Quantitative pcr in soil-transmitted helminth epidemiology and control programs: Toward a universal standard

Abstract

There is an increased interest to apply quantitative PCR (qPCR) in epidemiological studies and large-scale deworming programs targeting soil-transmitted helminths (STHs), due to important advantages over the current practice of microscopic stool examination (e.g., higher sensitivity and hookworm differentiation) [1]. Accordingly, qPCR should provide information on STH prevalence and infection intensity to evaluate progress toward WHO program goals (<2% moderate-to-heavy intensity infections) [2] and the ultimate goal to break transmission in targeted geographical areas [1,3]. However, qPCR is an umbrella term for a plethora of different procedures in which the results are most often expressed in units that do not allow interlaboratory comparison [4]. Reporting qPCR results in a universal unit that allows comparison is essential to standardize and compare protocols and subsequently implement qPCR in a programmatic decision algorithm. However, there is currently no consensus on such a universal unit, let alone on how to make international accepted standards available for the community. In this viewpoint, we first delineate the ideal characteristics of such a universal unit (to be used in combination with universal standards, i.e., standardized material that contains a known amount of STH material) in an STH programmatic setting. We then provide a brief overview of the different units that are used to report qPCR results and discuss their benefits and limitations. Subsequently, we illustrate the use of genome equivalents per mL (GE/mL) as a potential universal unit. We end this viewpoint by proposing some necessary steps to pave the way toward a universal standard for nucleic acid amplification techniques in an STH programmatic setting. A discussion on other challenges for standardizing qPCR, such as the existence of different methods to preserve stool samples, DNA extraction, and qPCR protocols, each with their own efficiency, is not within the scope of the present viewpoint. For a more detailed discussion on these challenges, we refer to previously published work [4–8].