Does occult hepatitis C virus infection exist?

Occult hepatitis C virus (HCV) infection has been found in anti-HCV and serum HCV RNA-negative patients with abnormal results of liver function tests of unknown origin and in patients with spontaneous or treatment-induced recovery from hepatitis C (2–6, 9–11). In a recent issue of the Journal of Clinical Microbiology, Halfon and coworkers questioned the existence of occult HCV infection as they have not detected HCV RNA in the peripheral blood mononuclear cells (PBMCs) of patients with cryptogenic liver diseases (7). However, negativity for HCV RNA in PBMCs does not exclude the existence of occult HCV infection because the “gold standard” method to identify this occult infection is by detection of viral RNA in liver cells. Thus, the authors should have tested liver samples in order to refute the original report (3). If no liver samples are available (as seems to be the case in Halfon’s report), already published alternative approaches, such as detection of HCV RNA in ultracentrifugated serum samples (1), should be performed in addition to HCV RNA detection in PBMCs to confirm negative results. There are also several concerns about the method used by Halfon and coworkers. First, the authors have not provided any information on how PBMC samples were preserved for RNA isolation. This is an important issue when testing for HCV RNA because improper storage of samples hinders viral RNA detection (8). Positivity of HCV RNA in PBMCs of patients with chronic hepatitis C does not ensure good preservation of samples because the amount of HCV RNA in PBMCs is higher in these patients than in occult HCVinfected patients. Consequently, viral RNA detection would be less affected by the quality of the isolated RNA. In addition, even though the authors used 1 million PBMCs for RNA isolation, the efficiencies of RNA extraction may differ among samples, which would affect HCV RNA detection. Thus, it is important to quantify total RNA to perform each PCR assay with the same amount of RNA. Second, when negative data are reported, the sensitivity of the assay is critical to ensure that the lack of HCV RNA detection is not due to the low sensitivity of the technique used. In this regard, the method employed to determine the sensitivity of HCV RNA detection in PBMCs was inadequate. Adding an HCV RNA-positive serum to uninfected PBMC lysates only ensures that the lysate by itself does not interfere with viral RNA detection. However, it does not indicate whether intracellular HCV RNA from infected PBMCs would be detected. A more appropriate method would be to use PBMCs from a patient with chronic HCV infection for RNA isolation and HCV RNA detection and then to test serial dilutions of the isolated RNA. Finally, the patient population studied was too small and heterogeneous to dispute previous results on occult HCV infection. In conclusion, the data reported by Halfon and coworkers (7) do not provide any evidence against the existence of occult HCV infection. REFERENCES