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How Do You Test for Anthrax?

Chris Suellentrop
Posted Tuesday, Oct. 16, 2001, at 7:25 PM ET

The Washington Post reported today that "sophisticated tests" confirmed the presence of anthrax on a letter mailed to Senate Majority Leader Tom Daschle. Yesterday, two "preliminary tests" detected anthrax in the same letter. What's the difference between these two types of anthrax tests?

Explainer doesn't know what specific tests were used by the U.S. Capitol Police or the U.S. Army Medical Research Institute of Infectious Diseases in Fort Detrick, Md. But generally speaking, anthrax tests can be divided into two categories: screening tests and confirmatory tests.

Screening tests are usually done on-site at the discretion of local law enforcement. By their nature, screening tests (which can include the small testing kits that this New York Times story says resemble home-pregnancy tests) are sensitive but not specific--meaning they're good at identifying whether a substance contains an organism or contaminant, but they're not as good at identifying the specific organism. For that you need confirmatory tests. Because they're so sensitive, screening tests are much more likely to give false positive results than confirmatory tests.

Confirmatory tests are done in a laboratory. They take longer, but they're more accurate. They often involve growing a culture in a laboratory and then identifying just what it is that's been grown. This Reno Gazette-Journal article describes three of the confirmatory tests used on a letter sent to Microsoft's Reno office (a "phage" test, a fluorescent antibody test, and a capsule stain test). Two of those tests were positive and one was negative. DNA analysis is another type of confirmatory test. DNA analysis can determine whether the strain of anthrax that's been discovered is virulent or harmless.

For more information on anthrax tests, Explainer recommends this Washington Post article and this New York Times article.

Next question?

Explainer thanks Bob Salcido of the Nevada State Health Division.

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Chris Suellentrop, a former Slate staffer, writes "The Opinionator" for the New York Times.
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