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

Self-Study Modules on Tuberculosis

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Module 3: Diagnosis of Tuberculosis Infection and Disease

Answers To Case Studies

3.1. Which of the following patients have a positive tuberculin skin test reaction?
  1. *Mr. West, 36 years old, HIV infected, 08 mm of induration
  2. Ms. Hernandez, 26 years old, native of Mexico, 07 mm of induration
  3. *Ms. Jones, 56 years old, has diabetes, 12 mm of induration
  4. *Mr. Sung, 79 years old, resident of a nursing home, 11 mm of induration
  5. Mr. Williams, 21 years old, no risk factors, 13 mm of induration
  6. *Ms. Marcos, 42 years old, chest x-ray findings suggestive of previous TB, 06 mm of induration
  7. *Ms. Rayle, 50 years old, husband has pulmonary TB, 09 mm of induration
(a), (c), (d), (f), (g)

3.2. A 30-year-old man who recently immigrated from India is given a tuberculin skin test and found to have 14 millimeters of induration. He reports that he was vaccinated with BCG as a child. He also says that his wife was treated for pulmonary TB disease last year.

  • Can you tell for sure whether this man has TB infection?
  • No. This man does have a positive reaction to the tuberculin skin test (10 or more millimeters is considered a positive reaction for a foreign-born person). However, this may be a false-positive reaction because he has been vaccinated with BCG.

  • What factors make it more likely that this man's positive reaction is due to TB infection?

    First, this man has a fairly large reaction (14 mm). Second, he was vaccinated a long time ago, when he was a child. Third, this man is from an area of the world where TB is common, so he was probably exposed to TB in his native country. Therefore, he is at increased risk for TB infection. Fourth, his wife has had pulmonary TB, which further increases the probability that he has been exposed to TB.

  • Because he has a positive skin test reaction, this man should be further evaluated for TB disease.

3.3. Mr. Bell comes to the TB clinic for a tuberculin skin test. He believes that he has been exposed to TB, and he knows he is at high risk for TB because he is HIV infected. He is given a tuberculin skin test, and his reaction is read 48 hours later as 0 millimeters of induration.

  • What are three ways to interpret this result?
  • There are three possible reasons why Mr. Bell had no reaction to the tuberculin skin test.

    • First, he may not have TB infection.
    • Second, he may be anergic. People who are HIV infected are more likely to be anergic than persons who are not HIV infected. If Mr. Bell is anergic, he would be unable to react to tuberculin even if he did have TB infection. To determine whether he is anergic, a clinician can test him with two substances other than tuberculin.
    • Third, it may be less than 10 weeks since he was exposed to TB. After TB has been transmitted, it takes 2 to 10 weeks before TB infection can be detected by the tuberculin skin test. Mr. Bell should be retested 10 weeks after he was last exposed to TB.

3.4. Ms. Wilson is a 60-year-old nurse. When she started a job at the local hospital, she was given a tuberculin skin test, her first test in 25 years. Her reaction was read 48 hours later as 0 millimeters of induration. Six months later, she was retested as part of the TB screening program in the unit where she works. Her skin test reaction was read 48 hours later as 11 millimeters of induration.

  • What are two ways to interpret this result?
  • There are two possible explanations for this result.

    • One explanation is that Ms. Wilson may have been exposed to and infected with M. tuberculosis sometime in the 6 months between her first and second skin tests.
    • The other explanation is the booster phenomenon. If Ms. Wilson was infected with M. tuberculosis many years ago, her ability to react to tuberculin may have decreased. This would explain why she did not react to the first tuberculin skin test. Then the first tuberculin test may have boosted the ability of her immune system to react to tuberculin. This would explain why she had a positive reaction to the second test, which was given within a year of the first test. If this scenario is true, Ms. Wilson's positive reaction would not mean that she was recently infected with M. tuberculosis.

    This problem in interpreting Ms. Wilson's reaction would have been avoided if she had been tested with a two-step procedure when she first joined the hospital. In any event, because she has a positive reaction, Ms. Wilson should be evaluated for TB disease.

3.5. Mr. Lee has a cough and other symptoms of TB disease, and he is evaluated with a chest x-ray. However, he is unable to cough up any sputum on his own for the bacteriologic examination.

  • What should be done?
  • If a patient cannot cough up a sputum specimen, other techniques can be used to obtain sputum. First, clinicians can try to obtain an induced sputum sample. If they cannot obtain an induced sputum sample, a bronchoscopy or gastric washing may be done.

3.6. Ms. Thompson gave three sputum specimens, which were sent to the laboratory for smear examination and culture. The smear results were reported as 4+, 3+, and 4+.

  • What do these results tell you about Ms. Thompson's diagnosis and her infectiousness?
  • These results show that Ms. Thompson's sputum specimens contain many acid-fast bacilli. Because the smears are positive, clinicians should suspect that Ms. Thompson has TB disease. They should also consider her infectious. However, it is possible that these acid-fast bacilli are mycobacteria other than tubercle bacilli. Therefore, the diagnosis of TB disease cannot be proven until the culture results are available.

3.7. Mr. Sagoo has symptoms of TB disease and a cavity on his chest x-ray, but all of his sputum smears are negative for acid-fast bacilli.

  • Does this rule out the diagnosis of pulmonary TB disease?
  • No.

  • Why or why not?
  • M. tuberculosis may grow in the cultures even though there were no acid-fast bacilli on the smear. Mr. Sagoo's symptoms and his abnormal chest x-ray suggest that he does have pulmonary TB disease.

3.8. In the public health clinic, you see a patient, Ms. Sanchez, who complains of weight loss, fever, and a cough of 4 weeks' duration. When questioned, she reports that she has been treated for TB disease in the past and that she occasionally injects heroin.

  • What parts of Ms. Sanchez's medical history lead you to suspect TB disease?
  • Ms. Sanchez has symptoms of TB disease (weight loss, fever, and a persistent cough). Also, in the past she has been treated for TB disease. We don't know whether she completed therapy, but until we can prove otherwise, we should assume that she has TB disease again. Her history of injecting illicit drugs (heroin) is another risk factor for TB.

  • What diagnostic tests should be done?
  • People who have TB symptoms should be evaluated for TB disease. Because she has symptoms of pulmonary TB disease (coughing), Ms. Sanchez should be given a chest x-ray. In addition, a sputum specimen should be collected for smear and culture, and drug susceptibility testing should be done if the culture is positive for M. tuberculosis. A tuberculin skin test may be helpful for the diagnosis of TB, but it is not necessary.


Released October 2008
Centers for Disease Control and Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Division of Tuberculosis Elimination -

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