Among major advances in solid organ transplantation is the development of a rational basis for the prevention, diagnosis and management of viral infections (i.e. cytomegalovirus (CMV) (1). These strategies are the result of the availability and deployment of effective antimicrobial agents coupled with quantitative molecular and antigen-based microbiologic assays for these pathogens. Nucleic acid-based tests (NAT) have been developed for many pathogens, including Epstein Barr virus (EBV) and BK polyomavirus, and are available commercially, at public health laboratories, and as ‘home brew’ diagnostic tools. These assays have become the basis of ‘pre-emptive’ antiviral therapies which use sensitive, quantitative assays to detect presymptomatic infections as a basis for the initiation of antimicrobial therapy. Quantitative molecular assays are routinely used to monitor responses to antiviral therapies for e.g. CMV, BK and EBV-associated processes. The presence of viral infections is also often used as an indirect indicator of excessive immunosuppression.
Transplant infectious disease physicians have long suspected that quantitative CMV and EBV assay results could not be compared between laboratories using different substrates (e.g. whole blood vs. serum), assay platforms or molecular primers. The degree to which these data varied has now been documented in a pair of important studies published in this issue by Preiksaitis et al. and Pang et al. (2,3). These studies are fairly unique in that they included laboratories from Canada and the United States and were sponsored by professional societies (the American Society of Transplantation and the Canadian Society of Transplantation), indicating the importance of NAT testing to the current clinical practice of organ transplantation. The studies indicate that ‘only 47.0% of all results fell within acceptable standards of variation’ for EBV viral load detection and ‘only 57.6% of results fell within acceptable standards' for CMV viral load detection (2,3). Variability in commercial assays was lower than that for laboratory-developed assays. The greatest variation was in the sensitivity or lower limits of detection for each assay (i.e. the ability to detect early viral infection), which must be sufficient to allow ‘early’ detection and treatment of infection while avoiding false positive tests or the detection of clinically irrelevant levels of infection.
The development of internationally accepted reference standards would be a major step in standardization of assays now used routinely in clinical care. Such standards could be used in the development of internal controls for clinical assays, the production of proficiency panels for laboratory quality assessment and as a basis (‘gold standard’) for the development of new diagnostic technologies. The availability of laboratory proficiency testing also implies the development of performance standards to assure technical competence and consistency within clinical laboratories. Thus, standards for checking, recording and reporting of assay results would limit the risk of miscommunication between clinical laboratories and clinical centers—a flaw recently associated with at least one cluster of donor-derived infections due to HIV (4). Until such standardization is achieved, viral infections in individual patients must be monitored using a single, quantitative assay from a single clinical laboratory.
These data raise a series of challenges for the transplant community and opportunities for commercial development of diagnostic assays. Recent clusters of organ donor-derived infections, including HIV, hepatitis C virus and lymphocytic choriomeningitis virus (LCMV), have demonstrated the need for improved and expanded testing of organ donors (5). Improved microbiological screening of organ donors will depend on the development of a consensus regarding a ‘list’ of pathogens to assay and regarding the optimal assay characteristics for each pathogen. In particular, studies must be conducted of organ donors to determine the level of sensitivity for each assay that will protect recipients while minimizing the exclusion of useable organs and tissues by false positive microbiologic assays. Such assays could be multiplexed (multiple simultaneous) NAT tests or based in other detection technologies. They must be rapid, easily performed, cost-effective and standardized. Such microbiological assay panels would largely supplant the ‘high risk donor’ exclusion criteria based in social and epidemiological characteristics and which are currently in use. The standards for such assays must span organ and tissue procurement systems.
Microbiologic assays are undergoing evolution. In addition to improving pathogen detection, investigational assays will, ultimately, allow clinicians to define the individual's risk for specific infections in terms of the presence or absence of protective immunity to each organism. Some of these tools are presented in Table 1. Such assays could allow antimicrobial prophylaxis to be provided only to those who need it, i.e. those without established protective immunity against specific pathogens. Until such assays are available for clinical use, each immunocompromised host remains an experiment in the balance between immunosuppression and infection.
|• Quantitative viral load assays (nucleic acid detection)|
|• Antigen detection|
|• Cytotoxic lymphocyte assays|
|• Mixed lymphocyte cultures|
|• HLA-linked tetramers|
|• Intracellular cytokine staining|
|• Interferon release assays|
|• Genomics/proteomics (patterns of gene expression) in infection|