What is the proportion of samples reported as positive among reactive samples in Siemens Atellica IM analyzer HIV and HCV serological screening?

• 69% of reactive HIV Ag/Ab samples with Atellica IM analyzer were reported positive • 96% of reactive anti-HCV samples with Atellica IM analyzer were reported positive • Reactive samples over measurement range were positive with high posterior probability Background: Serological screening of transmissible blood-borne viruses, like HIV and HCV, is a seminal part of medical laboratory workflow. Reliable screening methods are vital to provide impetus for confirmation testing, since false reactive results occur with HIV and HCV immunoassays. Objectives: We analyzed what proportion of the reactive HIV Ag/Ab and anti-HCV serum samples with Siemens Atellica IM Analyzer were finally reported as positive after confirmation testing. Study design: Reactive samples with Atellica IM Analyzers using HIV Ag/Ab Combo Assay (HIV Ag/Ab) or Atellica IM Hepatitis C assay (anti-HCV) analyzed between 1/2021-4/2023 were queried from our laboratory system. Index cut-off for reactive sample was ≥1.00 for HIV Ag/Ab and ≥0.80 for anti-HCV. All reactive samples underwent confirmation testing and were interpreted by clinical microbiologists. We calculated the proportion of positive results amidst reactive samples with Atellica IM Analyzers. We used index categorization based on our laboratory workflow for confirmation testing. Results: Of 802 reactive HIV Ag/Ab and 3100 reactive anti-HCV samples, 69% (95% CI 66-72%) and 96% (95% CI 95-97%) were eventually reported positive. Reactive samples with an index over measurement range were almost exclusively reported as positive with high posterior probabilities. Reactive HIV Ag/Ab samples within a very low or low reactivity category were seldom reported as positive. Conclusions: Most reactive samples with Atellica IM Analyzer were reported positive, nonetheless up to 31% of HIV Ag/Ab and 4% of anti-HCV samples were false reactives. These data can be utilized in designing algorithms for confirmation testing in clinical laboratories.