Evaluation of the core antigen assay as a second-line supplemental test for diagnosis of active hepatitis C virus infection

Abstract

The British Columbia Center for Disease Control laboratory performs approximately 95% of all hepatitis C virus (HCV) antibody tests for the province's 4 million inhabitants. In 2002, the laboratory tested 96,000 specimens for anti-HCV antibodies, of which 4,800 (5%) were seroreactive and required confirmation of active infection. Although HCV RNA assays with a sensitivity of 50 IU/ml or less are recommended for the confirmation of active HCV infection, given the large number of seroreactive specimens tested annually, we evaluated the Ortho trak-C assay (OTCA) as a second-line confirmatory test and determined its limit of detection (LoD). Of 502 specimens from treatment-naïve anti-HCV-positive individuals, 478 had sufficient volumes for evaluation by the OTCA and HCV RNA tests. Core antigen was not detected in 147 of 478 (30.8%) of these specimens, of which 37 of 147 (25.2%) were shown to be viremic by the VERSANT HCV (version 3.0) (branched-DNA) assay and/or the VERSANT HCV qualitative assay. Testing of 144 replicates of a World Health Organization standard dilution series indicated that the LoD of OTCA was approximately 27,000 IU/ml. This LoD is consistent with the inability of OTCA to detect core antigen in clinical specimens with low viral loads. We conclude that OTCA has limited value as a confirmatory test for the diagnosis of active HCV infection because 37 of 367 (10%) of viremic specimens had undetectable core antigen. Qualitative HCV RNA testing remains the present standard for the confirmation of active HCV infection in the diagnostic setting.

FIG. 1.

Percentage of active infections demonstrated by detection of HCV core antigen by OTCA versus detection of HCV RNA by the quantitative HCV bDNA assay and/or the qualitative HCV TMA.

FIG. 2.

Viral load distribution and HCV core antigen detection status of clinical specimens confirmed to be positive for anti-HCV antibodies.

FIG. 3.

Correlation between the concentrations of HCV core antigen measured by OTCA and HCV RNA measured by the HCV bDNA assay. The results of both assays for 328 clinical specimens were analyzed by Deming regression analysis.

FIG. 4.

LoD of OTCA estimated by application of a linear logistic regression model to the results obtained with the WHO dilution panel. The dashed lines represent the upper and lower 95% confidence levels (CL). The estimated LoD of 27,000 IU of HCV RNA/ml is indicated by the x coordinate, where the calculated curve intersects the 95% detection line.