FAQ: Nucleic acid testing for HBV
Questions
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What testing improvements is the Blood Service planning to introduce in 2010?
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How will the introduction of NAT affect the window period for detection of HBV?
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What are the implications of identifying existing donors with OBI?
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Why doesn’t the Blood Service routinely perform anti-HBc screening on all donations?
- What about plasma derived products?
For which viruses does the Australian Red Cross Blood Service routinely screen and what screening tests are currently used?
We screen every blood donation for the presence of markers for viral infection with human immunodeficiency virus (HIV), hepatitis C virus (HCV), hepatitis B virus (HBV) and human T-lymphotropic virus (HTLV) using:
Serological assays for
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HBsAg
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HIV-1 and HIV-2 antibodies
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HCV antibodies
- HTLV-1 and HTLV-2 antibodies
Nucleic acid testing (NAT) assays for
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HIV-1 RNA
- HCV RNA
What testing improvements is the Blood Service planning to introduce in 2010?
In 2010, we are replacing the current semi-automated NAT system and duplex assay (Procleix HIV-1/HCV Assay) with a fully automated testing system (Procleix Tigris) and a new triplex assay for the simultaneous detection of HIV-1 RNA, HCV RNA and HBV DNA (Procleix Ultrio Assay).
Why is the Blood Service introducing this change?
NAT automation is the most advanced NAT technology available and will enable us to maintain international best practice in business efficiency and safety when testing blood donations.
The major benefits of implementing NAT automation are as follows:
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The ability to test large numbers of donations using a highly sensitive assay in the individual donation test format.
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Introduction of NAT for HBV with the Ultrio assay.
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Provision of a platform that will allow us to rapidly introduce tests for new or emerging pathologies.
- Improvement in the quality of testing processes by increasing the efficiency and process control over the current semi-automated system.
How does HBV NAT work?
Tests for HBV DNA, such as NAT, screen for the infectious virus itself and are more sensitive compared with HBsAg (the test we currently use) when performed on single donations in countries with low prevalence of HBV.
The recent availability of fully automated testing platforms for HBV NAT allow for rapid individual donation screening.
How will the introduction of HBV NAT reduce the residual risk of transfusion-transmitted HBV infection?
Introduction of HBV NAT will reduce the residual risk of transfusion-transmitted HBV infection by:
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Improving the detection of acute HBV infection
- As HBV DNA is the first detectable marker in acute HBV infection (ie, before HBsAg), introduction of HBV NAT will reduce the infectious window period (see diagram below).
- Detecting individuals with occult HBV infection (OBI)
Serologic and clinical patterns observed during acute HBV infection
Source: Hollinger (2008).(1)
How will the introduction of NAT affect the window period for detection of HBV?
The implementation of individual donation HBV NAT will reduce the infectious window period for HBV from approximately 38 days to approximately 24 days.
What is occult HBV infection (OBI)?
Acute HBV infection is self-limiting in approximately 97% of immunocompetent adults, leading to recovery and immunity.
However, in 3% of cases, individuals remain chronically infected (ie, carrier state), usually testing HBsAg positive and anti-HBc positive.
Some of these chronically infected individuals can have very low and/or intermittently detectable levels of HBV DNA. Upon testing, they are found to be HBsAg negative, anti-HBc positive, HBV DNA positive or negative, and anti-HBs positive or negative. This is known as occult HBV infection (OBI).
What do we know about OBI in blood donors?
The prevalence of OBI in blood donors varies markedly by region. One study of Asian blood donors quotes a median prevalence of 1% with a range of 0 to 4.6% of donors.(2) The HBV DNA viral load is generally low and therefore intermittently detectable.
Synthesized data for cases of confirmed HBV transfusion-associated transmission from international studies showed the rate of HBV transmission by components collected from donors in the window period of acute HBV infection was greater than those collected from donors with OBI (81% versus 19%, respectively).(3)
With the introduction of HBV NAT, is the Blood Service likely to find existing blood donors who have OBI?
Our HBV screening algorithm combining universal HBsAg screening and a ”history of hepatitis” (HOH) protocol has substantially reduced the risk of transfusion transmitted HBV infection in Australia.
The current estimated residual risk being 1 in 739,000. This residual risk benchmarks well against other comparable international blood service.
All donors must complete a comprehensive medical questionnaire and sign a declaration form relating to risk factors for transfusion transmissible infections. Each donor also participates in a confidential interview to determine their eligibility to donate.
Potential donors who declare symptoms or a history of hepatitis undergo additional testing (ie, anti-HBc and anti-HBs and, if necessary, HBV DNA testing) to further determine their eligibility to donate.
This HOH protocol does partially address the risk of having a donor with OBI. However, as individuals who have HBV infection may be asymptomatic (50% to 70% of cases), it is possible that a donor with OBI may not to be identified through the donor questionnaire and confidential interview and the additional testing will not be performed.
The addition of HBV NAT will increase the probability of detecting donors with OBI. It is estimated that, following implementation of HBV NAT, a small number of potential new donors and existing donors with OBI may be identified.
What are the implications of identifying existing donors with OBI?
All donors identified as having OBI would need to be notified of their test results, counseled and referred for further medical management.
Blood component recall and lookback investigations would need to be undertaken for any previous donations provided by existing donors who are subsequently identified as having OBI.
Why doesn’t the Blood Service routinely perform anti-HBc screening on all donations?
We do not screen potential donors for anti-HBc if they have indicated past exposure to hepatitis. However, the test is not used routinely for all donations because of its relatively high rate of false reactivity.
This would result in the unacceptable loss of many healthy donors whose donations are reactive in the test but who are not infectious.
Are other countries screening for anti-HBc?
Other countries that have a higher prevalence of HBV in the community than Australia, such as the USA, Canada and France, screen donors for anti-HBc.
How will the implementation of NAT for HBV affect the residual risk estimates for transfusion-transmitted HBV?
The following table shows the change in infectious window period, the percentage of window period closure and the residual risk estimates for HBV following the implementation of HBV NAT.
The window period closure of 37.6% for HBV will reduce the residual risk of transfusion transmitted HBV from 1 in 739,000 to less than 1 in 1 million.
| Virus | Assay | NAT Pool Size | Infectious WP* (Days) | % WP* Closure |
Estimate of Residual Risk (per unit) |
| HBV | HBsAg | No NAT | 38 | 1 in 739,000 | |
| ULTRIO | 1 | 24 | 37.6% |
Less than 1 in 1 million |
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| Note: *WP = window period | |||||
How does the Australian residual risk estimate for transfusion-transmitted HBV compare internationally?
The current Blood Service residual risk estimate for transfusion transmitted HBV (based on 2007/08 data) is lower than comparable international blood services.
| Country |
Residual risk estimate for transfusion transmitted HBV |
Source |
| Australia | Approximately 1 in 739,000 | Seed et al (2008) |
| USA | 1 in 205,000 to 488,000 | Zou et al (2009) |
| Canada | 1 in 153,000 | O’Brien et al (2007) |
| Germany | 1 in 360,000 | Houfar et al (2008) |
| France | 1 in 640,000 | Pillonel and Laperche (2005) |
| UK | 1 in 600,000 | Soldan et al (2005) |
The introduction of HBV NAT will further reduce the Australian residual risk estimate to less than 1 in 1 million.
What was the process for the introduction of the HBV NAT?
We actively monitor emerging technologies and the donor screening approaches used across the industry.
We have previously undertaken a cost/benefit evaluation regarding the implementation of HBV NAT, which recommended that routine implementation of hepatitis B NAT would be of benefit provided the testing was able to be undertaken on single donations given the estimated prevalence in Australia.
This has now been made possible with the recent availability of fully automated NAT testing platforms.
We have since participated in an international study evaluating the two commercially available automated NAT platforms which both provide HBV NAT.
A business case and formal tender process was completed and, in line with government funding approval, the implementation planning to introduce this technology is now well advanced.
We are working closely with the Therapeutic Goods Administration (TGA) to ensure that the testing is introduced at the earliest opportunity.
It is anticipated that testing will be provided for the Australian blood supply from mid-2010 with all fresh blood components that we supply being tested using HBV NAT by early September 2010.
When will all fresh blood components be tested by HBV NAT?
From early September 2010, all red cells and platelets that we supply will have been tested for HBV DNA using the NAT triplex assay.
As frozen clinical plasma (ie, fresh frozen plasma, cryoprecipitate and cryodepleted plasma) have a longer shelf-life, management of these components during the transition period will be more complex.
However, we aim to be able to supply only HBV NAT tested clinical plasma components from early September 2010 and are implementing a number of strategies to facilitate this.
We advise Approved Health Providers (AHPs) to review their current inventory holdings of clinical plasma components, as well as your inventory management practices, to enable a more rapid turnover of product during the transition period.
It is recommended that AHPs hold a maximum of 2 months' supply of clinical plasma in the lead-up to the commencement of HBV NAT testing. It should be noted that our inventory holdings of clinical plasma will be increased during this period to compensate for the reduced AHP stock holdings.
We will be working closely with the Therapeutic Goods Administration (TGA), the National Blood Authority (NBA), governments and the clinical community to provide the optimal outcome with respect to blood component inventory management during the transition period.
What about plasma derived products?
Plasma derived products such as immunoglobulin products, albumin solutions and coagulation factor concentrates undergo two dedicated pathogen inactivation and/or removal steps during their manufacture, which have been validated to result in multiple log reductions in potential pathogens.
Additionally, the fractionation process includes physical separation using precipitation and chromatography. This also contributes to non-specific reduction of viruses and other pathogens.
No confirmed transmissions of viral agents have occurred from products used in Australia since introduction of effective, dedicated pathogen inactivation and removal steps.
Based on the fact that there is no change to the risk profile of plasma-derived products, the recommended framework for the introduction of HBV NAT testing is as follows:
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Retrospective HBV NAT testing of retained samples from plasma pools need not be performed on the basis that it would not be useful, because the level of HBV DNA is almost certain to be below the current level of sensitivity of the test.
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Prospective HBV NAT testing of manufacturing pools that include only HBV NAT tested donations will be introduced to ensure consistency with HIV and HCV testing procedures.
- Recall and lookback for plasma-derived products are not indicated based on the current risk evaluation.
References
- Hollinger FB. Hepatitis B virus infection and transfusion medicine: science and the occult. Transfusion 2008;48(5):1001–1026.
- Liu CJ, Chen DS, Chen PJ. Epidemiology of HBV infection in Asian blood donors: emphasis on occult HBV infection and the role of NAT. J Clin Virol 2006;36(Suppl 1):S33–44.
- Candotti D, Allain JP. Transfusion-transmitted hepatitis B virus infection. J Hepatol 2009;51(4):798–809.
