Anthrax Q & A: Laboratory Testing
Can I get screened or tested to find out whether I have been exposed to anthrax?
There is no screening test for anthrax; there is no test that a doctor can do for you that says you've been exposed to or carry it. The only way exposure can be determined is through a public health investigation. Nasal swabs and environmental tests, are not tests to determine whether an individual should be treated. These kinds of tests are used only to determine the extent of exposure in a given building or workplace.
If patients are suspected of being exposed to anthrax, should they be quarantined or should other family members be tested?
Anthrax is not known to spread from one person to another person. Therefore, there is no need to quarantine individuals suspected of being exposed to anthrax or to immunize or treat contacts of persons ill with anthrax, such as household contacts, friends, or coworkers, unless they also were also exposed to the same source of infection.
CDC collect samples to test the bacteria?
CDC is engaging its partners in the Laboratory Response Network (LRN) in states all across the United States. The LRN is a collaborative partnership and multilevel system linking state and local public health laboratories with advanced capacity laboratories—including clinical, military, veterinary, agricultural, water, and food-testing laboratories—to rapidly identify threat agents, including anthrax. Local clinical laboratory testing is confirmed at state and large metropolitan public health laboratories. CDC conducts the definitive or highly specialized testing for major threat agents. There are 100 laboratories in the network; none of them are commercial labs.
an area is tested for the presence of Bacillus anthracis, how long
does it take to get the results?
Before testing can begin, samples must be collected in a form suitable for testing. The length of time it takes to get test results depends on both the kind of test to be performed and the laboratory’s workload. Some tests may take only a short time to perform, but confirmation takes longer. It may take many days to get the test results.
Testing is a two-step process. The first test, a screening test, may be positive within 2 hours if the sample is large and contains a lot of Bacillus anthracis, the organism that causes the disease anthrax. However, a positive reading on this first test must be confirmed with a second, more accurate test. This confirmation test, conducted by a more sophisticated laboratory, takes much longer. The length of time needed depends in part on how fast the bacteria grow, but results are usually available 1 to 3 days after the sample is received in the laboratory.
CDC recommend the use of home test kits for anthrax?
Hand-held assays (sometimes referred to as “Smart Tickets”) are sold commercially for the rapid detection of Bacillus anthracis. These assays are intended only for the screening of environmental samples. First responder and law enforcement communities are using these as instant screening devices and should forward any positive samples to authorities for more sensitive and specialized confirmatory testing. The results of these assays should not be used to make decisions about patient management or prophylaxis. The utility and validity of these assays are unknown.
At this time, CDC does not have enough scientific data to recommend the use of these assays. The analytical sensitivity of these assays is limited by the technology, and data provided by manufacturers indicate that a minimum of 10,000 spores is required to generate a positive signal. This number of spores would suggest a heavy contamination of the area (sample). Therefore a negative result does not rule out a lower level of contamination. Data collected from field use also indicate specificity problems with some of these assays. Some positive results have been obtained with spores of the non-anthrax Bacillus bacteria that may be found in the environment.
For these reasons, CDC has been asked to evaluate the sensitivity and specificity of the commercially available, rapid, hand-held assays for B. anthracis. When this study is completed, results will be made available. Conclusions from this study are not expected in the near future.
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.
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.
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.
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.
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.
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.
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.
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.
is the turnaround time for an anthrax test of an environmental sample?
Before testing can begin, samples must be collected and arrive in a form suitable for testing. The length of time necessary to get results of tests depends on transportation to the laboratory and the specific tests to be done. Testing is a two-step process. Initial screening tests (such as Gram stain) may be positive within two hours if the sample is large and the concentration of bacteria is high. These tests are used to narrow the definition of the sample. The confirmation tests take much longer, depending in part on how fast the bacteria grow, but are usually available 24-48 hours after the sample is received by the laboratory.
Mayo Clinic/Roche Rapid Anthrax Test a new test?
This is not a "new test." The Laboratory Response Network (LRN) has been using a validated real-time polymerase chain reaction (PCR) assay on the LightCycler for some time. CDC has also developed and validated real-time PCR assays for Bacillus anthracis for the SmartCycler™, ABI/PE 7700 and 5700. In addition, Idaho Technology has a real-time PCR assay for B. anthracis that can be used with the R.A.P.I.D.™, which is similar to the LightCycler™. SmartCycler is a trade name of Cepheid; R.A.P.I.D. is a trade name of Idaho Technology; LightCycler is a trade name of Roche; Idaho Technology is the name of a company.
Mayo Clinic assay the same as the assay available to Laboratory Response
Network (LRN) laboratories?
No. The Mayo Clinic assay targets the Lethal Factor (lef) gene on the virulence plasmid p0X1 and the Protective Antigen gene (pag) on p0X2. This assay has been tested with DNA from 32 strains of Bacillus anthracis, 26 Bacillus species, and 21 different bacterial genera commonly encountered in human specimens. The Mayo Clinic assay has not been validated in multiple laboratories.
The CDC assay uses targets on p0X1, p0X2, and the chromosome. A total of 100 Bacillus anthracis isolates were used to evaluate the sensitivity of the assay. Of the 100, 77 were selected to provide the best possible representation with respect to geographic origin and date isolated. The strains were obtained from infected animals, humans, and from industrial sites associated with anthrax outbreaks; they span 58 years (1939-1997) and are from various countries. In addition, five p0X1-cured strains (including the Sterne strain) and one p0X2-cured strain (Pasteur strain) were included. For evaluation of the specificity of the assay, 54 Bacillus species were used (B. subtilis, 9; B. cereus, 23; B. thuringiensis, 12; and B. megaterium, 10) as well as 250 other DNAs of various viruses and bacteria from human, animal, and insect sources. The assay was validated in a multicenter study by using state public health laboratories that had the specific platform.
are the limitations of the Mayo Clinic test?
Because the Mayo Clinic assay uses only two plasmid targets, it cannot identify Bacillus anthracis strains such as Sterne or Pasteur that may be present in environmental specimens. This would not be a problem if the assay were used to confirm the identity of a gram-positive, non-motile, non-hemolytic rod.
going to validate this assay?
CDC is testing samples from the current anthrax outbreak. When we have sufficient time, we will study this and other anthrax assays.
aware of any sensitivity or specificity issues with the Mayo Clinic test?
Should we expect a large number of false positive/negative results?
On the basis of the data provided by the Mayo Clinic, the assay appears to be sensitive and specific. However, the results are only as good as the method used to prepare the sample for analysis. There have been no data provided to indicate the types of samples that can be assayed or how they are to be processed. The Food and Drug Administration (FDA) has not seen the package insert for this test. The CDC assay has been validated for different types of samples and sample processing methods.
furnishing CDC-tested equipment and reagents to laboratories?
Through the bioterrorism cooperative agreement, CDC has funded the purchase of platforms for real-time polymerase chain reaction (PCR) assays for the Laboratory Response Network (LRN). To date, 61 instruments have been purchased or ordered. Among these are 17 LightCyclers,™ 23 SmartCyclers™, 13 7700s, and 5 5700s. Reagents for real-time anthrax assays are made at CDC and placed in inventory. They are available at no charge to LRN laboratories. Currently, reagents for the LightCycler™ anthrax assay are available; reagents for the other platforms will be available soon. All of the assays have undergone the same rigorous validation procedure.
laboratory asks our opinion on whether to use Mayo Clinic/Roche Rapid
test, what is our answer?
The Food and Drug Administration (FDA) considers this an investigational assay. As such it should be used only in conjunction with other tests, such as culture tests. Currently, polymerase chain reaction (PCR) assays are not considered confirmatory assays.
CDC accept results from laboratories that use this assay?
Currently, polymerase chain reaction (PCR) assays are not considered confirmatory tests for anthrax. PCR-positive specimens (or cultures) should be forwarded to the nearest LRN laboratory for confirmation.
- Page last reviewed: November 19, 2012
- Page last updated: November 19, 2012
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