Author’s Reply

Editor – We appreciate Dr. Birungi’s correspondence and question regarding the appropriate laboratory testing for hepatitis C virus (HCV).¹ Her question brings up interesting points for discussion, both clinically and from a laboratory perspective.

Since the discovery of HCV in 1989, there have been at least three generations of enzyme immunoassay (EIA) tests developed to aid in clinical diagnosis and patient management. These tests are used to detect antibody to HCV. Each new generation of test has led to improvements in sensitivity and specificity with the primary goals being reduction of false positives and a decrease in the time it takes to detect seroconversion. As Dr. Birungi correctly points out, there are parts of the world where the performance characteristics of these test kits have not yet been established. This is important because one of the primary determinants of test performance is the prevalence of disease within a population. In 1997, the World Health Organization estimated that 1.17% of the population in Uganda was infected with HCV. In contrast, during the same time period, the estimate was 1.8% for the United States and 18.0% for Egypt.² One would expect to have a higher rate of false positive EIAs in populations with a low prevalence of disease. Laboratory technique is also an important factor. Although EIAs are generally robust tests that can be used successfully in a wide range of laboratories, technical problems can exist and close attention must be directed towards performing the assay as directed by the manufacturer and incorporating quality control procedures. We are not aware that the performance characteristics of the newer generation EIAs are adversely affected by HCV genotype, however, it is known that the most prevalent genotype in Central Africa has historically been type 4.³

Unlike the EIA, qualitative and quantitative HCV RNA tests are technically demanding, and both false positive and false negative results can occur. It has been difficult to standardize nucleic acid testing for HCV between laboratories, and results have been variable. However, as laboratories gain experience with these assays, it is expected that proficiency will improve. Early generations of quantitative HCV assays did show some variation based on the genotype that was present.⁴

The recombinant immunoblot assay (RIBA) seems to have limited utility except to confirm positive EIA results in very low prevalence populations, such as blood donors (figure 1⇓). Compared to EIA, RIBA has a better specificity for HCV. RIBA results are reported as positive (two or more antigens present), indeterminate (one positive antigen or two or more positive antigens with positive yeast marker) or negative. Patients with a positive or indeterminate RIBA should undergo qualitative HCV RNA testing, since this test only signifies evidence for HCV exposure and not necessarily active infection. In most cases, it is more cost-effective to confirm positive EIA results with a sensitive HCV RNA test. For patients with a negative screening EIA, HCV RNA testing should be performed if patients are immunosuppressed and/or have acute hepatitis.

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Figure 1.

Diagnostic algorithm for HCV testing. Adapted with permission from figure 4, page 1488, Shuhart MC, Gretch DR. Hepatitis C and G viruses. In: Murray PR, Baron EJ, Jorgensen JH, Pfaller MA, Yolken RH, eds. Manual of clinical microbiology. 8th ed. Washington DC: ASM Press; 2003. 1480–1494.