Ann Lab Med 2018; 38(5): 466-472  
The Role of the Signal-to-Cutoff Ratio in Automated Anti-HCV Chemiluminescent Immunoassays by Referring to the Nucleic Acid Amplification Test and the Recombinant Immunoblot Assay
Moon Suk Choi, M.D.1,2, Kyunghoon Lee, M.D.1,3, Yun Ji Hong, M.D.1,3, Eun Young Song M.D.3, Dal Sik Kim M.D.2, and Junghan Song M.D.1,3
Department of Laboratory Medicine1, Seoul National University Bundang Hospital, Seongnam; Department of Laboratory Medicine2, Chonbuk National University College of Medicine, Jeonju; Department of Laboratory Medicine3, Seoul National University College of Medicine, Seoul, Korea
Corresponding author: Yun Ji Hong
Department of Laboratory Medicine, Seoul National University Bundang Hospital, 82 Gumi-ro 173beon-gil, Bundang-gu, Seongnam 13620, Korea
Tel: +82-31-787-7695
Fax: +82-31-787-4015
Received: April 25, 2017; Revised: July 21, 2017; Accepted: May 6, 2018; Published online: September 1, 2018.
© Korean Society for Laboratory Medicine. All rights reserved.

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Background: Following discontinuation of the recombinant immunoblot assay (RIBA), the only available supplementary test for the detection of hepatitis C virus (HCV) is the nucleic acid amplification test (NAAT). However, the NAAT does not adequately detect past HCV. Consequently, it is hard to distinguish between past HCV infection and biological false positivity with an anti-HCV result alone. We assessed the diagnostic performance of two immunoassays: the ARCHITECT anti-HCV chemiluminescent microparticle immunoassay (CMIA; Abbott Diagnostics, Wiesbaden, Germany) and the Access HCV Ab PLUS chemiluminescent immunoassay (CIA; Bio-Rad, Marnes-la-Coquette, France). We also explored an optimized algorithm to determine the anti-HCV results.
Methods: We tested 126,919 patients and 44,556 individuals who underwent a medical checkup. RIBA and NAAT were conducted for samples that tested anti-HCV-positive using CMIA and CIA. We assessed the optimal signal-to-cutoff (S/CO) ratio in HCV-positive samples.
Results: In total, 1,035 blood samples tested anti-HCV-positive. Of these, RIBA was positive in 512, indeterminate in 160, and negative in 363 samples. One hundred sixty-five samples were NAAT-positive. Diagnostic sensitivity and positive predictive value (PPV) were 96.7% and 52.1%, respectively, for CMIA, and 94.7% and 72.3%, respectively, for CIA. The optimal S/CO ratio was 5.2 for CMIA and 2.6 for CIA at 95% PPV. In total, 286 samples tested positive in CMIA and 444 in CIA, while 443 samples tested positive in both assays.
Conclusions: It is hard to determine anti-HCV positivity based on the S/CO ratio alone. However, this study elucidated the role of the S/CO ratio by using the NAAT and RIBA.
Keywords: Hepatitis C virus, Chemiluminescent immunoassay, Recombinant immunoblot assay, Nucleic acid amplification test, Signal-to-cutoff ratio

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