Dr. Tim Johnson Explains Anthrax Tests
B O S T O N, Oct. 19, 2001 -- 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.
Cultures — Culturing, or growing, anthrax bacteria is relatively simple, but does take some time. First the sample is plated, or placed, onto a lab petri dish containing a solid layer of gelatin, called agar, which contains nutrients for the bacteria to grow. Once plated, it takes 18 to 24 hours before the bacteria have grown enough to be analyzed. Scientists then study the bacteria that have grown on the plates.
PCR: Polymerase Chain Reaction — PCR allows scientists to amplify, or create copies, of small amounts of anthrax DNA into quantities large enough to analyze. The process only amplifies the specific DNA that you are looking for. This test can be done quickly in the field to determine if anthrax is present, and also again more thoroughly in the lab to determine if the bacteria contains specific genes, such as ones that might make the bacteria antibiotic-resistant.
Antibody tests — These tests indicate if your body has begun to fight off infection. One such test, sometimes called an ELISA, indicates if your body has produced antibodies against the anthrax bacteria. When you become infected with anything your body produces antibodies that help fight off the infection. If you have antibodies against the anthrax bacteria, and you were never exposed to it before, it would indicate that you are infected.
Microscopy — Anthrax bacteria have many distinguishing physical characteristics that scientists use to identify them from other types of bacteria under the microscope. For example, the anthrax are shaped like small rods, are non-motile, meaning they do not move around on their own, and are can be stained by certain dyes.
DNA fingerprinting — This test is usually done after all the others to find out the specific type of anthrax bacteria involved. By closely scrutinizing the bacteria's DNA, scientists can determine if two anthrax samples originate from the same strain. For fingerprinting, labs use PCR technology as well as other genetic techniques to study bacterial genes. Each bacterium has its own unique DNA fingerprint. By sequencing certain genes scientists can compare samples to an existing library of anthrax bacteria sequences. Figuring out where a bacterium came from can very difficult, especially if it turns out to be a strain commonly found in research institutions.