Dr. Tim Johnson Explains Anthrax Tests

Due to the recent anthrax exposures, the American public has been rapidly learning the scientific complexities of the disease. However, since most of us are not microbiologists, the medical and scientific jargon is at times understandably confusing. We hope the following questions and answers on anthrax testing help you sort it out.

Watch the ABCNEWS Anthrax Q&A with Dr. Timothy Johnson

Q: What is the difference between a screening test and a diagnostic test?

A: A screening test, such as a nasal swab, tests whether a person has been exposed to anthrax by testing for bacteria spore presence. If spores are found in mucous from the nose, or from a skin sample, it simply means you have been exposed to the bacteria; it does not mean that you have the anthrax disease, which is when the bacteria have successfully started an infection. If no bacteria spores are found, it is likely that you were not exposed; however it is possible to have been exposed and yet have a negative nasal swab test.

A diagnostic test is done to determine if somebody has actual anthrax disease. These tests tell doctors if the bacteria have entered inside the body and actively infected a patient's blood and/or lungs. Tests can also tell if the patient has begun to fight off the infection.

Q: What is a nasal swab?

A: A nasal swab can be used as a screening test for anthrax. To get a sample of nasal mucous, a health care worker rotates a cotton swab around the inside of the nose to pick up any bacteria spores that might be present. Again, because it is only a screening test, it cannot be used to diagnose actual anthrax disease.

Q: What is a field test for anthrax?

A: A field test is one that can be done outside of the lab. Investigators use these tests on the scene of suspected exposure to determine if anthrax is present, for example, on surfaces within a building or on a person's clothes. As a general rule, field tests are usually preliminary, and additional lab tests are usually necessary to confirm field test results.

Q: How does a field test work?

A: Right now, there are at least two tests available to identify the anthrax bacteria in the field. The first type is very similar to a home pregnancy test. A sample from a wiped surface is added to a test strip covered with proteins and chemicals that detect the bacteria. If the bacteria are present, the result is a visual indicator on the strip that is detected by eye. More sensitive machines can also be used to read the strips.

Another field test available, but not common, uses polymerase chain reaction (PCR) technology (see below for definition). Samples can be placed into a machine on site, and the results will indicate if anthrax bacteria DNA is present.

Q: What types of samples are tested?

A: Anthrax bacteria can be isolated from blood, fluid from around the lungs, skin samples, or from swabs taken from the surfaces of objects or people (like the nasal swab).

Q: How do scientists know if a sample contains anthrax?

A: There are several tests capable of detecting and analyzing the anthrax bacteria, including: cultures, PCR, antibody tests, microscopy, and DNA fingerprinting.

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