Low signal-to-cutoff ratio (S/Co) in the diagnosis of hepatitis C: A diagnostic dilemma?

Abstract

Dear Editor, Hepatitis C is an important public health problem affecting more than 185 million people around the world, of whom 3, 50,000 die each year [1]. India has a low to moderate (1 % to 1.5 %) prevalence of hepatitis C virus (HCV) with approximately 12–18 million infected population which accounts for a significant share of global HCV infections [2]. Laboratory diagnosis of HCV is mainly by serologic tests to detect antibodies and although there are accurate third-generation immunoassays available for detecting antibodies, false-positive anti-HCV results occur at unacceptable frequencies (15 % to 62 %) [3]. A reactive HCVantibody result should be followed by a nucleic acid test (NAT) for HCV RNA for detecting current infection. If HCV RNA is negative, that indicates either past, resolved infection, or false antibody positivity [1, 4]. At present, most of the laboratories report anti-HCV results based on initial screening serological assay only. The recommended anti-HCV testing algorithm has also mentioned the use of the signal-to-cutoff (S/Co) ratio of screening test—positive results as an alternative to a supplemental test in some circumstances, providing a result that has a high probability of reflecting the person’s true antibody status. S/Co ratio is calculated by dividing the optical density (OD) value of the sample being tested by the OD value of the assay cutoff for that run. A specific S/Co ratio can be identified for each test, that would predict a true antibody-positive result (as defined by the results of supplemental testing) ≥95 % of the time, regardless of the anti-HCV prevalence or characteristics of the population being tested [4]. We retrieved samples with low S/Co (≥1 to <4) by a screening chemiluminescence (CLIA) assay (third generation, Architect, Abbott, Germany) and reevaluated them with a second, more specific, fourth-generation ELISA (Monolisa HCV Ag-Ab Ultra, BioRad, France) and a confirmatory assay, i.e. HCV RNA real-time PCR (COBAS TaqMan HCV test v2.0, Roche Molecular Systems Inc, USA). Retrospective analysis of 8500 serum samples that were screened at the virology laboratory for anti-HCV antibody from October 2012 to September 2014 was done. Results of the serological testing were grouped as samples with S/Co—<1: nonreactive, ≥1 and <4: low positive, ≥4: high positive. Of total, 454 (5.3 %) were anti-HCV reactive. Mean age of reactive cases was 46 (±1.8) years, 289 (63.6 %) were males, median bilirubin was 1.8 (0.3–35.5) mg/dL, median ALTwas 48 (15–1592) IU/mL. Among these, 114 (25.1 %) showed low-positive results. Out of these, samples positive by both the assays were 46 (41.8 %), the remaining 68 (59.1 %) were negative by the second assay. Only 7.01 % (8/114) of all lowpositive samples had detectable HCV RNA; positivity was higher, 15.2 % (7/46), when both the serological assays were positive as compared to samples where only one of the assay was positive, 1.47 % (1/68) (p<0.001). Therefore, this study shows that a low-positive anti-HCV report should be communicated with a lot of caution, in view of high false positivity. Communication of a false-positive report has a significant psychological impact on the patient. The inclusion of a more sensitive and specific assay like fourth-generation ELISA or HCV antigen detection is essential for confirming the diagnosis of low-positive serum samples. Fourth-generation ELISA tests detect simultaneously HCV capsid antigen as well as antibodies to the core, NS3, NS4, and NS5 regions of the virus [5]. These assays have * Ekta Gupta ektagaurisha@gmail.com