Standardization of Nucleic Acid Tests for Clinical Measurements of Bacteria and Viruses

Abstract

f Nucleic acid-based tests for infectious diseases currently used in the clinical laboratory and in point-of-care devices are diverse. Measurement challenges associated with standardization of quantitative viral load testing are discussed in relation to human cytomegalovirus, BK virus, and Epstein-Barr virus, while the importance of defining the performance of qualitative methods is illustrated with Mycobacterium tuberculosis and influenza virus. The development of certified reference materials whose values are traceable to higher-order standards and reference measurement procedures, using, for instance, digital PCR, will further contribute to the understanding of analytical performance characteristics and promote clinical data comparability. Molecular approaches, such as nucleic acid (NA) amplification-based tests (NAATs) and sequence analysis, are increasingly replacing conventional microbiological methods, such as pathogen propagation in culture and techniques for the detection of antigens, for (i) viral load quantification, (ii) the detection of pathogenic viruses and bacteria, (iii) monitoring viral and bacterial resistance to therapeutic agents, and (iv) monitoring transmission across communities, as they often enable faster, more accurate, or more sensitive measurements (1, 2). However, commercial and in-house NAATs for infectious diseases utilize different technologies, reaction chemistries, and calibration materials, leading to demonstrable variability in the test results in terms of (i) numerical values (e.g., numbers of genome copies) for quantitative methods or (ii) presence or absence of the pathogen for qualitative methods (3-5).The In Vitro Diagnostics Directive (IVDD) in Europe has promoted assay standardization in the clinical laboratory community, through requiring manufacturers to provide information about the traceability of their calibrators (for definitions of terms in italics, see Table 1), in compliance with ISO 17511 (6). The concept of metrological traceability in clinical chemistry and laboratory medicine has been comprehensively discussed in several articles (7-9). For small-molecule measurements in clinical chemistry, such as those of blood glucose, creatinine, cortisol, and electrolytes, reference measurement procedures of high metrological quality are available which relate the quantity value of the calibrators and reference materials used in a calibration hierarchy traceable to the International System of Units (SI) as described in ISO 17511 (6, 8). However, for the majority of the more complex biomeasurements, including those based on NA testing and infectious pathogen detection, reference materials whose values are traceable to the SI and reference measurement procedures (with well-understood uncertainty) are not yet available. ISO 17511 indicates recognition of the fact that, for measurements of, for example, viral NAs (for human cytomegalovirus [CMV], human immunodeficiency virus [HIV], and hepatitis B virus [HBV]), typically WHO international standards (IS) are available, w...

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