Editorials

Occult Hepatitis B Virus Infection: A Hidden Menace?

SEE ARTICLE ON PAGE 194

Recovery from an acute hepatitis B virus (HBV) infection isassociated with loss of HBV DNA from serum, hepatitis B eantigen seroconversion, hepatitis B surface antigen (HBsAg)seroconversion, and normalization of serum aminotrans-ferases. These changes generally imply clearance of virus, butclinical observations have shown that reactivation of HBVinfection can occur either spontaneously or after immunosup-pression.1-3 Recent studies showed that immune response toHBV remains vigorous long after an acute infection. In addi-tion, HBV DNA can be detected by polymerase chain reaction(PCR) assays in serum, liver, and peripheral blood mononu-clear cells more than a decade after an apparent recovery fromHBV infection.4-6 These findings suggest that recovery fromacute hepatitis B may not result in complete virus elimination,but rather the immune system keeps the virus at very lowlevels. There is, however, no clear evidence that patients whohave persistently low levels of HBV after recovery from acutehepatitis B develop progressive liver disease.

The availability of PCR assays for HBV DNA allows thedetection of 102 copies/mL compared with 106 copies/mL us-ing hybridization assays. Using PCR assays, HBV DNA hasbeen detected in some subjects who are HBsAg negative in-cluding those with no serologic markers of HBV infection. Inthis issue of the HEPATOLOGY, Brechot et al. review the preva-lence, virologic basis, and clinical significance of occult HBVinfection.7 For the clinician, several issues regarding occultHBV infection are pertinent: Does it exist? How common is it?What is the risk of transmission? What is the risk of progres-sive liver disease? How can it be diagnosed? Is antiviral ther-apy indicated? Before these issues can be addressed, it must berecognized that there is currently no standardized definitionor diagnostic criteria of occult HBV infection. A simple defi-nition would be the detection of HBV DNA in HBsAg-negativesubjects. However, more specific information must be pro-vided, as occult HBV infection is a heterogeneous clinical en-tity.

Brechot et al. provide very strong evidence that occult HBVinfection exists and that most cases are related to very lowlevels of HBV rather than to HBV mutants that do not expressor produce aberrant surface proteins and therefore are unde-

tected by standard testing.7 Because HBV-DNA detection isthe key to diagnosis of occult HBV infection, the type of assayused and its sensitivity must be specified. The sensitivity ofPCR assays for HBV DNA in studies on occult HBV infectionvaries from 101 to 103 copies/mL.8 However, most PCR assaysincluding commercially available assays are not standard-ized.9 Other factors that may affect the detection rates of HBVDNA include the volume of sample used and the materialtested. Thus, the limit of detection can be increased by using alarger volume of serum as in the case of the hybrid captureassay. Most studies on occult HBV infection have reportedhigher rates of HBV-DNA detection in liver or peripheralblood mononuclear cells compared with serum or plasma. Inaddition, snap-frozen liver tissue has a higher rate of HBV-DNA detection than paraffin-embedded liver tissue. More im-portantly, specificity and reproducibility of assay results mustbe ensured. This requires meticulous steps to prevent contam-ination of samples, inclusion of negative controls, and perfor-mance of assays in duplicate using two independent sets ofHBV primers.

Occult HBV infection should also be reported in the contextof other HBV serology markers. Broadly, individuals shouldbe classified as being seropositive or seronegative. Seroposi-tive subjects are positive for antibodies to hepatitis B coreantigen (anti-HBc) and can be further divided into 2 sub-groups: with and without anti-HBs. Seronegative subjectsare negative for both anti-HBc and anti-HBs. As indicated inthe review by Brechot et al.,7 the HBV-DNA detection rate ishighest in subjects who are anti-HBc positive/anti-HBs nega-tive; some of these individuals probably have low-level HBVinfection with subdetectable HBsAg. The HBV-DNA detectionrate is intermediate in subjects who are positive for both anti-HBc and anti-HBs. These individuals may have recoveredfrom previous infection but may have persistent low levels ofHBV. The HBV-DNA detection rate is lowest in seronegativesubjects. These individuals have recovered from previous in-fection but lost all serologic markers of HBV infection. Rarely,they may be infected with HBV mutants that do not expressHBV serologic markers.

Geographic differences in the prevalence of occult HBVinfection are most likely related to the endemicity of HBVinfection. Thus, occult HBV infection is most commonly re-ported in high endemic areas where 70% to 90% of the popu-lation have been exposed to HBV and infrequently reported inlow endemic areas where 5% to 20% of the population hadprior infection with HBV.10-12 The prevalence of occult HBVinfection also depends on the population studied, being morecommon in patients with chronic liver disease and less com-mon among healthy blood or organ donors. Patients withfulminant hepatitis B may be misdiagnosed as occult HBVinfection because of rapid virus clearance with undetectableHBsAg at the time of presentation.

Transmission of HBV infection has been documented fromHBsAg-negative, anti-HBcpositive blood and organ donors.The risk is variable (0.4%-90%)13-17; it is highest when livers

Abbreviations: HBV, hepatitis B virus; HBsAg, hepatitis B surface antigen; PCR, poly-merase chain reaction; anti-HBc, antibody to hepatitis B core antigen; HCC, hepatocel-lular carcinoma.

From the Division of Gastroenterology, University of Michigan Health System, AnnArbor, MI.

Received March 14, 2001; accepted March 30, 2001.Address reprint requests to: Anna Lok, M.D., Division of Gastroenterology, University

of Michigan Health System, 3912 Taubman Center, Box 0362, Ann Arbor, MI 48109-0362. E-mail: aslok@umich.edu; fax: 734-936-7392.

Copyright 2001 by the American Association for the Study of Liver Diseases.0270-9139/01/3401-0028$35.00/0doi:10.1053/jhep.2001.25225

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from anti-HBcpositive donors are transplanted to seronega-tive recipients. The possibility of HBV transmission from HBsAg-negative donors raises several questions: Should blood/organdonors be screened for anti-HBc? Should blood/organs fromHBsAg-negative, anti-HBcpositive donors be used? WouldPCR testing for HBV DNA differentiate infectious from non-infectious donors? In the United States, blood and organ do-nors are routinely screened for anti-HBc. In general, bloodand organs from HBsAg-negative, anti-HBcpositive donorsare not used. The shortage of donor organs has led to a wide-spread practice of transplanting organs from anti-HBcposi-tive donors to HBsAg-positive recipients. The transplantationof such organs (especially livers) to HBsAg-negative recipientshas been reported to result in de novo HBV infection.14-17 Ad-ditional testing for HBV DNA by PCR in the setting of solidorgan transplantation is logistically difficult and several stud-ies found that PCR testing of donor blood is not sensitiveenough to identify donors who are infectious.17,18 Thus, use oforgans from anti-HBcpositive donors in HBsAg-negative re-cipients in the United States should be restricted to dire emer-gencies only. However, anti-HBc screening and exclusion ofanti-HBcpositive donors is impractical in countries whereHBV infection is prevalent and greater than 20% of the popu-lation are anti-HBc positive. Whether anti-HBc screening anddirection of organs from anti-HBcpositive donors to seropos-itive recipients should be implemented deserves careful study.The risk of transmission of HBV infection from seronegativeindividuals has not been well studied. Although there havebeen case reports of such transmission, the overall risk islikely to be negligible and does not warrant routine screeningof seronegative blood/organ donors for HBV DNA.

Occult HBV infection is usually associated with very lowlevels of HBV DNA. A key question is whether the presence ofsmall amounts of HBV will lead to progressive liver disease.Available data suggest that the likelihood is extremely lowalthough definitive data are lacking. Among patients with per-sistent low levels of HBV DNA after apparent recovery fromacute hepatitis B, there is no evidence that they have increasedrisks of cirrhosis or hepatocellular carcinoma (HCC). Cirrho-sis and HCC have been reported in patients with seropositiveoccult HBV infection, but it is not clear if the cirrhosis or HCCis a result of persistent low levels of HBV or other causes ofliver disease. In patients with other etiologies of chronic liverdisease such as alcoholism and hepatitis C virus infection,HBV may be a bystander or a cofactor in the pathogenesis ofliver disease. Other patients may have chronic HBV infectionfor decades leading to liver damage but HBsAg is no longerdetectable when cirrhosis or HCC is diagnosed.19-21 The roleof occult HBV infection in the etiology of liver disease inseronegative individuals is less clear. Data are scanty andbased on case reports or case series. Thorough evaluation forother causes of liver disease and duplicate testing for HBVDNA under stringent conditions must be performed. At therecent National Institutes of Health Conference on HepatitisB, an arbitrary HBV DNA level of 105 copies/mL was chosen asa diagnostic criterion for chronic hepatitis B and an indicationfor antiviral therapy.22 However, more studies using standard-ized assays are needed to determine if a threshold HBV-DNAlevel exists for pathogenicity of liver disease and to define thislevel.

When should occult HBV infection be considered? In pa-tients with cryptogenic acute or chronic liver disease, it is not

unreasonable to test for HBV DNA particularly in anti-HBcpositive individuals but routine testing of seronegative indi-viduals is not necessary. HBV-DNA testing is also not recom-mended in HBsAg-negative patients with other identifiableetiologies of liver disease. An exception may be in patientswho are hepatitis C antibody positive but hepatitis C virusRNA negative on repeat testing. HBV-DNA testing should beperformed on anti-HBcpositive donors regardless of amino-transferase levels if their blood or organs may be used. How-ever, the logistics of obtaining reliable results in a timely man-ner are difficult and a negative test result has a very lowaccuracy in predicting infectivity. Thus, blood or organs fromanti-HBcpositive donors should in general be used in HBsAg-positive or seropositive recipients only. Based on the limiteddata available, universal screening for HBV DNA among sero-negative donors is not warranted.

There are no data on the use of antiviral treatment in pa-tients with occult HBV infection. Intuitively, the benefit isanticipated to be low as the vast majority of patients withoccult HBV infection have very low levels (103 copies/mL)of HBV DNA, which may not be sufficient to cause progressiveliver disease. However, the threshold level of HBV DNA thatcauses liver disease has not been defined and may be differentin different clinical situations. Thus, patients with active liverdisease and no other identified etiology should have HBVDNA levels monitored. Antiviral treatment may be consideredin the context of clinical trials in patients with HBV DNAlevels that are persistently or intermittently above 103 copies/mL.

In summary, we agree with Brechot et al. that occult HBVinfection exists and, in most instances, is associated with verylow levels of HBV rather than HBV mutants. We propose thatoccult HBV infection be defined as the detection of HBV DNAby PCR or other amplification assays in HBsAg-negative indi-viduals. The prevalence of occult HBV infection is unclear butappears to be more common in endemic areas and amongseropositive individuals (anti-HBc positive with or withoutanti-HBs). There is, as yet, no proof that occult HBV infectionexists in individuals with anti-HBs only. Standardized defini-tion and diagnostic criteria of occult HBV infection are neededfor future research to determine the prevalence and clinicalsignificance of occult HBV infection and the role of antiviraltherapy. Reports of occult HBV infection should specify thetype and sensitivity of HBV-DNA assay, the material used fordetection of HBV DNA, the HBV serology profile, and thepresence or absence of liver disease as determined biochemi-cally and/or histologically.

HARI S. CONJEEVARAM, M.D.ANNA S. LOK, M.D.Division of GastroenterologyUniversity of Michigan Health SystemAnn Arbor, MI

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