Transfusion Transmitted Infection FAQs

Transfusion-Transmitted Infection FAQs

Residual Risk Estimates for Viral Transfusion-Transmitted Infection - Frequently Asked Questions  

The following frequently asked questions and answers have been developed to provide health professionals with a better understanding of how the ARCBS residual risk estimates for viral transfusion-transmitted infections are calculated and the rationale for using these calculations.  

What TTI residual risk estimates do ARCBS publish and where are they published? 

ARCBS publishes estimates of the residual risks of transfusion-transmitted infections in the Blood Component Information (BCI) booklet, each Medilink newsletter and on the ARCBS clinical website (www.transfusion.com.au) as a service to clinicians to guide transfusion decision-making and informed consent processes.  The viral risk estimates presented in the table below have recently been revised based on ARCBS data from 1 January 2007 to 31 December 2008.   

Residual risk estimates for transfusion-transmitted infections 

 ^a  HIV, HCV, HBV risk estimates are based on ARCBS data from 1 January 2007 to 31 December 2008. HTLV risk estimate based on data from 1 January 2004 to 31 December 2008. For other agents refer below. Viral estimates: Seed CR, Kiely P and Keller AJ. Residual Risk of Transfusion Transmitted Human Immunodeficiency Virus, Hepatitis B Virus, Hepatitis C Virus and Human T Lymphotrophic Virus. Intern Med J 2005; 35(10): 592-8.Malaria: Seed CR. Residual Risk Estimates for Transfusion Transmitted Malaria (TTM). ARCBS DPARC: November 9/10 2005 meeting. There have been no reported cases of transmission by transfusion of classical Creutzfeldt-Jakob Disease (cCJD), and retrospective studies suggest that the possibility of such transmission of cCJD is remote.To date, there have been no reported cases of vCJD in Australia.  In the UK, there have been a small number of reported cases of putative transfusion transmission since 2004.  In Australia, as a precaution, people who have spent a cumulative period of 6 months in the UK between 1 January 1980 and 31 December 1996 and/or had a transfusion in the UK between 1 January 1980 and the present time are not accepted as blood donors. 

How are the ARCBS TTI residual risk estimates for HIV, HCV, HBV and HTLV calculated? 

ARCBS estimates of residual risk of transfusion-transmitted viral infection are based on published models and represent the median risk estimate derived using 3 models.

These estimates are updated annually. It should be noted that, as the order of magnitude of these risks is very small, the calculated median risk estimate may fluctuate from year to year. Furthermore the estimates are conservative since they are based on the ‘worst case’ assumption that an infectious donation is always issued for transfusion and, that if transfused will always lead to infection in the recipient (i.e. infectivity is 100%). 

In very simple terms, what is the basis of the model calculations? 

Viral point estimates are derived by determining the probability of an undetected ‘window period’ (WP) (defined as the interval between infection and first positive test marker in the bloodstream) donation in a given time period.  

The models essentially assess the rate of seroconversion (i.e. positive donors who have previously tested negative at ARCBS for the same marker) in the repeat donor (RD) population as a measure of viral incidence (i.e. early or acute infection).

In order to incorporate the incidence in first time donors (FTD) (who have no previous testing at ARCBS), one model uses a separate calculation whereas the other two use a correction factor for the RD incidence based on the proportion of NAT positive/antibody negative (i.e. NAT yield) donors in the FTD and RD populations respectively.

Two models also incorporate the average inter-donation interval for all seroconverters (in days) between the positive result and previous negative result. The longer this interval for an individual donor, the less risk the donor was in the WP at the time of donation i.e. the inter-donation interval is inversely proportional to the risk.  

What is the rationale for using the above calculations? 

The models presume that the risk of collecting blood from an infectious donor predominantly relates to them being in the WP (i.e. incident infection) and the best estimate of incidence is the rate of seroconversion in the RD population.

While the assumption that WP donors account for the majority of risk seems to hold true for HIV, HCV and HTLV, HBV is problematic because of 'chronic' infection (i.e. HBsAg negative/anti-HBc positive).

Whereas one model includes a correction factor for the incidence to compensate for chronic infection the other two do not.

This is a potential confounder for HBV RR estimation with the relative impact dependent on the proportion of acute versus chronic HBV infection in the donor population.  

How do these residual risk estimates compare to the risks associated with everyday living? 

When considering the significance of specific risks, it is often useful to compare them to the risks associated with everyday living.

The risk estimates listed above are very small when compared to everyday risks (refer to the Calman scale below). The chance of dying in a road accident, for example, is about 1 in 10,000 per year which is considered a ‘low’ risk. Comparatively, all the viral risk estimates are well below this level, being considered as either ‘minimal’ (HBV) or ‘negligible’ (HIV and HCV).    

The CALMAN Chart (Calman 1996*) for explaining risk (UK risk per 1 year) 

• Negligible: < 1,000,000 e.g. death from a lightning strike
• Minimal: 1:100,000 - 1:1,000,000 e.g. death from a train accident
• Very low:  1:10,000 - 1:100,000 e.g. death from an accident at work
• Low:  1:1000 - 1:10,000 e.g. death from a road accident
• Moderate:  1:100 - 1: 1000 e.g. death from smoking 10 cigarettes per day
• High:  > 1:100 e.g. transmission of chickenpox to susceptible household contacts  

* Calman K. The Health of the Nation. Br J Hosp Med 1996; 56: 125-6.