Recent insights in testing for transfusion transmissible viral infections M. Vermeulen, 1 N. Lelie 2 & R. Reddy 1 1 South African National Blood Service, Johannesburg, South Africa 2 Lelie Research, Paris, France Background Since the advent of HIV in the early 80s there has been a rapid evolution in the detection of transfusion transmissible viral infections (TTVIs), first based on serologic screening methods and more recently on molecular diagnostics or nucleic acid amplification technology (NAT). For many years, transfusion centres have relied on serologic testing for HBsAg, anti-HIV and anti-HCV to minimize the risk of TTVIs, but since the late 1990s NAT for detection of HIV- RNA, HCV-RNA and HBV-DNA has been introduced in a growing number of countries to provide a second layer of safety. Initially, NAT was performed on larger minipools (MPs) of typically 16–96 donations, but since 2005 auto- mated triplex NAT systems have become available for test- ing donations in smaller test pools (<10) or even in individual donation (ID) format. The main benefit of intro- ducing NAT has been the detection of donations in the highly infectious window period (WP), but as ‘WP dona- tions’ are rarely found in the low prevalence countries in the western world, the cost effectiveness of this interven- tion is questioned and has been subject of intensive debate. It is in the context of this discussion that a World Health Organization expert group has drafted Recommendations for screening for TTIs [1], and proposed the use of combo ELISAs as an alternative and perhaps more cost-effective method to reduce transfusion risk of WP donations, partic- ularly in developing countries where circumstances do often not allow for introducing NAT [2]. Nowadays, HIV antibody-p24 antigen combo assays have been widely introduced and are known to reduce the infectious WP by approximately 5 days [3]. Although the HIV combo assays have undoubtedly improved the early diagnosis of HIV infection, these assays are known to suffer from a certain risk of a second diagnostic WP when p24 antigen can be neutralized by antibody [4–7]. The HCV combo assays that simultaneously detect anti-HCV and core antigen have not yet been introduced at a large scale. Studies in seroconver- sion panels of plasma donors have shown that the core antigen detection component in the assays is capable of substantial reduction of the prolonged plateau viraemia phase in the anti-HCV-negative WP and detect HCV approximately 5 days later than NAT in the rapid HCV ramp up phase [8]. However, more recent studies have shown that half of HCV-RNA-positive WP donations have HCV-RNA levels below the detection limit of the combo assays [9]. Although technically, a HBV combo assay for simultaneous detection of anti-HBc and HBsAg could be developed, such assays would not be useful in large parts of the world where HBV is endemic and >10% of the popula- tion has been exposed to the virus. Nevertheless, as anti- HBc-positive blood is known to be infectious some of the time (<10%) if anti-HBs is not detectable [10,11] anti-HBc screening has been introduced in a number of countries with a low HBV prevalence, well before NAT was intro- duced. More recently, a few European countries have intro- duced anti-HBc testing after MP-NAT had already been introduced [12], when it became clear that HBV transmis- sion cases by blood from donors with occult HBV infection (OBI) could still occur [13]. HBV-DNA levels in donors with OBI are generally low and can fluctuate around the detec- tion limit of the NAT systems in use. Recently, more sensi- tive ID-NAT technology for detection of HBV has been introduced, but it is unclear which proportion of HBV transmission cases caused by occult HBV carriers would have been detectable with these assays. HBV transmission reports have shown that HBV-DNA positive blood in OBI can be infectious if anti-HBs titres are below 10 mIU ⁄ ml [13], but the few systematic look back studies that have been performed so far indicate that the risk of HBV trans- mission from occult HBV carriers is relatively low [11,14]. If anti-HBs antibodies are detectable the risk of anti-HBc positive blood to be infectious is negligible [10,11], unless viraemia breaks at relatively high level through a low anti- HBs titer as was observed in one case reports [15]. This latter case report showed that re-appearance of persistently present HBV-DNA in OBI when it increased to approxi- mately 900 copies ⁄ ml at the time anti-HBs titres had declined to 12 mIU ⁄ ml was infectious in two recipients. Correspondence: Marion Vermeulen, South African National Blood Service, 1 Constantia Boulevard Constantia Kloof, Extension 22, Weltevreden Park 1715, Johannesburg, South Africa E-mail: marion.vermeulen@sanbs.org.za ISBT Science Series (2011) 6, 229–233 STATE OF THE ART 5A-S27 ª 2011 The Author(s). ISBT Science Series ª 2011 International Society of Blood Transfusion 229