Chapter 9. Laboratory testing for determination of population immune status

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Other than testing that is performed for case confirmation, the laboratories in the GMRLN are often responsible for providing laboratory support for serological surveillance conducted on a selected population or community. These seroprevalence studies are performed to assess the impact of vaccination programmes and aid in the identification of susceptible groups. Seroprevalence studies involve aspects of study planning and design as well as the serologic testing and analysis of results of specimens collected from a defined population over a specified period of time. The publication, Guidance on conducting serosurveys in support of measles and rubella elimination in the WHO European Region, provides additional information including sampling design [1].

While EIA testing may be performed to confirm the presence of measles- or rubella-specific IgG antibody to assess the immunity of individuals (e.g., health-care personnel), detection of IgG by EIA is also the most common method utilized for the estimation of population immunity to measles and rubella. Population-based seroprevalence studies can help characterize the immune profile of target populations, however, there are serious limitations and challenges. Careful study design is required to generate quality data that are useful for identification of susceptible populations and at-risk groups. Adequate resources and support are needed to ensure results are available in a timely fashion.

Depending on the scope of the study and the resources available, residual serum specimens submitted for diagnostic testing (convenience or opportunistic sample) are often used, or serum specimens may be collected according to the sampling strategy. While other sources of antibody have been used in seroprevalence studies such as oral fluid, breast milk, or whole blood, these fluids have a reduced concentration of antibody compared to serum and the sample volume used in an assay to measure IgG or a test that measures neutralizing antibody is difficult to adjust to accommodate that variation [2].

This chapter includes a discussion of the relationship between the measurement of virus-specific IgG and the determination of measles or rubella immunity. Guidelines are provided for the evaluation and interpretation of equivocal results in the calculation of seroprevalence. The last section provides an overview of the critical elements to consider in the planning, preparation, and coordination of seroprevalence studies.

Note: In this manual, commercial measles and rubella IgG assays are referred to as EIAs for simplicity but may include assays with alternative detection systems.