HBV Diagnostics

Question 1

Architect used for HBsAg testing

Anti-Hbs binds to HBsAg

What are the targets?

Answer 1

The Major hydrophilic region (MHR) of HBsAg contains a highly conformational B-cells epitope cluster that is the main target of neutralizing antibodies to HBsAg

HBsAg includes 220 amino acids
MHR is from amino acids 99 - 169

Question 2

Some patients can have occult HBV infection

HBsAg neg - due to mutations of HBsAg
HBV DNA detected

What is the process by this occurs

Answer 2

Lack of proofreading by HBV DNA polymerase causes frequent mutations in S domain of HBsAg

Mutations in the Major Hydrophilic Region (MHR) alters antigenicity, and ability for anti-HBs to bind

This leads to failure of HBV detection - diagnostic escape, in convential assays

So patients who clinically have HBV infection, may have HBsAg neg, HBV DNA pos

The first vaccine-induced natural mutation (G145R) was found in a newborn of an HBsAg carrier mother, and mutations in this surface protein have also been shown to lead to a failure of immune prophylaxis in infants receiving HBV vaccines (“immune escape”) and in a liver transplant patient receiving hepatitis B immune globulin (HBIG)

Question 3

HBsAg nucelotide sequence is 226 amino acids

Which are the two most common mutations?

Answer 3

Substitute for the following amino acids -

• 120k
• 123D

If both present, renders HBsAg undetectable in comemrical EIA kits.
If only one substitution present, there is partial reduced binding capacity of HBsAg against HBsAg

Ensure assay used covers for common known resistance mutations - Architect covers these

Question 4

What are the uses for HBsAg quantitative assay?

Answer 4

Newer test which may be helpful in some circumstances

For patient on IFN, if quantitative value decreases, it may provide better indication that patient will clear HBV infection
If HBsAg is not decreasing, then unlikely patient will successfully clear infection

Question 5

What are clinical implications of mutant HBsAg for PEP?

Answer 5

HBV vaccine and HBIG will have no effect on preventing infection in the recipient, as the antibodies produced do not bind successfully to the virus

For example, pregnant mother. Vaccine/ HBIG will not protect baby
So ensure mother is on treatment to reduce risk of transmision

Question 6

HBV PCR Viral load

What are the gene targets

Answer 6

Highly conserved regions -

• ORF - S gene (N-terminal)
• ORF - preCore region
• ORF - X gene

Question 7

What is the utility of measuring HBV core antigen (HBcrAg)?

Answer 7

HBcrAg levels strongly correlate with serum HBV DNA levels.

HBcrAg can be detected even when HBV DNA cannot be detected, this seems to predict risk of viral relapse after discontinuation of treatment

the assay needs to be more sensitive, as many HBeag-negative patients have undetectable HBcrag, which limits its clinical relevance.

Question 8

What are the EASL names for HBV profiles?

Answer 8

Acute HBV infection

Chronic
HBeAg-positive chronic infection
HBeAg-positive chronic hepatitis

HBeAg-negative chronic infection
HBeAg-negative chronic hepatitis