Self-Study Modules on Tuberculosis
This is an archived document. The links are no longer being updated.
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
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
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
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
- 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
- People who inject illicit drugs but who are known to be HIV
- 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
- 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
- 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
- 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
3.21. What are the four steps in diagnosing TB disease?
The four steps in diagnosing TB disease are
- The medical history
- The tuberculin skin test
- The chest x-ray
- 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
- Been exposed to a person who has infectious TB,
- Symptoms of TB disease,
- Had TB infection or TB disease before, or
- 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
- Pain in the chest when breathing or coughing
- Coughing up sputum or blood
The general symptoms of TB disease (pulmonary or extrapulmonary)
- Weight loss
- 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
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
- 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
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.