Self-Study Modules on Tuberculosis
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3: Diagnosis of Tuberculosis Infection and Disease
In this module, you will learn about the diagnosis of TB infection
and disease. TB infection is diagnosed with the tuberculin skin test.
The purpose of diagnosing TB infection is to identify (1) people with
TB infection who may be given treatment to prevent them from developing
TB disease and (2) people who may have TB disease and who need treatment
to be cured. In most cases, TB disease is diagnosed with certain laboratory
tests (bacteriologic examination); for patients who may have pulmonary
TB disease, a chest x-ray is also useful for diagnosis. It is important
to evaluate people who have symptoms of TB disease; if they are found
to have TB disease, they need treatment to be cured and to avoid spreading
TB to others. For this reason, the diagnosis of TB disease is crucial
to controlling the spread of TB in homes and communities.
After working through this module, you will be able to:
- Explain the purpose of the tuberculin skin test.
- Describe how the Mantoux tuberculin skin test is given.
- Explain when the patient's arm is examined and how the induration
- Explain why the Mantoux skin test is preferable to multiple-puncture
- Explain how the reaction to the Mantoux skin test is classified.
- Describe the factors that can cause a false-positive reaction
to the tuberculin skin test.
- Explain how reactions to the tuberculin skin test are interpreted
for BCG-vaccinated persons.
- Describe the factors that can cause a false-negative reaction
to the tuberculin skin test.
- Discuss why and for whom anergy testing should be considered.
- Describe the booster phenomenon.
- Discuss why and when two-step tuberculin testing should be done.
- List the four steps in diagnosing TB disease.
- List the parts of the medical history that should lead a clinician
to suspect TB.
- Describe the symptoms of TB disease.
- Explain the purposes of the chest x-ray in diagnosing TB disease.
- List the four parts of a bacteriologic examination.
- Outline the procedures for collecting sputum specimens.
- Explain the purpose and significance of the acid-fast bacilli
- Explain the purpose and significance of the culture.
- Explain the purpose and significance of drug susceptibility
Look for the following new terms in this module and in the glossary.
acid-fast bacilli (AFB) mycobacteria that stay
stained even after they have been washed in an acid solution; may
be detected under a microscope in a stained smear
anergy the inability to react to a skin test
because of a weakened immune system, often caused by HIV infection
or severe illness (see anergy testing)
anergy testing giving skin tests using two substances
other than tuberculin; done to determine whether a person is anergic.
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.
bacteriologic examination tests done in a mycobacteriology
laboratory to diagnose TB disease; includes examining a specimen
under a microscope, culturing the specimen, and doing drug susceptibility
baseline skin test the tuberculin skin test
given to employees or residents in certain facilities when they
start their job or enter the facility (see TB screening program
and two-step testing)
BCG bacille Calmette-Guιrin (BCG), a vaccine
for TB disease that is used in many countries but rarely used in
the United States; may cause a false-positive reaction to the tuberculin
boosted reaction a positive reaction to a tuberculin
skin test, due to a boosted immune response from a skin test given
up to a year earlier; occurs in people who were infected a long
time ago and whose ability to react to tuberculin had lessened.
Two-step testing is used in TB screening programs to tell the difference
between boosted reactions and reactions caused by recent infection
(see booster phenomenon and two-step testing)
booster phenomenon 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 the first skin
test boosts the immune response. Two-step testing is used in TB
screening programs to tell the difference between boosted reactions
and reactions caused by recent infection (see two-step testing)
bronchoscopy a procedure used to obtain pulmonary
secretions or lung tissue with an instrument called a bronchoscope;
used only when patients cannot cough up sputum on their own and
an induced specimen cannot be obtained
cavity a hollow space within the lung, visible
on a chest x-ray, that may contain many tubercle bacilli; often
occurs in people with severe pulmonary TB disease
clinician a physician, physician assistant,
colonies groups of mycobacteria that have grown
in a culture
culture organisms grown on media (substances
containing nutrients) so that they can be identified; a positive
culture for M. tuberculosis contains tubercle bacilli,
whereas a negative culture contains no detectable tubercle bacilli
drug susceptibility pattern the list of drugs
to which the strain of tubercle bacilli is susceptible and to which
it is resistant
erythema redness around the site of the injection
when a Mantoux skin test is done; erythema is not considered when
the reaction size is measured, because redness does not indicate
that a person has TB infection
exposure to TB time spent with someone who has
infectious TB disease
false-negative reaction a negative reaction
to the tuberculin skin test in a person who has TB infection; may
be caused by anergy, recent infection (within the past 10 weeks),
or very young age (younger than 6 months old)
false-positive reaction a positive reaction
to the tuberculin skin test in a person who does not have TB infection;
may be caused by infection with nontuberculous mycobacteria or by
vaccination with BCG
gastric washing a procedure done by inserting
a tube through the patient's nose and passing it into the stomach;
may be useful for obtaining sputum from children, who produce little
or no sputum when they cough
induced sputum sputum that is obtained by having
the patient inhale a saline (salt water) mist, causing the patient
to cough deeply; this procedure is used to help patients cough up
sputum if they cannot do so on their own
induration swelling that can be felt around
the site of injection after a Mantoux skin test is done; the reaction
size is the diameter of the swollen area (excluding any redness),
measured across the forearm
infiltrate a collection of fluid and cells in
the tissues of the lung; visible on a chest x-ray in people with
pulmonary TB disease
isolate a group of organisms isolated, or separated,
from a specimen; in an M. tuberculosis isolate, the organisms
have been identified as M. tuberculosis (a positive culture
for M. tuberculosis)
malaise a feeling of general discomfort or illness
Mantoux tuberculin skin test the preferred method
of testing for TB infection; done 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;
the reaction to this test, usually a small swollen area (induration),
is measured 48 to 72 hours after the injection and is classified
as positive or negative depending on the size of the reaction and
the patient's risk factors for TB
media substances containing special nutrients
for growing cultures of bacteria found in specimens
medical history the part of a patient's life
history that is important for diagnosing and treating TB infection
or disease, including history of exposure, symptoms, diagnosis of
TB infection or disease, and risk factors for TB disease
multiple-puncture test tuberculin skin test
done by puncturing the skin of the forearm with a set of short prongs
or tines to inject tuberculin (for example, Tine test); although
easy to give and convenient, these tests are not accurate and should
not be used to determine whether a person has TB infection
mycobacteriology laboratory a laboratory that
deals specifically with M. tuberculosis and other mycobacteria
PPD skin test a tuberculin skin test
purified protein derivative (PPD) the type of
tuberculin used in the Mantoux skin test
resistant able to grow in the presence of a
skin test conversion a change in a skin test
reaction from negative to positive between screening intervals
smear a specimen that has been smeared onto
a glass slide, stained, washed in an acid solution, and then placed
under the microscope for examination; used to detect acid-fast bacilli
in a specimen
sputum phlegm from deep in the lungs, collected
in a sterile container for processing and examination
susceptible able to be killed by a particular
symptoms of TB disease conditions caused by
TB disease. The symptoms of pulmonary TB disease include coughing,
pain in the chest when breathing or coughing, and coughing up sputum
or blood. The general symptoms of TB disease (pulmonary or extrapulmonary)
include weight loss, fatigue, malaise, fever, and night sweats.
The symptoms of extrapulmonary TB disease depend on the part of
the body that is affected by the disease
TB screening program a program in which employees
and residents of a facility are periodically given tuberculin skin
tests; done to identify people who have TB infection and possibly
TB disease and to determine whether TB is being transmitted in the
tuberculin protein from tubercle bacilli that
have been killed by heating; used to determine whether a person
has TB infection. Tuberculin is not a vaccine.
tuberculin unit a standard strength of tuberculin
used in the United States and Canada; a strength of 5 tuberculin
units is used for the Mantoux tuberculin skin test
two-step testing a strategy used in TB screening
programs to distinguish a boosted reaction (caused by TB infection
that occurred many years before the skin test) from a reaction caused
by recent infection. If a person has a negative reaction to an initial
skin test, a second test is given 1 to 3 weeks later; a positive
reaction to the second test probably represents a boosted reaction,
not recent infection. Two-step testing is used in many TB screening
programs for skin testing employees when they start their job.