Type of test: Fluid analysis
 Normal findings
 Negative
Test explanation and related physiology
Infections in prosthetic joints can occur any time after the joint replacement surgery. Infection is a common cause of failed arthroplasties. The diagnosis of periprosthetic joint infection (PJI) remains a serious clinical challenge. Treatment may require prolonged use of parenterally administered antibiotics followed by complex surgery. Therefore it is important that diagnosis is accurate and timely. Diagnosis of PJI is often hampered by administration of antibiotics before or during joint fluid sampling. These antibiotics can inhibit bacterial growth and cause false-negative results.
PJI is usually considered when joint pain worsens after replacement. Joint fluid analysis is not very reliable because of lack of sensitivity and specificity. Advanced joint fluid stains, serology (IgG, IgM), and molecular testing for bacterial DNA sequencing to identify bacteria or the body’s response to bacteria have been developed to aid the diagnosis of PJI. Unfortunately, these tests require significant pretest care of the specimen, are time consuming, and too expensive to apply to the population of those potentially affected by PJI. Other testing, such as C-reactive protein  and sed rate , are not specific enough. Imaging (i.e., CT scanning), while more reliable has a low specificity.
Alpha defensin is a protein that is produced by WBCs in joint fluid in response to PJI. An immunoassay to identify this bio marker can accurately identify nearly all incidences of PJI. This testing, however, cannot identify the specific infecting agent or its sensitivity to antibiotics. While synchronously occurring inflammatory disease (e.g., rheumatoid arthritis) can confound other testing for PJI, alpha defensin levels are not affected. A positive alpha defensin test result in the presence of a low C-reactive protein may represent a false-positive result and should be questioned.
Interfering factors
 • Fresh blood in the synovial fluid can alter test results.
Procedure and patient care
 Before
* Explain the procedure to the patient. Fasting is not required.
 • Obtain an informed consent if indicated.
During
 • After aseptically cleaning and locally anesthetizing the area, a needle is inserted into the affected joint space to remove fluid.
 • A few cubic centimeters of synovial fluid is injected into tubes provided by the central laboratory.
 • The specimen is sent to the central laboratory.
 After
* Assess the joint for pain, fever, and swelling. Teach the patient to look for signs of infection at home.
• Apply ice to decrease pain and swelling.
 Abnormal findings
 PJI