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Education Materials > Publications > Self-Study Modules on TB > Module 3 > Study Questions

Self-Study Modules on Tuberculosis

This is an archived document. The links are no longer being updated.

Module 3: Diagnosis of Tuberculosis Infection and Disease

Answers To Study Questions

3.1. What is the tuberculin skin test used for?
The tuberculin skin test is used to determine whether a person has TB infection.

3.2. How is the Mantoux tuberculin skin test given?

The Mantoux skin test is given by using a needle and syringe to inject 0.1 ml of 5 tuberculin units of liquid tuberculin between the layers of the skin (intradermally), usually on the forearm.

3.3. With the Mantoux skin test, when is the patient's arm examined?

The patient's arm is examined 48 to 72 hours after the tuberculin is injected.

3.4. How is the induration measured?

The diameter of the indurated area is measured across the forearm; erythema (redness) around the indurated area is not measured, because the presence of erythema does not indicate that a person has TB infection.

3.5. Why is the Mantoux skin test preferable to multiple-puncture tests?

The Mantoux skin test is preferable to multiple-puncture tests because it is more accurate. In multiple-puncture tests, the amount of tuberculin that actually enters the skin cannot be measured. In the Mantoux test, however, the amount of tuberculin can always be measured. Therefore, the Mantoux test is more accurate, and it is the preferred method.

3.6. What two factors are considered when classifying a skin test reaction as positive or negative? What additional factor is considered for people who may be exposed to TB on the job?

Whether a reaction to the Mantoux tuberculin skin test is classified as positive depends on the size of the induration and the person's risk factors for TB.

For people who may be exposed to TB on the job, an additional factor is considered: the risk of exposure to TB in the person's job.

3.7. For which groups of people is 5 or more millimeters of induration considered a positive reaction? Name four.

An induration of 5 or more millimeters is considered a positive reaction for
  • People with HIV infection
  • Close contacts of people with infectious TB
  • People with chest x-ray findings suggestive of previous TB disease
  • People who inject illicit drugs and whose HIV status is unknown

3.8. For which groups of people is 10 or more millimeters of induration considered a positive reaction? Name seven.

An induration of 10 or more millimeters is considered a positive reaction for
  • People born in areas of the world where TB is common (foreign-born persons)
  • People who inject illicit drugs but who are known to be HIV negative
  • Low-income groups with poor access to health care
  • People who live in residential facilities (for example, nursing homes or correctional facilities)
  • People with medical conditions that appear to increase the risk for TB (not including HIV infection), such as diabetes
  • Children younger than 4 years old
  • People in other groups likely to be exposed to TB, as identified by local public health officials

3.9. For which group of people is 15 or more millimeters of induration considered a positive reaction?

An induration of 15 or more millimeters is considered a positive reaction for people with no risk factors for TB.

3.10. Name two factors that can cause false-positive reactions to the tuberculin skin test.

Two factors that can cause false-positive reactions are infection with nontuberculous mycobacteria (mycobacteria other than M. tuberculosis) and vaccination with BCG.

3.11. Is there a reliable way to distinguish a positive tuberculin reaction caused by vaccination with BCG from a reaction caused by true TB infection?

No, there is no reliable way to distinguish a positive tuberculin reaction caused by vaccination with BCG from a reaction caused by true TB infection. However, the reaction is more likely to be due to TB infection if any of the following are true:
  • The reaction is large
  • The person was vaccinated a long time ago
  • The person comes from an area of the world where TB is common
  • The person has been exposed to someone with infectious TB disease
  • The person's family has a history of TB disease

3.12. Name three factors that can cause false-negative reactions to the tuberculin skin test.

False-negative reactions may be due to
  • Anergy
  • Recent TB infection (within the past 10 weeks)
  • Very young age (younger than 6 months old)

3.13. What is anergy?

Anergy is the inability to react to skin tests because of a weakened immune system. Many conditions, such as HIV infection, cancer, or severe TB disease itself, can weaken the immune system and cause anergy. HIV infection is a main cause of anergy.

3.14. How is anergy testing done?

Anergy testing is done by giving skin tests using two substances other than tuberculin. The recommended substances for anergy testing are mumps, Candida (a type of fungus), or tetanus extracts. Most healthy people will have a skin test reaction to one or more of these substances.

People who do not react to any of the substances, including tuberculin, after 48 to 72 hours (that is, people who have less than 3 millimeters of induration to all of the skin tests), are considered anergic. People who have a reaction (3 or more millimeters of induration) to any of the substances are NOT anergic. If a person being evaluated for anergy has a reaction of 5 or more millimeters of induration to tuberculin, he or she is considered to have TB infection, regardless of the reaction to the other substances.

3.15. After TB has been transmitted, how long does it take before TB infection can be detected by the tuberculin skin test?

After TB has been transmitted, it takes 2 to 10 weeks before TB infection can be detected by the tuberculin skin test.

3.16. What should be done if a patient has a negative skin test reaction but has symptoms of TB disease?

Any patient with symptoms of TB disease should be evaluated for TB disease, regardless of his or her skin test reaction. In fact, people with symptoms of TB disease should be evaluated for TB disease right away, at the same time that the tuberculin skin test is given.

3.17. What is the booster phenomenon?

The booster phenomenon is a phenomenon in which people (especially older adults) who are skin tested many years after becoming infected with M. tuberculosis may have a negative reaction to an initial skin test, followed by a positive reaction to a skin test given up to a year later. This happens because in some people who have TB infection, the ability to react to tuberculin lessens over time. The first skin test "jogs the memory" of the immune system, boosting its ability to react to tuberculin.

3.18. What is the purpose of two-step testing?

The purpose of two-step testing is to tell the difference between boosted reactions and reactions caused by recent infection. Because it provides accurate information about each employee's baseline skin test reaction, two-step testing is used in many TB screening programs for skin testing employees when they start their job.

3.19. In what type of situation is two-step testing used?

Two-step testing is used in many TB screening programs for skin testing employees when they start their job.

3.20. How is two-step testing done?

If a person has a negative reaction to an initial skin test, he or she is given a second test 1 to 3 weeks later.
  • If the reaction to the second test is positive, it is considered a boosted reaction (due to TB infection that occurred a long time ago).
  • If the reaction to the second test is negative, the person is considered uninfected. In this person, a positive reaction to a skin test given later on will probably be due to recent TB infection.

3.21. What are the four steps in diagnosing TB disease?

The four steps in diagnosing TB disease are
  1. The medical history
  2. The tuberculin skin test
  3. The chest x-ray
  4. The bacteriologic examination

3.22. What parts of a patient's medical historymshould lead a clinician to suspect TB?

Clinicians should suspect TB disease in patients who have
  1. Been exposed to a person who has infectious TB,
  2. Symptoms of TB disease,
  3. Had TB infection or TB disease before, or
  4. Risk factors for developing TB disease.

3.23. What are the common symptoms of pulmonary TB disease? What are the general symptoms of TB disease (pulmonary or extrapulmonary)?

Pulmonary TB disease usually causes one or more of the following symptoms:
  • Coughing
  • Pain in the chest when breathing or coughing
  • Coughing up sputum or blood

The general symptoms of TB disease (pulmonary or extrapulmonary) include

  • Weight loss
  • Fatigue
  • Malaise
  • Fever
  • Night sweats

The symptoms of extrapulmonary TB disease depend on the part of the body that is affected by the disease. For example, TB of the spine may cause pain in the back; TB of the kidney may cause blood in the urine.

3.24. For patients with symptoms of TB disease, should clinicians wait for tuberculin skin test results before starting other diagnostic tests?

No. For patients with symptoms of TB disease, clinicians should not wait for tuberculin skin test results before starting other diagnostic tests.

3.25. Name the two purposes of the chest x-ray.

The purposes of the chest x-ray are to
  • Help rule out the possibility of pulmonary TB disease in a person who has a positive reaction to the tuberculin skin test
  • Check for lung abnormalities in people who have symptoms of TB disease

3.26. Can the results of a chest x-ray confirm that a person has TB disease? Why or why not?

No, the results of a chest x-ray cannot confirm that a person has TB disease. This is because a variety of illnesses may produce abnormalities whose appearance on a chest x-ray resembles TB. Although an abnormality on a chest x-ray may lead a clinician to suspect TB, only a bacteriologic culture that is positive for M. tuberculosis proves that a patient has TB disease.

3.27. What are the four ways to collect sputum specimens? Indicate which procedure is the cheapest and easiest to perform.

  • Usually, patients who are suspected of having pulmonary TB disease simply cough up sputum and the sputum is collected in a sterile container for processing and examination. This is the cheapest and easiest procedure. If a patient cannot cough up sputum on his or her own, other techniques can be used to obtain a specimen.
  • An induced sputum sample can be obtained by having the patient inhale a saline (salt water) mist, which causes the patient to cough deeply.
  • Bronchoscopy can be used to obtain pulmonary secretions or lung tissue. In this procedure, an instrument called the bronchoscope is passed through the mouth directly into the diseased portion of the lung, and some sputum or lung tissue is removed.
  • Gastric washing involves inserting a tube through the patient's nose and passing it into the stomach. The idea is to get a sample of sputum that has been coughed into the throat and then swallowed.

3.28. What do laboratory personnel look for in a smear?

Laboratory personnel use the microscope to look for acid-fast bacilli (AFB) on the smear. AFB are mycobacteria that stay stained even after they have been washed in an acid solution. Tubercle bacilli are one kind of AFB.

3.29. What does a positive smear indicate about a patient's infectiousness?

The results of the smear examination can be used to help determine the infectiousness (contagiousness) of the patient. Patients who have many tubercle bacilli in their sputum have a positive smear. Patients who have positive smears are considered infectious because they can cough many tubercle bacilli into the air.

3.30. Why is it necessary to culture a specimen?

Culturing the specimen is necessary to determine whether the specimen contains M. tuberculosis and to confirm a diagnosis of TB disease. (However, in some cases, patients are diagnosed with TB disease on the basis of their signs and symptoms, even if their specimen does not contain M. tuberculosis.)

3.31. What does a positive culture for M. tuberculosis mean? How is this important for the TB diagnosis?

A positive culture for M. tuberculosis means that M. tuberculosis has been identified in a patient's culture. A positive culture for M. tuberculosis confirms the diagnosis of TB disease.

3.32. Why are drug susceptibility tests done?

Drug susceptibility tests are done to determine which drugs will kill the tubercle bacilli that are causing disease in a particular patient. The results of drug susceptibility tests can help clinicians choose the appropriate drugs for each patient.

3.33. How often should drug susceptibility tests be done?

Drug susceptibility tests should be done when a patient is first found to have a positive culture for M. tuberculosis. In addition, drug susceptibility tests should be repeated if a patient has a positive culture for M. tuberculosis after 2 months of treatment or if a patient does not seem to be getting better.

 


Released October 2008
Centers for Disease Control and Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Division of Tuberculosis Elimination - http://www.cdc.gov/tb

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