Be Identified Using Arthrocentesis?
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
- hemarthrosis (bleeding into joint space)
What Specifically Points to Osteoarthritis in Arthrocentesis?
In the joint fluid analysis, the cell count and differential count distinguish between non-inflammatory and inflammatory effusions. In other words, the number and type of blood cells found in the joint fluid determine whether the effusion (abnormal amount of fluid on the joint) was caused by an inflammatory or non-inflammatory condition. 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 mucin
- Crystals: Basic calcium phosphate (BCP) crystals, Apatite crystals
- White 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.
Arthrocentesis of the Knee. Thomsen TW et al. The New England Journal of Medicine. May 11, 2006.
Synovial Fluid Analysis. p.140-143. Primer on the Rheumatic Diseases. Edition 12. Arthritis Foundation. Accessed April 23, 2008.