HBsAg

HBsAg

HBsAg is the most common hepatitis B test. The presence of HBsAg in serum indicates that the patient has contracted HBV infection. The test is used to identify those at risk of spreading the disease, e.g., blood donors, pregnant women, intravenous drug abusers, healthcare workers, institutionalized people, transplant donors and recipients, and donors of semen for artificial insemination. A quantitative HBsAg test can be used to monitor the treatment of chronically infected patients. HBsAg screening assays are normally supported by confirmatory tests, which are used to confirm repeatable reactive (positive) results. Typically the confirmatory test involves neutralization of the HBsAg in the sample by >50% using a human anti-HBs antibody.

Hepatitis B Virus

HBsAg positive patients should undergo hemodialysis in a separate room designated for HBsAg positive patients only. They should use dedicated machines, equipment, and supplies, and most importantly staff members should not care for both HBsAg positive and susceptible patients at the same time (shift) or while the HBsAg positive patient is in the treatment area. Dialyzers should not be reused on HBsAg-positive patients because HBV is efficiently transmitted through occupational exposure to blood and reprocessing dialyzers from HBsAg-positive patients might place HBV-susceptible staff members at increased risk for infection.

HBV chronically infected patients (i.e., those who are HBsAg positive, total anti-HBc positive, and IgM anti-HBc negative) are infectious to others and are at risk for chronic liver disease. They should be counseled on how to prevent transmission to others, especially for those who are their household and sexual partners. Household contacts and sexual partners should be advised to receive hepatitis B vaccine. The HBsAg positive patient should also be evaluated (by consultation or referral, if appropriate) for the presence or development of chronic liver disease according to current medical practice guidelines. It is recommended that individuals with chronic liver disease be vaccinated against the hepatitis A virus (HAV), if susceptible, to prevent any additional injury to the liver.

HBV chronically infected patients do not require any routine follow-up testing for purposes of infection control. However, annual testing for HBsAg is reasonable to detect the small percentage of HBV-infected patients who might lose their HBsAg.

HBsAg variants

HBsAg variants arise as an escape mechanism during infection in the presence of anti-HBs. In babies given hyperimmune hepatitis B globulin (HBIG) and active immunization to reduce the risk of infection, but who are exposed to maternal virus during birth, the immune selection pressure from the HBIG and vaccine may result in the selection of HBsAg escape mutants. This is seen particularly in countries where universal childhood HBV vaccination has been practiced for many years, leading to breakthrough infection despite vaccination. Another scenario where immune selection pressure is intense is following liver transplantation for chronic hepatitis B. Immune selection pressure is exerted through the post-transplantation use of HBIG to protect the new liver from being re-infected with HBV.

HBsAg escape mutants mainly affect the ‘a’ determinant of HBsAg, the principal target of anti-HBs. The most common mutation observed is in position 145 of the HBsAg protein with an amino acid change from glycine to arginine (G145R). HBsAg escape mutants are transmissible and the widespread occurrence of HBsAg mutants would create considerable problems for the hepatitis B vaccination programme. In addition, many existing diagnostic assays for HBsAg use specific monoclonal antibodies that detect epitopes associated with the ‘a’ determinant. The presence of such mutations could lead to false negative diagnostic results when the monoclonal antibodies fail to bind to the mutated epitopes. Some cases of so-called ‘occult HBV infection’ are in fact due to false negative HBsAg, with detectable HBV DNA and no detectable antigenaemia.

Screening

Serum HBsAg and anti-HBs are the most useful screening tests for chronic HBV infection or immunity to HBV. HBsAg is present in most chronically infected persons, even when HBV DNA levels are undetectable. Lack of anti-HBs in an HBsAg-negative person indicates susceptibility to HBV infection.

Routine HBsAg screening is recommended for the following people: (1) all pregnant women; (2) all neonates born to HBsAg-positive women after completing the vaccination series; (3) immigrants from regions of high or intermediate HBV endemicity (e.g., most areas of Asia, Africa, the Middle East, and Pacific Islands); (4) U.S. born people, who were not vaccinated as infants, whose parents were born in regions with HBsAg prevalence ≥8%; (5) people with elevated serum ALT/AST levels of unknown etiology; (6) people with behavioral risks including MSM and injection drug users; and (7) and hemodialysis patients.⁶⁹

Screening for HBsAg is recommended at the first prenatal visit for all pregnant women.¹⁰⁹ Women in labor without HBsAg test information should have HBsAg serology upon arrival. In addition, pretested women who have a history of high-risk behaviors (e.g., HBsAg-positive sexual partner, injection-drug use) should be retested at the onset of labor. A copy of the original HBsAg test result should be placed in the infant's medical chart (in addition to the mother's chart) to avoid errors in communication or interpretation of the laboratory report.¹¹⁰

All HBsAg-positive tests should be reported to the state or local health department, including HBsAg-positive tests from pregnant women for referral to the Perinatal Hepatitis B Prevention Program.

1 HBsAg and the Dane particle

Hepatitis B

HBsAg-positive hepatitis can be transmitted by both hetero- and homosexual practices but there is a particularly high risk of transmission of hepatitis B amongst homosexuals. The risk of acquiring hepatitis B is greatest in ‘passive’ homosexuals and in those with many sexual partners.

Serum, suitably labelled with ‘hepatitis risk’ labels and transported to the laboratory in a plastic bag, is collected from patients with possible symptoms of hepatitis and from close contacts of these patients. The tests indicated include HBsAg. ‘e’ antigen may be looked for in HBsAg positive cases as carriage of ‘e’ antigen is associated with a higher infectivity. (See Chapter 15.)

There is no specific treatment but liver function tests are indicated in HBsAg positive cases. A referral to a physician for possible liver biopsy is occasionally necessary. In a few instances alpha-interferon has been administered to patients with chronic hepatitis B. When the patient is an asymptomatic HBsAg positive carrier, it is difficult to do more than monitor his close contacts. Ideally some restraint should be exercised by promiscuous homosexuals who are ‘e’ antigen positive to reduce the number of possible new cases of hepatitis B.

Homosexual patients who lack serological evidence of previous hepatitis B infection should be offered active hepatitis B immunization to prevent hepatitis B.

Laboratory diagnosis

HBsAg

Sensitivities and specificities of current testing methods for HBsAg are high, but prevalence of detectable HBsAg in blood donors is low, and thus positive predictive value is also low.

Laboratory Findings and Diagnostic Tests

Serum HBsAg and anti-HBs are the most useful screening tests for chronic HBV infection or immunity to HBV. HBsAg is present in most chronically infected persons. Lack of anti-HBs in an unvaccinated HBsAg-negative person indicates susceptibility to HBV infection.³

Routine screening for HBV infection is recommended for the following people: (1) all pregnant women; (2) all infants born to HBsAg-positive women after completing the vaccination series; (3) immigrants from regions of high or intermediate HBV endemicity (e.g., most areas of Asia, Africa, the Middle East, and Pacific Islands); (4) US-born people, who were not vaccinated as infants, whose parents were born in regions with HBsAg prevalence ≥8%; (5) people with elevated serum ALT or aspartate transaminase levels of unknown etiology; (6) people with behavioral risks, including MSM and injection-drug users; (7) hemodialysis patients, (8) HIV-positive persons; (9) persons needing immunosuppressive therapy; (10) donors of blood, plasma, organs, tissues, or semen; (11) household, needle-sharing, or sexual contacts of persons known to be HBsAg positive; and (12) persons who are the source of blood or body fluids for exposures that might require postexposure prophylaxis.⁶⁵

Screening for HBsAg is recommended at the first prenatal visit for all pregnant women.^3,109 Women in labor without HBsAg test information should have HBsAg serology on arrival. In addition, pretested women who have a history of certain risk factors (i.e., HBsAg-positive sexual partner or more than one sexual partner in the previous 6 months, evaluation or treatment for a sexually transmitted disease, injection-drug use, and clinical hepatitis) should be retested at the time of admission to the hospital for delivery.^3,110

Interpretation of Diagnostic Tests

Hepatitis B surface antigen (HBsAg) is the first marker of HBV detectable in serum in acute infection. By the time clinical and biochemical hepatitis is present after an incubation period of up to 140 days, other serologic markers of HBV infection appear—including antibody to HBV core antigen (anti-HBc). Hepatitis B core antigen, a marker of viral replication found in infected hepatocytes, does not circulate in serum. However, its corresponding antibody (anti-HBc) does. Documented HBsAg positivity in serum for 6 or more months suggests chronic HBV with a low likelihood of subsequent spontaneous resolution. Chronic HBV is diagnosed by the absence of IgM anti-HBc antibody. IgM anti-HBc antibody is a marker of acute or recent acute hepatitis B and is detectable for 6 months after infection, whereas IgG anti-HBc is lifelong. If acute HBV resolves, neutralizing antibody against HBsAg (anti-HBs) develops. If HBV infection becomes chronic, other HBV markers—including HBV viremia (HBV DNA) and hepatitis e antigen (HBeAg)—should be sought. Both of these markers imply viral replication and thus greater infectivity, although any patient who is HBsAg positive is potentially infectious.