Rapid Alpha Defensin Test Helpful for Diagnosing PJI
Quick and accurate: that’s what orthopaedic surgeons want in diagnostic tools to help them determine whether patients presenting with pain after total joint arthroplasty have an infection. A prospective Level I study by Gehrke et al. in the January 3, 2018 issue of The Journal of Bone & Joint Surgery determined that a new lateral flow version of the Synovasure Alpha Defensin Test meets those requirements.
Alpha defensin is a protein secreted by neutrophils in response to bacterial infection, prior to the development of specific immune responses. Earlier research established alpha defensin in synovial aspirates to be an excellent biomarker for periprosthetic joint infection (PJI). The original ELISA-based alpha defensin test is usually sent out for 24-hour processing, limiting its intraoperative utility. However, the lateral flow version of the test (akin to an over-the-counter pregnancy test) was approved for use in Europe—and its results are available in 10 to 15 minutes.
Gehrke et al. compared the rapid test’s results to the diagnostic criteria promulgated by the Musculoskeletal Infection Society (MSIS). According to MSIS criteria, there were 76 joints with PJI among 191 study subjects on whom 195 joint aspirations were performed. Using that as the benchmark for diagnosis, the authors analyzed results from the rapid alpha defensin tests and found the following performance:
- 92.1% sensitivity
- 100% specificity
- 100% positive predictive value
- 95.2% negative predictive value
- 96.9% overall accuracy
Although the rapid test does not provide information about the identity of specific pathogens, the authors conclude that it “enables surgeons to start proper therapy without delay.” That ability comes at a price, however. In Germany, where this study was performed, each rapid test costs about 400 Euros, which is nearly $500 US.
In a commentary on the study, Garth Ehrlich and Michael Palmer cite another possible cost with the rapid-test scenario. Prior to using any alpha defensin test, physicians must rule out metallosis with MRI, because that non-infectious entity triggers false-positive results.