• Extensively Drug-Resistant Tuberculosis (XDR TB) >

Q and A Related to CDC July 3, 2007 Press Conference with National Jewish Medical and Research Center

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Press Briefing Transcript

How can drug susceptibility testing have different results on specimens from the same patient?

• TB bacteria can show variable resistance to second-line TB medications. Thus, a change or difference in test results can, and does, happen when it comes to MDR and XDR TB. Different specimens/samples from the same person can produce different results. The patient may also be infected with more than one strain of TB. TB that consists of more than one strain is not uncommon. For example, in one recent study, 19% of patients were infected with two different strains of TB at the same time. [Reference: Warren RM et al. Patients with Active Tuberculosis often Have Different Strains in the Same Sputum Specimen. Am J Respir Crit Care Med 2004;169:610-4.]

How do these results affect the follow-up of persons exposed to this patient?

• The public health response to drug resistant TB infections, whether MDR or XDR, is the same under the World Health Organization’s TB and Airline Travel guidelines.
• It is important to remember that a patient who has drug-resistant TB represents a significant public health concern.  MDR-TB is a rare version of TB and is resistant to the most commonly used drug therapies.  It is a serious illness that can be transmitted to others, and thus put others at risk for getting a difficult-to-treat disease.
• People with these infections should not be flying on commercial airlines and if it is discovered that such travel has taken place, an effort should be made to notify and evaluate passengers who were seated near them.
• CDC continues efforts to ensure the well being of persons who may have been exposed and infected by this patient.

What is multidrug-resistant tuberculosis (MDR TB)?

• Multidrug-resistant tuberculosis (MDR TB) is TB that is resistant to at least two of the best anti-TB medications, isoniazid and rifampin. These medications are considered first-line drugs and are used to treat all persons with drug-susceptible TB disease.
• A more serious form of MDR TB is called extensively drug-resistant TB (XDR TB). XDR TB is a relatively rare type of TB that is resistant to nearly all medicines used to treat TB disease. Because XDR TB is resistant to the most effective TB medicines used to treat TB, patients are left with very limited useful treatment options.
• Treatment for MDR TB is considerably less effective, more toxic, and more expensive than for drug-susceptible TB. MDR TB, as is XDR TB, is a difficult to treat disease that is often fatal.
• Treatment of MDR TB requires a patient to take 18–24 months of medication, including taking multiple second-line medications, to kill the bacteria.

What is extensively drug resistant tuberculosis (XDR TB)?

XDR TB is a more serious form of multidrug-resistant TB (MDR TB). In both cases, the TB is resistant to the “first-line” antibiotics available for treatment, as well as some of the second-line antibiotics. Both MDR TB and XDR TB, are difficult to treat and are often fatal. XDR TB is defined as TB which is resistant to isoniazid and rifampin, plus resistant to any fluoroquinolone and at least one of three injectable second-line drugs (i.e., amikacin, kanamycin, or capreomycin). Because XDR TB is resistant to first-line and second-line drugs, patients are left with very limited treatment options that are even less effective than for MDR TB that does not qualify as XDR TB.

What is drug susceptibility testing?

• Drug susceptibility testing uses laboratory techniques to determine which medicines will kill the TB bacteria in the patient’s specimen.  The results of drug susceptibility tests can help clinicians choose the appropriate treatment regimen for each patient.

Why are National Jewish Medical Center’s test results different from CDC’s results?

• It is not unusual to have differing drug susceptibility results from multiple specimens from the same patient.  Reasons for differing results include
  • The TB bacteria in a sputum specimen may come from different parts of the lung or different lesions in the lung.
  • The TB bacteria in a sputum specimen may have different types of bacteria such as drug-susceptible bacteria (bacteria that can be killed by TB medicines) and drug-resistant (bacteria that cannot be killed by most TB medicines).
  • The amount of the different types of bacteria may vary over time and from specimen to specimen.
• When drug susceptibility test results from different specimens from one patient are not the same, a treatment regimen is chosen that is most likely to be effective on the most predominant TB bacteria.
• CDC's laboratory functions as a national and supranational (i.e., for other countries) reference facility to provide access to drug susceptibility testing for second-line drugs.

How is drug susceptibility testing conducted?

• CDC receives samples of M. tuberculosis that have been obtained from cultures performed in labs outside of CDC (for example, state public health, international, and private laboratories).
• Before sending samples to CDC, the labs smear cultures on media and keep the cultures warm media until bacteria grow (colonies).
• Colonies of bacteria are scraped from the surface of the culture media, taking care to sample all parts of this growth (colonies).
• The colonies are placed into a broth and kept warm for approximately one week.
• At one week, the broth is diluted to obtain the number of organisms needed to perform susceptibility testing.
• The diluted broth is spread onto solid agar media in Petri plates (drug plates) that contain the drugs used for tuberculosis. There is also a growth control (a plate with no drug) included.
• The drug plates and growth control plates are kept warm for 21 days and then examined for growth. The number of colonies on the growth control plate and on each drug plates is determined. A sample is resistant to a drug when the number of colonies that grow on the drug-containing agar are greater than 1% of the colonies that grow on the growth control.

What does CDC do to ensure laboratory quality control for drug susceptibility testing?

• The CDC drug susceptibility testing is performed using standard techniques with quality control and data review before results are released.
• CDC uses a standardized inoculum with quality control testing to confirm that the correct drugs and drug concentrations are used.
• Quality control testing is also performed on the culture media used to grow the TB bacteria to ensure consistency.
• The CDC laboratory performs M. tuberculosis drug susceptibility testing as described by the Clinical and Laboratory Standards Institute (CLSI; formerly National Committee for Clinical Laboratory Standards). Testing is performed and results are reported in compliance with Clinical Laboratory Improvement Amendments (CLIA) regulations.

What are the first- and second-line drugs used to treat TB?

• When drug susceptibility test results from different specimens from one patient are not the same, a treatment regimen is chosen that is most likely to be effective on the most predominant TB bacteria.
• There are 10 drugs currently approved by the U.S. Food and Drug Administration (FDA) for treating TB. Of the approved drugs, the first-line anti-TB agents that form the core of treatment regimens include
  • isoniazid (INH)
  • rifampin (RIF)
  • ethambutol (EMB)
  • pyrazinamide (PZA)
• Second-line drugs used to treat TB include
  • fluoroquinolones (levofloxacin, moxifloxacin, gatifloxacin)
  • three injectable drugs (amikacin, kanamycin, or capreomycin)
  • ethionamide
  • cycloserine
• Second-line drugs are less effective and more toxic than first-line drugs used to treat TB.

What can be done to improve our ability to prevent and reduce the numbers and cases of TB?

New tools for TB diagnosis, treatment, and prevention are needed to achieve the goal of TB elimination.  New diagnostic tools for TB detection, especially drug-resistant TB, are needed. Rapid diagnostic tests, such as those that have been developed for HIV, are still unavailable for drug-resistant TB.

Last Reviewed: 05/18/2008
Content Source: Division of Tuberculosis Elimination
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention

 
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