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

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. 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. 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. 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.