Prevaccination screening may be cost effective when the expected prevalence of prior HBV infection exceeds 30% , such as in the high-risk adult populations for whom hepatitis B vaccine is indicated. Occasionally potential recipients of hepatitis B vaccine test positive for antibody to hepatitis B core antigen (anti-HBc) but negative for both HBsAg and anti-HBs. Isolated anti-HBc may be found due to suppression of HBV replication by hepatitis C virus in co-infected patients [21,22], or in the setting of low level infection with undetectable HBsAg but a positive HBV DNA by polymerase chain reaction (PCR). Alternatively, an individual with isolated positive anti-HBc may have cleared HBsAg and had detectable anti-HBc and anti-HBs, but subsequently lost anti-HBs. Finally, an isolated anti-HBc may be a false-positive result especially in a low-risk population [23,24].
Anyone found to have isolated an anti-HBc should be retested. A diagnostic algorithm can be used for those who are persistently positive for anti-HBc only [25,26]. To distinguish among the three possibilities mentioned previously (low-level HBV infection, prior immunity without detectable anti-HBs, or false-positive result), the patient may be given a single dose of hepatitis B vaccine with follow-up testing with a quantitative anti-HBs in 1 month. Should anti-HBs become positive in high titer indicating an anamnestic response , a convalescent state is proven and no further vaccine injections are necessary. On the other hand, if anti-HBs remains negative after the single dose of vaccine, HBV DNA should be tested by a PCR technique. If HBV DNA is detectable indicating a low level of viremia, the patient has HBV infection and further vaccination is not necessary. On the other hand, a negative HBV DNA in combination with undetectable anti-HBs would establish that the patient is not infected nor has been previously exposed, indicating the need to complete the three-shot vaccine series.
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