Simple Screening Tests

Simple laboratory tests are used to screen biologic samples for the presence of biological agents or other substances that might indicate a disease, an infection with a biologic agent, or a contamination with a toxin or other chemical. The classical approach to identification of biologic agents involves direct examination of stained materials by microscopic examination for the presence of agents (DHHS/CDC, n.d.; York, 2003). Microscopic examination relies on staining characteristics and the size and shape of the organisms found.With few exceptions, it is rarely possible to identify an agent based only on microscopic characteristics. Direct stains may help eliminate organisms from further consideration, but they are usually not sufficient without further testing to identify a pathogen. Wet mounts are used for the direct microscopic examination of clinical materials for fungi and parasitic agents. Additional simple assays take advantage of the ability of an organism to metabolize chemicals or produce chemical reactions that result in color changes of a substrate. Recently, many assay kits have been developed for waived tests. These waived test kits can produce reliable results in most cases; however, one must understand the limitations of these kits and the need for confirmatory testing. Field test kits, often referred to as handheld assays, have become popular with first responders who have a need to know if an unknown material contains a biologic agent or toxic chemical. Simple immunologic reactions, coupled with a colorimetric indicator, form the detection systems for many handheld assays. Validating the performance of handheld assays in comparative studies is a task generally reserved for governmental agencies or contract laboratories (Emanuel et al., 2003). The major advantage of the simple assays is that they are quick, as a result may be available in 5 to 20 minutes. The short turnaround time makes these tests ideal for use by surveillance systems provided one understands the limitations of the tests and that a method for confirmatory testing is available.

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