Self-Study Modules on Tuberculosis
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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?
- *Mr. West, 36 years old, HIV infected,
08 mm of induration
- Ms. Hernandez, 26 years old, native of Mexico, 07 mm of induration
- *Ms. Jones, 56 years old, has diabetes, 12 mm of induration
- *Mr. Sung, 79 years old, resident of a nursing home, 11 mm of
- Mr. Williams, 21 years old, no risk factors, 13 mm of induration
- *Ms. Marcos, 42 years old, chest x-ray findings suggestive of
previous TB, 06 mm of induration
- *Ms. Rayle, 50 years old, husband has pulmonary TB, 09 mm of
(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
- 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.
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
- 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
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
- Does this rule out the diagnosis of pulmonary TB disease?
- 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
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.