Sunday, August 14, 2011

Laboratory Diagnosis of Hepatitis B virus

• Serology
The serologic changes that follow exposure to HBV represent a complex interaction between host, virus, and specific antigens. HBsAg is first detected 7 - 9 weeks. It reaches a peak concentration during the acute stage and then slowly declines to undetectable levels within 4 - 6 months. HBV DNA is detected 3 - 5 weeks after infection and up to 40 days before the appearance of HBsAg (average, 6 -15 days), although newer, more sensitive HBsAg assays are closing this gap. The HBV DNA rises slowly and circulates at relatively low levels during the early HBsAg seronegative window period (Busch and Kleinman, 2000).
HBeAg and IgM anti-HBc are usually detected subsequent to the appearance of HBsAg. Because the IgM-specific anti-HBc is produced in high titer in response to the synthesis of the nucleocapsid protein of the virus, its appearance in the serum is indicative of viral replication. These elevated titers decline to undetectable levels regardless of whether the disease resolves or becomes chronic, but the antibody may reappear during reactivation of HBV. IgG-specific anti-HBc generally remains detectable for a lifetime. During the early stages of convalescence and before HBsAg disappears, anti-HBe replaces HBeAg which indicate reduction in viral replication and the beginning of the resolution of the disease (Kuhns et al., 1998).
Termination of acute HBV infection occurs with the disappearance of HBsAg and the appearance of anti-HBs. Anti-HBs generally persists for a lifetime in over 80% of the patients. Those patients with acute hepatitis who maintain high levels of HBsAg concentration throughout the infection, or those whose serum HBeAg persists 8 -10 weeks after symptoms begin to resolve, are most likely to become healthy carriers or to develop chronic liver disease. By convention, HBV carriers are identified by the persistence of HBsAg for at least 6 months (Baker et al., 1991).
• Diagnostic Assays
A number of assays are available to differentiate acute from chronic HBV infection, to evaluate relative infectivity and prognosis, and to assess the immune status of the patient. Among the many commercially licensed assays offered, RIA and ELISA are the most sensitive and specific. Nucleic acid hybridization procedures and PCR assay for the detection of HBV DNA in serum (Yotsuyanagi et al., 1998).
PCR assay can detect less than 100 HBV DNA genomes per milliliter. Using this technique, virus has been detected 2 - 3 weeks before the appearance of HBsAg in experimentally infected chimpanzees and has continued to be detectable for 1-3 weeks after anti-HBs seroconversion (Lai et al., 1998).

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