Diagnostic Utility of a Novel Point-of-Care Test of Calprotectin for Revision Total Knee Arthroplasty

Calprotectin is a biomarker closely associated with leucocytes in general, and is present in high volumes in neutrophil cells. Calprotectin is also produced by infiltrating monocytes and macrophages, where calprotectin is released upon phagocytosis. In neutrophils, calprotectin is stored intracellularly and are released upon activation of the cell. The determination of number of neutrophils and proportion of neutrophils out of total number of inflammatory cells is a diagnostic strategy commonly used in diagnosis of infection. Upon encounter with a pathogen, neutrophils have several strategies to fight infections , and produce high-levels of calprotectin. Activation of neutrophils, and release of calprotectin, can be for any reason causing activation of the complement system and aseptic Inflammatory responses. Moreover, Calprotectin is a danger associated molecular patterns (DAMP) signal influencing the inflammatory responses. The level of activated neutrophils in PJI provide basis of the presence of calprotectin in the synovial fluid of PJI patients, and thus, for calprotectin as a potential biomarker for PJI. Calprotectin-levels in the synovial fluid do not merely reflect the level of leucocytes and neutrophils present in the synovial fluid, but levels are correlated to the WBC content. Calprotectin is likely to reflect the number of activated cells and surpass the diagnostic accuracy of total WBC counts and neutrophil percentage for PJI diagnosis.

A level of calprotectin of 50 mg/L in the synovial fluid has very good diagnostic accuracy for PJI, supported by area under the curve values of more than 0.9. In a subgroup analysis for patients with chronic PJI, a NPV of 97% was observed. The excellent NPV may assist in the orthopaedic clinic to rule out the presence of infection and consider diagnostic alternatives for aseptic loosening and pain revision of the joint patient. A rapid and accurate distinction between these two causes is important as PJI and aseptic loosening are managed differently with regards to surgical Intervention and followup.

Point of Care Test diagnostics by lateral flow devices provides reliable test results within minutes of sample collection. Currently, Calprotectin can be detected by such lateral flow devices (developed by Orthogenics, Tromsø, Norway). The speed and ease of use of this test allows for diagnosis at patient's bed side. These tests can be applied in the physician's office, operating room, an ambulance, the home, the field, or in the hospital. As the results are timely they allow rapid diagnostic and identifies treatment alternatives for the patient. This technology empowers clinicians to make decisions at the "point-of-care" and can have significant impact on health care delivery and ability to address challenges of health disparities. However, it is important to validate the diagnostic utility of calprotectin POC in a diverse set of patients undergoing revision arthroplasty.