Anthrax Laboratory Testing

by CDC

(Page 4 of 10)

Is a nasal swab test an approved diagnostic tool for determining whether a person has been exposed to anthrax?

No. At present, CDC does not recommend the use of nasal swab testing on a routine basis to determine whether a person has been exposed to B. anthracis or as a diagnostic tool. At best, a positive result may be interpreted only to indicate exposure; a negative result does not exclude the possibility of exposure. Nasal swab screening may be used by public health officials to assist in an epidemiological investigation of potentially exposed persons to evaluate the dispersion of spores.

Are health department laboratories capable of conducting testing?

All state health departments are capable of obtaining results of tests on suspected infectious agents. Laboratories are usually classified as Level A, B, C, or D. Level A laboratories are those typically found in community hospitals, and these laboratories should be able to perform initial testing on all clinical specimens (usually blood or some other body fluid). Public health laboratories are usually Level B; these laboratories are valuable for confirming or refuting preliminary test results and can usually perform antimicrobial susceptibility tests on bacteria and viruses. Level C laboratories, which are reference facilities and can be public health laboratories, can perform more rapid identification tests. Level D laboratories are designed to perform the most sophisticated tests and are located in federal facilities such as CDC. Every state has a Laboratory Response Network (LRN) contact. The LRN links state and local public health laboratories with advanced-capacity laboratories, including clinical, military, veterinary, agricultural, water, and food-testing laboratories. Laboratorians should contact their state public health laboratory to identify their local LRN representative. CDC's Emergency Preparedness and Response website provides access to CDC's Centers for Public Health Preparedness, a national network of academic institutions and local health departments whose goal is to ensure that local public health workers are fully prepared to respond to current and emerging health threats, including bioterrorism.

How effective and reliable are anthrax tests?

There are many kinds of tests, and the reliability of each has not been determined. In general, findings from culturing environmental samples are specific; that is, a positive result reflects the true presence of Bacillus anthracis, and a negative result likely means that no B. anthracis is present.

What is subtyping?

Subtyping is a laboratory process to identify different subtypes of organisms, which is not possible with standard microbiological testing. Most Bacillus anthracis subtyping is done by examining the organism's molecular structure for certain genetic characteristics that can then be compared with those of other B. anthracis organisms to determine whether they are the same or different. Differences between these two organisms would indicate different strains.

Is subtyping different from polymerase chain reaction (PCR)?

Polymerase chain reaction (PCR) is a laboratory method used to detect and amplify genetic material from organisms. It can be used to diagnose disease by identifying genetic material (DNA) commonly found in all Bacillus anthracis strains or it can be used to subtype the organism by amplifying specific genetic material and comparing it with known strains of B. anthracis to see if it matches or if it is different. When PCR is used for subtyping, the amplified genetic material is usually further analyzed by other molecular methods, such as DNA sequencing.

What method does CDC use to subtype Bacillus anthracis?

CDC uses a method called MLVA, which is the acronym for multi-locus variable-number of tandem (consecutive) repeat analysis.

How does MLVA (multi-locus variable-number of tandem [consecutive] repeat analysis) identify different strains of anthrax? MLVA examines a number of DNA segments within the chromosome or plasmids of Bacillus anthracis that have specific repeat patterns of nucleotides (fundamental DNA units). These repeats may differ by sequence and length, as well as the number of times that they are repeated. Different types of these repeats and the number of times that they are repeated provide a specific pattern that will identify different strains of the organism. More than 100 different strains of B. anthracis have been identified using this method.

When is environmental sampling performed?

Environmental sampling is the sampling of the air, soil, dust, water, and physical surfaces to identify the presence or absence of bacteria, chemicals, and radiological materials (see "Procedures for Collecting Surface Environmental Samples for Culturing Bacillus anthracis"). In the case of anthrax, this is used to identify its location and presence in the environment. Environmental sampling would be conducted if there were a threat or possibility of Bacillus anthracis contamination. However, the presence of B. anthracis in an environmental sample does not mean the person will get the disease.

Why is environmental sampling performed?

• To identify the site or source of B. anthracis that could lead to exposure and disease,
• To trace the route of an exposure (e.g., a letter),
• To guide clean-up efforts in a facility with known exposure, and
• To assess biosafety procedures in laboratories processing anthrax specimens.

What is a nasal swab test?

A nasal swab involves placing a swab inside the nostrils and taking a culture. The CDC and the U.S. Department of Health and Human Services do not recommend the use of nasal swab testing by clinicians to determine whether a person has been exposed to Bacillus anthracis, the bacteria responsible for anthrax, or as a means of diagnosing anthrax. At best, a positive result may be interpreted only to indicate exposure; a negative result does not exclude the possibility of exposure. Also, the presence of spores in the nose does not mean that the person has inhalation anthrax. The nose naturally filters out many things that a person breathes, including bacterial spores. To have inhalation anthrax, a person must have the bacteria deep in the lungs, and also have symptoms of the disease.

Another reason not to use nasal swabs is that most hospital laboratories cannot fully identify anthrax spores from nasal swabs. They are able to tell only that bacteria that resemble anthrax bacteria are present.

About the Author

www.cdc.gov
The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.