title: Low-positive anti-hepatitis C virus enzyme immunoassay results: an important predictor of low likelihood of hepatitis C infection.
abstract: 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 <or=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.
authors.affiliation: Pathology and Laboratory Medicine Service, Veterans Affairs Medical Center, Washington, DC 20422, USA. d.robert.dufour@med.va.gov
authors.forename: D Robert
authors.initials: DR
authors.lastname: Dufour
authors.forename: Mageli
authors.initials: M
authors.lastname: Talastas
authors.forename: Maria D A
authors.initials: MD
authors.lastname: Fernandez
authors.forename: Barbara
authors.initials: B
authors.lastname: Harris
authors.forename: Doris B
authors.initials: DB
authors.lastname: Strader
authors.forename: Leonard B
authors.initials: LB
authors.lastname: Seeff
mesh_terms: D000328:Adult; D000368:Aged; D015415:Biomarkers / Q000097:blood; D005189:False Positive Reactions; D006526:Hepatitis C / Q000175:diagnosis*; D018937:Hepatitis C Antibodies / Q000097:blood*; D006801:Humans; D015151:Immunoblotting / Q000379:methods; D007124:Immunoenzyme Techniques; D008875:Middle Aged; D012367:RNA, Viral / Q000097:blood; D012189:Retrospective Studies; D012307:Risk Factors; D012698:Serologic Tests; D014728:Veterans
chemical_list: D015415:Biomarkers; D018937:Hepatitis C Antibodies; D012367:RNA, Viral