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1 Pathology and Laboratory Medicine Service and
2 Medical Service, Veterans Affairs Medical Center, Washington, DC 20422.
3 National Institute of Digestive, Diabetes, and Kidney Diseases, Bethesda, MD 20892.
4 Department of Pathology, The George Washington University Medical Center, Washington, DC 20037.
5 Department of Medicine, Georgetown University Medical Center, Washington, DC 20008.
aAddress correspondence to this author at: Pathology and Laboratory Medicine Service-113, Veterans Affairs Medical Center, 50 Irving St. NW, Washington, DC 20422. Fax 202-745-8284; e-mail d.robert.dufour{at}med.va.gov.
Background: Tests for hepatitis C antibodies (anti-HCV enzyme immunoassays) are usually described as positive or negative. Several studies, mainly in blood donors, have found that specimens with low signal/cutoff (S/C) ratios are commonly negative when tested with a recombinant immunoblot assay (RIBA) or for HCV RNA.
Methods: We retrospectively reviewed 17 418 consecutive anti-HCV results from a screening program for high-risk veterans; 2986 (17.1%) samples were anti-HCV-positive, and 490 (16.4%) had S/C ratios 
3.7 (low positive). Additional tests were performed in 1814 anti-HCV-positive individuals.
Results: RIBA was performed in 263 patients with low-positive anti-HCV; results were negative in 86%, indeterminate in 12%, and positive in 2%. Only 16 of 140 individuals (11%) with low-positive anti-HCV values were HCV RNA-positive, whereas HCV RNA was positive in 90% of 1435 individuals with high-positive anti-HCV values (P <0.0001). Compared with those with high-positive anti-HCV, individuals with low-positive anti-HCV values were older (P <0.0001) and were less likely to have risk factors for HCV (P <0.0001 for most), multiple increased alanine aminotransferase (ALT) activity values (30% vs 81%; P <0.0001), or positive anti-hepatitis B core antigen (19% vs 59%; P <0.0002). Among 634 individuals with high anti-HCV titers and multiple increased ALT activity values, 95% were HCV RNA-positive.
Conclusions: The S/C ratio is important even in high-risk individuals; laboratories should report the S/C ratio along with anti-HCV EIA results and perform supplemental RIBA testing in those with low-positive values to avoid reporting false-positive results.
The following articles in journals at HighWire Press have cited this article:
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  F.-B. Wu, H.-Q. Ouyan, X.-Y. Tang, and Z.-X. Zhou Double-antigen sandwich time-resolved immunofluorometric assay for the detection of anti-hepatitis C virus total antibodies with improved specificity and sensitivity J. Med. Microbiol., August 1, 2008; 57(8): 947 - 953. [Abstract] [Full Text] [PDF]
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 I. S. Fox Discordant Results of Two Automated Assays for Detection of Hepatitis C Virus-Specific Antibody J. Clin. Microbiol., April 1, 2006; 44(4): 1599 - 1600. [Full Text] [PDF]
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M. Oethinger, D. R. Mayo, J. Falcone, P. K. Barua, and B. P. Griffith Efficiency of the Ortho VITROS Assay for Detection of Hepatitis C Virus-Specific Antibodies Increased by Elimination of Supplemental Testing of Samples with Very Low Sample-to-Cutoff Ratios J. Clin. Microbiol., May 1, 2005; 43(5): 2477 - 2480. [Abstract] [Full Text] [PDF]
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 D. R. Dufour Lot-to-Lot Variation in Anti-Hepatitis C Signal-to-Cutoff Ratio Clin. Chem., May 1, 2004; 50(5): 958 - 960. [Full Text] [PDF]
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D. R. Dufour, M. Talastas, M. D.A. Fernandez, and B. Harris Chemiluminescence Assay Improves Specificity of Hepatitis C Antibody Detection Clin. Chem., June 1, 2003; 49(6): 940 - 944. [Abstract] [Full Text] [PDF]
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