Evaluation of the AIX1000 automated rapid plasma reagin assay in a high prevalence setting

BACKGROUND: Manually performed nontreponemal assays, such as rapid plasma reagin (RPR), are labor intensive and time consuming. Recently, commercial automated RPR assays gained attention. The aim of this study was to compare the qualitative and quantitative performance of the AIX1000TM (RPR-A) (Gold Standard Diagnostics) to a manual RPR test (RPR-M) (Becton Dickinson Macrovue) within a high prevalence setting.

METHODS: A retrospective panel of 223 samples was selected for comparison between RPR-A and RPR-M, including 24 samples from patients with known syphilis stages and 57 samples from 11 patients in follow-up. Additionally 127 samples obtained during routine syphilis diagnosis with RPR-M were analyzed prospectively with AIX1000TM.

RESULTS: Overall qualitative concordance (% agreement) between both assays was 92.0% in the retrospective and 89.0% in the prospective panel. Out of 32 discordances, 28 were explained by a treated syphilis infection still positive in one assay and already negative in the other. One sample was false positive with RPR-A, one infection remained undetected by RPR-M and two by RPR-A. A hook effect was apparent on the AIX1000TM at RPR-A titers from 1/32 onwards, however no infections were missed. Accepting a ± 1 titer difference, quantitative concordance between both assays reached 73.1% and 98.4% for the retrospective and prospective panel respectively with an upper limit of reactivity for RPR-A at 1/256.

CONCLUSIONS: The AIX1000TM showed a similar performance to Macrovue RPR with the exception of a negative deviation for high titer samples. Within the reverse algorithm used in our high prevalence setting AIX1000TM's main advantage is automation.