Analytical Comparison of an In-House FARR Fluorescence Immunoassay with Commercial Analytical Assays for the Detection of Anti-Double-Stranded DNA Antibodies

Background/Objectives: Anti-double-stranded DNA antibodies (anti-dsDNA) are a specific biomarker for systemic lupus erythematosus (SLE). We aimed to analytically compare the in-house Farr fluorescence immunoassay (Farr-FIA), developed based on classical Farr-RIA, for the detection of anti-dsDNA with commercially available assays, including a chemiluminescence assay (CLIA) and the Crithidia luciliae immunofluorescence test (CLIFT). Additionally, we evaluated analytical agreement for diagnostic combinations of these methods. Methods: We analyzed 182 samples from patients at the Department of Rheumatology, UMC Ljubljana, using Farr-FIA, CLIFT (Immuno Concepts—CLIFT 1 and Inova Diagnostics—CLIFT 2), and the QUANTA Flash dsDNA CLIA on the BIO-FLASH analyzer (Inova Diagnostics). Agreement between methods and diagnostic combinations was assessed using Cohen’s kappa. Results: Overall agreement between methods using manufacturer determined cut-offs ranged from 78.6% to 91.8% (κ = 0.582–0.836). The in-house Farr-FIA showed 78.6–86.8% comparability with commercial assays (κ = 0.582–0.730). Analytical agreement between combinations of two methods ranged from κ = 0.788 to 1.000, indicating moderate to almost perfect agreement. Our routine combination of CLIFT 1 followed by Farr-FIA showed high agreement (κ = 0.788–0.938) with other two-step approaches. Regardless of initial screening (CLIFT 1, CLIFT 2, or CLIA), applying Farr-FIA as a confirmatory method yielded highly consistent final classifications (97.3–100% agreement). However, the screening methods differed in the proportion of samples requiring confirmatory method (e.g., CLIFT 2 identified more positives requiring Farr-FIA), consequently affecting workflow and costs, although the final diagnostic results remain the same. Conclusions: Farr-FIA demonstrated moderate to substantial analytical agreement with established commercial assays (CLIA and CLIFT). Several diagnostic combinations demonstrated high analytical agreement; however, the choice of initial screening method substantially influences the number of samples requiring confirmatory testing and, therefore, affects laboratory workload.