Synovial Calprotectin in Suspected Periprosthetic Joint Infection After Total Knee Arthroplasty: Diagnostic Accuracy and Exploratory Adjunctive Value to Preoperative ICM Classification

Background/Objectives: Diagnosis of periprosthetic joint infection (PJI) after total knee arthroplasty remains challenging when standard criteria are inconclusive. This study evaluated the diagnostic accuracy of synovial calprotectin and its prespecified exploratory adjunctive value to preoperative 2018 International Consensus Meeting (ICM) classification. Methods: This prospective single-center diagnostic accuracy study included 35 consecutive patients with primary or revision total knee arthroplasty who underwent synovial calprotectin testing followed by surgery for suspected PJI. Calprotectin was measured using a lateral flow assay with a prespecified threshold of ≥50 mg/L. Diagnostic performance was assessed against a multidisciplinary composite postoperative reference standard that did not incorporate calprotectin. Results: Twenty-one patients were classified as infected and 14 as non-infected. Calprotectin yielded 18 true-positive, 11 true-negative, 3 false-positive, and 3 false-negative results, corresponding to 85.7% sensitivity, 78.6% specificity, 85.7% positive predictive value, 78.6% negative predictive value, and an area under the curve of 0.83. Preoperative ICM correctly classified 26 patients, misclassified one, and left eight inconclusive. Among the inconclusive cases, calprotectin correctly classified all six non-infected patients but neither of the two infected patients. The combined strategy classified all 35 patients, correctly classifying 32 and misclassifying three. Among 10 antibiotic-exposed patients—seven infected and three non-infected—no misclassifications occurred; this finding was descriptive. Conclusions: Synovial calprotectin showed good overall diagnostic performance in this surgically managed cohort. When applied to preoperative ICM-inconclusive cases, it increased classification yield but missed both infected cases and should not be used alone to exclude PJI. Larger blinded multicenter studies are required before routine implementation.