Evaluation of the AIX1000 automated rapid plasma reagin test in a tertiary academic medical center.

Objectives: To evaluate the analytical performance of the automated AIX1000 RPR assay compared to the manual Macro-Vue RPR Card Test for syphilis monitoring in a high-prevalence, reverse screening setting.

Methods: Serum samples were tested using both AIX1000 and manual RPR in retrospective (n = 75) and prospective (n = 279) cohorts. Qualitative and quantitative concordance were assessed, along with AIX1000's accuracy, precision, specificity, carry-over, and freeze-thaw stability.

Results: In the retrospective cohort, overall qualitative agreement was 90.7% (κ = 0.80), with 77.3% of samples showing titers within one dilution. In the prospective cohort, qualitative agreement was 86.0% (κ = 0.72), with 90.7% of samples within one dilution. AIX1000 results were often 1-2 dilutions lower than manual RPR, especially in high-titer samples (≥1:16), where titer concordance (±1 dilution step) dropped to 38.7% (prospective) and 35.3% (retrospective). Accuracy analysis using external quality controls revealed a consistent one dilution-step bias for both assays: manual RPR read higher and AIX1000 lower, explaining the observed discrepancies. Analytical performance met predefined criteria: within and between runs were consistent, specificity showed no cross-reactivity, and no carry-over contamination was observed. Freeze-thaw testing had minimal effect on results.

Conclusion: The AIX1000 showed good concordance with manual RPR, particularly at lower titers, with reliable analytical performance and operational advantages. However, systematic titer underestimation compared to manual RPR, particularly in high-titer samples, may affect clinical interpretation if tests are transitioned. Clinicians should be informed of this discrepancy when transitioning platforms.