High demand for diagnostics for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes coronavirus disease 2019 (COVID-19), combined with an emphasis on decreasing exposures to people infected with the virus, led the US Food and Drug Administration (FDA) to issue an Emergency Use Authorization for several RDTs and multiplex panels that include SARS-CoV-2. RDTs include rapid antigen diagnostics and the first molecular diagnostic for home use. These can be performed with self- or caregiver-collected samples. Some home test kits require that users download a smartphone application that provides test interpretation for the user and reports de-identified data for public health surveillance. These diagnostic kits perform best in symptomatic people; results in asymptomatic people should be interpreted with caution.
People who become ill while traveling might seek medical care abroad; development and availability of RDTs for diagnosis of tropical infectious diseases has expanded greatly in recent years, and travelers might return home having been diagnosed based on results from these tests. RDTs for tropical infections typically are lateral-flow immunochromatographic tests that detect antigens from or antibodies to certain pathogens. Because only 1 such test (for malaria) is cleared for use in the United States, the diagnostic characteristics of RDTs used overseas are unfamiliar to most providers. Additionally, a variety of RDTs might be available for certain pathogens (e.g., dengue) in other countries, with widely varying or poorly studied performance characteristics. Institutions that do not have continuous access to a single brand of test further complicates interpretation of results provided by the laboratory.
The following is an illustrative, though by no means exhaustive, list of several common infections for which RDTs are available.
Dengue. Rapid, lateral-flow assays are available to detect the dengue nonstructural protein 1 (NS1) antigen, and IgM and IgG. Dengue tests have widely variable performance characteristics depending on the manufacturer, circulating dengue types, a patient’s past medical history, and symptom duration.
Emerging Infections. Emerging pathogens represent a diagnostic challenge. Rapid assays became available after outbreaks of chikungunya, Ebola, and Zika. Such assays might not be available or well-studied at the peak of an outbreak, however.
Leishmaniasis. Assays to detect antibodies against the rK39 antigen (visceral leishmaniasis) have demonstrated good specificity in endemic regions, and highest sensitivity for detecting disease in South Asia.
Leptospirosis. Because of the many pathogenic and intermediate Leptospira serotypes that result in human disease worldwide, the usefulness of serologic assays for diagnosing leptospirosis is limited.
Malaria. An FDA-cleared RDT for malaria is available, and malaria RDTs are widely used throughout the world. In general, these tests perform best for Plasmodium falciparum, with variable or poor performance for other Plasmodium species.
Typhoid. Rapid serologic tests have demonstrated only moderate accuracy to diagnose typhoid. Additionally, these tests are designed to detect Salmonella enterica serotype Typhi only.
The number of assays compatible with POC testing will undoubtedly continue to increase. Building upon testing milestones achieved during the COVID-19 pandemic, “at home” testing, including molecular testing, is expected to increase in the coming years for both respiratory viruses and other pathogens. Because of the wide breadth and diversity of infecting pathogens in returned travelers, use of POC testing for nondomestic infectious diseases might not be practical for most centers once test volume, personnel training, and cost are taken into consideration. POC testing for common syndromes that affect travelers and nontravelers alike (e.g., respiratory tract and gastrointestinal infections) could provide rapid diagnosis, inform triage decisions, and limit unnecessary laboratory testing.
The following authors contributed to the previous version of this chapter: Elizabeth Rabold, Jesse Waggoner
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