Tuberculosis is spread from person to person through the air
by droplet nuclei, particles 1 to 5
mm in diameter that contain
M. tuberculosis complex (4). Droplet nuclei are produced
when persons with pulmonary or laryngeal tuberculosis cough,
sneeze, speak, or sing. They also may be produced by aerosol
treatments, sputum induction, aerosolization during bronchoscopy,
and through manipulation of lesions or processing
of tissue or secretions in the hospital or laboratory. Droplet
nuclei, containing two to three M. tuberculosis organisms (5),
are so small that air currents normally present in any indoor
space can keep them airborne for long periods of time
If the bacillus is able to survive initial defenses,
it can multiply within the alveolar macrophage. The tubercle
bacillus grows slowly, dividing approximately every 25
to 32 h within the macrophage. Mycobacterium tuberculosis
has no known endotoxins or exotoxins; therefore, there is no
immediate host response to infection. The organisms grow for
2 to 12 wk, until they reach 103 to 104 in number, which is sufficient
to elicit a cellular immune response (19, 20) that can be
detected by a reaction to the tuberculin skin test.
Before the development of cellular immunity, tubercle bacilli
spread via the lymphatics to the hilar lymph nodes and
thence through the bloodstream to more distant sites
Events in the Infectious Process
Early events. As discussed above, M. tuberculosis usually
enters the alveolar passages of exposed humans in an aerosol
droplet, where its first contact is thought be with resident
macrophages, but it is also possible that bacteria can be initially
ingested by alveolar epithelial type II pneumocytes. This cell
type is found in greater numbers than macrophages in alveoli,
and M. tuberculosis can infect and grow in these pneumocytes
ex vivo (24, 190). In addition, dendritic cells play a very important
role in the early stages of infection since they are much
better antigen presenters than are macrophages (286) and presumably
play a key role in activating T cells with specific M.
tuberculosis antigens (31, 114). Since dendritic cells are migratory,
unlike differentiated macrophages (164), they also may
play an important role in dissemination of M. tuberculosis.
However, this discussion is limited to the much more extensively
studied and better understood M. tuberculosis-macrophage
interaction. The bacteria are phagocytosed in a process
that is initiated by bacterial contact with macrophage mannose
and/or complement receptors (254). Surfactant protein A, a
glycoprotein found on alveolar surfaces, can enhance the binding
and uptake of M. tuberculosis by upregulating mannose
receptor activity (107). On the other hand, surfactant protein
D, similarly located in alveolae, inhibits phagocytosis of M.
tuberculosis by blocking mannosyl oligosaccharide residues on
the bacterial cell surface (90), and it is proposed that this
prevents M. tuberculosis interaction with mannose receptors on
the macrophage cell surface. Cholesterol in cell plasma membranes
is thought to be important for this process, since removal
of this steroid from human neutrophils decreases the
phagocytosis of M. kansasii (221) and similar depletion experiments
prevented the entry of M. bovis BCG into mouse macrophages
(106). The human toll-like receptor 2 (TLR2) also
plays a role in M. tuberculosis uptake (201), and this important
interaction with bacterial components is discussed later in this
review. On entry into a host macrophage, M. tuberculosis and
other intracellar pathogens initially reside in an endocytic vacuoule
called the phagosome. If the normal phagosomal maturation
cycle occurs, i.e., phagosome-lysosome fusion, these
bacteria can encounter a hostile environment that includes
acid pH, reactive oxygen intermediates (ROI), lysosomal enzymes,
and toxic peptides. Reactive nitrogen intermediates
(RNIs) produced by activated mouse macrophages are major
elements in antimicrobial activity (197), and mice with mutations
in the gene encoding the macrophage-localized cytokineinducible
nitric oxide synthase gene are more susceptible to
various pathogens, including Leishmania major (311), Listeria
monocytogenes (169), and M. tuberculosis (168). The M. tuberculosis
result is consistent with the results of other experiments
showing that RNIs are the most significant weapon against
virulent mycobacteria in mouse macrophages (48, 50) and the
observation that resistance to RNIs among various strains of
M. tuberculosis correlates with virulence (48, 50, 202). The
presence of RNIs in human macrophages and their potential
role in disease has been the subject of controversy, but the
alveolar macrophages of a majority of TB-infected patients
exhibit iNOS activity (200).
Since most macrophage killing of bacteria occurs in the
phagolysosome (89), intracellular pathogens have evolved
many ways to avoid this hostile vacuolar microenvironment.
Listeria and Shigella physically escape the phagosome and replicate
in the cytoplasm (252), and Legionella inhibits phagosome-
lysosome fusion (134). Salmonella enterica serovar Typhimurium
phagosomes also are diverted from the normal
endocytic pathway of phagosome-lysosoma fusion (42, 233),
and this bacterium requires acidification of the phagosome to
survive in macrophages (234). Pathogenic mycobacteria also
inhibit phagosome-lysosome fusion (6, 98), but unlike the situation
for Salmonella, the mycobacterial phagosome is not
acidified (60). This is presumably due to the exclusion of proton
ATPases from the mycobacterial phagosome (281), but it is
not clear that the blocking of endosomal maturation is essential
for M. tuberculosis survival in macrophages. Live M. tuberculosis
cells can made to traffic to late endosomes by opsonization
with polyclonal antibodies against M. tuberculosis H37Rv,
which presumably directs bacterial binding to Fc receptors.
However, this rerouting has no effect on bacterial growth in
mouse peritoneal macrophages (6). On the other hand, a recent
study in which human monocyte-derived macrophages
(MDMs) were infected with M. tuberculosis Erdmann opsonized
with a polyclonal antibody raised against the M. tuberculosis
cell surface glycolipid lipoarabinomannan (LAM)
showed that this treatment causes 80% loss of bacteria as well
as increased trafficking to late, more acidic endosomes (175).
The different results in these two experiments have not been
resolved but could be in part due to the source of the macrophages,
the nature of the antibodies, and the bacterial strains
used. An interesting finding in the latter work is that Ca2_
signaling is inhibited when M. tuberculosis enters human macrophages
but not when killed M. tuberculosis or antibody-opsonized
M. tuberculosis cells are phagocytosed (175). This effect
was correlated with trafficking to late endosomes; i.e.,
elevated Ca2_ levels were associated with phagolysosome formation.
Since Ca2_ can stimulate many host responses to infection,
e.g., the respiratory burst as well as NO and cytokine
production, preventing increases in Ca2_ levels would help M.
tuberculosis avoid these host defense mechanisms. It has also
been postulated that a selective advantage to M. tuberculosis of
staying in an early endosome is that there would be less host
immunosurveillance by CD4_ T cells. In agreement with this
idea, there is a decrease in the expression of major histocompatibility
complex class II (MHC-I) proteins and in the
MHC-II presentation of bacterial antigens in macrophages
after M. tuberculosis infection (201). As discussed below, this
effect seems to be induced by presence of the secreted or
surface-exposed M. tuberculosis 19-kDa lipoprotein, which is
thought to interact with TLR2 in the early phase of bacterial
entry into macrophages (287). The mechanism by which virulent
mycobacteria prevent phagosomal maturation is not
known, but in the normal maturation of the mycobacterial
phagosome there is a successive recruitment of Rab proteins,
which are small GTPases involved in endosome trafficking; i.e.,
Rab5 associates with early endosomes, and Rab7 is found in
later endosomes. The M. tuberculosis phagosome that does
contain Rab5 does not recruit Rab7 (298). Also, TACO, a
member of the coronin family of actin binding proteins, is
preferentially recruited to the mycobacterial phagosome of
infected murine macrophages, where it was reported to be
retained in phagosomes containing live and not killed M. bovis
BCG (91). However, a more recent study, in which phagosomes
and other macrophage organelles were isolated, has
shown that the association of coronin with phagosomes containing
live M. bovis BCG in both murine and human macrophages
is transient and is retained only on phagosomes containing
clumped bacteria (257). These latter results suggest
that coronin is not involved with the arrest in phagosome
trafficking observed in M. tuberculosis infections of macrophages.
It is also not known whether the exclusion of Rab7
and/or the decreased Ca2_ signaling discussed above is directly
responsible for this block in phagosome maturation or is a
secondary consequence.
Later events. The relative ease of working with tissue culture
has provided many data on M. tuberculosis entrance and trafficking
in the macrophage and on other responses of the infected
cells, as discussed above, but there is much less information
on how the bacterium survives and grows during later
stages of infection in the lung. It is known that infected macrophages
in the lung, through their production of chemokines,
attract inactivated monocytes, lymphocytes, and neutrophils
(297), none of which kill the bacteria very efficiently (89).
Then, granulomatous focal lesions composed of macrophagederived
giant cells and lymphocytes begin to form. This process
is generally an effective means of containing the spread of the
bacteria. As cellular immunity develops, macrophages loaded
with bacilli are killed, and this results in the formation of the
caseous center of the granuloma, surrounded by a cellular zone
of fibroblasts, lymphocytes, and blood-derived monocytes (63).
Although M. tuberculosis bacilli are postulated to be unable to
multiply within this caseous tissue due to its acidic pH, the low
availability of oxygen, and the presence of toxic fatty acids,
some organisms may remain dormant but alive for decades.
The strength of the host cellular immune response determines
whether an infection is arrested here or progresses to the next
stages. This enclosed infection is referred to as latent or persistent
TB and can persist throughout a person’s life in an
asymptomatic and nontransmissible state. In persons with efficient
cell-mediated immunity, the infection may be arrested
permanently at this point. The granulomas subsequently heal,
leaving small fibrous and calcified lesions. However, if an infected
person cannot control the initial infection in the lung or
if a latently infected person’s immune system becomes weakened
by immunosuppressive drugs, HIV infection, malnutrition,
aging, or other factors, the granuloma center can become
liquefied by an unknown process and then serves as a rich
medium in which the now revived bacteria can replicate in an
uncontrolled manner. At this point, viable M. tuberculosis can
escape from the granuloma and spread within the lungs (active
pulmonary TB) and even to other tissues via the lymphatic
system and the blood (miliary or extrapulmonary TB). When
this happens, the person becomes infectious and requires antibiotic
therapy to survive (63).
Clinical Manifestations
Tuberculosis involving any site may produce symptoms and
findings that are not specifically related to the organ or tissue
involved but, rather, are systemic in nature. Of the systemic effects,
fever is the most easily quantified. The frequency with
which fever has been observed in patients with tuberculosis varies
from approximately 37 to 80% (33, 34). In one study (33),
21% of patients had no fever at any point in the course of hospitalization
for tuberculosis. Of the febrile patients, 34% were
afebrile within 1 wk, and 64% in 2 wk, of beginning treatment.
The median duration of fever after beginning treatment was
10 d, with a range of 1 to 109 d. Loss of appetite, weight loss,
weakness, night sweats, and malaise are also common but are
more difficult to quantify and may relate to coexisting diseases.
The most common hematologic manifestations of tuberculosis
are increases in the peripheral blood leukocyte count and
anemia, each of which occurs in approximately 10% of patients
with apparently localized tuberculosis (35, 36). The increase in
white blood cell counts is usually slight, but leukemoid reactions
may occur. Leukopenia has also been reported. An increase
in the peripheral blood monocyte and eosinophil counts
also may occur with tuberculosis. Anemia is common when the
infection is disseminated. In some instances, anemia or pancytopenia
may result from direct involvement of the bone marrow
and, thus, be a local, rather than a remote, effect.
Hyponatremia, which in one series was found to occur in
11% of patients (37), has been determined to be caused by
production of an antidiuretic hormone-like substance found
within affected lung tissue (38).
In many patients tuberculosis is associated with other serious
disorders. These include HIV infection, alcoholism, chronic
renal failure, diabetes mellitus, neoplastic diseases, and drug
abuse, to name but a few. The signs and symptoms of these
diseases and their complications can easily obscure or modify
those of tuberculosis and result in considerable delays in diagnosis
or misdiagnoses for extended periods of time, especially
in patients with HIV infection (39
Historically, pulmonary tuberculosis has been
divided into primary and postprimary tuberculosis,
with primary tuberculosis being considered a
disease of childhood and postprimary tuberculosis
a disease of adulthood. However, a reduction in
the prevalence of tuberculosis in most Western
countries (1,2) owing to effective treatment and
public health measures has resulted in large unexposed
adult populations who are at risk for contracting
primary tuberculosis. As a result, primary
tuberculosis now accounts for 23%–34% of all
adult cases of tuberculosis.
It can sometimes be difficult to differentiate
between primary and postprimary tuberculosis
both clinically and radiologically, since their features
can overlap. However, confirming the diagnosis
is more important than identifying the subtype
because it allows initiation of correct clinical
management.
Primary tuberculosis is seen in patients not previously
exposed to M tuberculosis. It is most common
in infants and children and has the highest
prevalence in children under 5 years of age
Parenchymal Disease.—Typically, parenchymal
disease manifests as dense, homogeneous
parenchymal consolidation in any lobe; however,
predominance in the lower and middle lobes is
suggestive of the disease, especially in adults. Its
appearance is often indistinguishable from that of
bacterial pneumonia; however, it can be differentiated
from bacterial pneumonia on the basis of
radiographic evidence of lymphadenopathy and
the lack of response to conventional antibiotics
(Fig 1). In approximately two-thirds of cases, the parenchymal
focus resolves without sequelae at conventional
radiography; however, this resolution
can take up to 2 years. In the remaining cases, a
radiologic scar persists that can calcify in up to
15% of cases, an entity that is known as a Ghon
Focus.
Lymphadenopathy.—Radiographic evidence of
lymphadenopathy is seen in up to 96% of children
and 43% of adults. Lymphadenopathy is
typically unilateral and right sided, involving the
hilum and right paratracheal region (Fig 2), although
it is bilateral in about one-third of cases.
Any nodes greater than 2 cm in diameter generally
have a low-attenuation center secondary to
necrosis at CT and are highly suggestive of active
disease. CT is more sensitive
than chest radiography for assessing lymphadenopathy.
The combination of calcified hilar
nodes and a Ghon focus is called a Ranke complex
and is suggestive of previous tuberculosis,
although it can also result from histoplasmosis.
Miliary Disease.—Clinically significant miliary
disease affects between 1% and 7% of patients
with all forms of tuberculosis. It is usually seen in
the elderly, infants, and immunocompromised
persons, manifesting within 6 months of initial
exposure. Chest radiography is usually normal at
the onset of symptoms, and hyperinflation may be
the earliest feature. The classic radiographic findings
of evenly distributed diffuse small 2–3-mm
nodules, with a slight lower lobe predominance,
are seen in 85% of cases. High-resolution
CT is more sensitive than conventional radiography,
with nodules seen in a random distribution
A pleural effusion is seen in
approximately one-fourth of patients with proved
primary tuberculosis (29). The effusion is often
the sole manifestation of tuberculosis and usually
manifests 3–7 months after initial exposure. Pleural
effusion is a very uncommon finding in infants.
Postprimary Tuberculosis
Postprimary tuberculosis remains primarily a disease
of adolescence and adulthood. It occurs in
patients previously sensitized to M tuberculosis.
The term postprimary tuberculosis is generally used
to refer to both reinfection with and reactivation
of tuberculosis. Primary tuberculosis is usually
self-limiting, whereas postprimary tuberculosis is
progressive, with cavitation as its hallmark, resulting
in hematogenous dissemination of the disease
as well as disease spread throughout the lungs.
Healing usually occurs with fibrosis and calcification.
the distinguishing
features of postprimary tuberculosis include a
predilection for the upper lobes, the absence of
lymphadenopathy, and cavitation.
At radiology, postprimary tuberculosis may
manifest as parenchymal disease, airway involvement,
and pleural extension.
Disease.—The earliest finding in
parenchymal disease is patchy, poorly defined
consolidation, particularly in the apical and posterior
segments of the upper lobes (28). In the majority
of cases, more than one pulmonary segment
is involved, with bilateral disease seen in onethird
to two-thirds of cases.
Cavitation, the hallmark of postprimary tuberculosis,
affects about 50% of patients. The cavities
typically have thick, irregular walls, which
become smooth and thin with successful treatment.
Cavities are usually multiple and occur
within areas of consolidation (Figs 4, 5). Resolution
can result in emphysematous change or scarring.
A minority of cavities demonstrate air-fluid
levels; however, these findings can indicate the
presence of superinfection.
If there is airway disease and, in particular,
endobronchial spread of infection, tree-in-bud
opacities may develop. These findings, which are
usually visible in the lung periphery and resemble
a branching tree with buds at the tips of the
branches, are indicative of active tuberculosis
Lymphadenopathy and pneumothoraces are
seen in only about 5% of patients.
Bilateral cavitations
Tree- bundle branching pattern
Airway involvement is
characterized by bronchial stenosis, leading to
lobar collapse or hyperinflation, obstructive pneumonia,
and mucoid impaction. Bronchial stenosis
is seen in 10%–40% of patients with active tuberculosis
(27) and is best demonstrated with CT,
which usually shows long segment narrowing with
irregular wall thickening, luminal obstruction,
and extrinsic compression.
Pleural Extension.—Pleural effusions occur
most often in primary tuberculosis but are seen in
approximately 18% of patients with postprimary
tuberculosis;
Success in isolating mycobacteria from clinical materials depends
on the manner in which specimens are handled after
collection.
For optimal results, specimens should be collected
in clean, sterile containers and held under conditions that in hibit growth of contaminating organisms, since most specimens will contain bacteria other than mycobacteria
Patients need to be instructed as to the proper
method of sputum collection. It is important that the patient
be informed that nasopharyngeal discharge and saliva are not
sputum; rather, the material brought up from the lungs after a
productive cough constitutes the material desired. Whenever
possible, attending personnel should observe the sputum collection.
A series of at least three single specimens (but usually
not more than six) should be collected initially (preferably on
different days) from sputum-producing patients. For optimal
results, sputum should be collected and processed in the same
container. Commercially available sputum collection devices
using a 50-ml plastic, single-use, disposable centrifuge tube is
recommended
Induced sputum. For patients who have difficulty producing
sputum, there are several methods of obtaining a specimen.
Inhalation of an aerosol of sterile hypertonic saline (3–
15%), usually produced by an ultrasonic nebulizer, can be
used to stimulate the production of sputum (79). Even though
aerosol-induced specimens may appear thin and watery, they
should be processed. The specimen should be clearly labeled
as “induced sputum”so it will not be discarded by the laboratory
as an inadequate specimen.
Gastric aspiration in children
LBA
Several quantitative
studies have shown that there must be 5,000 to 10,000 bacilli
per milliliter of specimen to allow the detection of bacteria in
stained smears (93). In contrast, 10 to 100 organisms are needed
for a positive cultura. Negative smears, however,
do not preclude tuberculosis disease. Various studies
have indicated that 50 to 80% of patients with pulmonary tuberculosis
will have positive sputum smears. There is no need to hospitalize
a person solely because they are infectious. Outpatients
should be instructed to remain at home, without visitors, until
they are no longer thought to be infectious.
In the research laboratory, these
procedures can produce a positive result from specimens containing
as few as 10 bacilli (79); however, in clinical laboratories,
the sensitivity is somewhat less.
In clinical respiratory
specimens that are AFB smear positive, the sensitivity of the
amplification methods is approximately 95%, with a specificity
of 98%. In specimens that contain fewer organisms and are
AFB smear negative, the nucleic acid amplification test is positive
in 48–53% of patients with culture-positive tuberculosis
and the specificity remains approximately 95% (99). Thus, the
CDC included a positive nucleic acid amplification test in the
setting of a positive smear as confirmation of the diagnosis of
tuberculosis.
In patients (AFB
smear positive and negative) where the clinician had an intermediate
or high suspicion of tuberculosis disease, the sensitivity
of the enhanced nucleic acid amplification test was 75–88%
and the specificity was 100%.
All clinical specimens suspected of containing mycobacteria
should be inoculated (after appropriate digestion and decontamination,
if required) onto culture media for four reasons:
(1) culture is much more sensitive than microscopy, being able
to detect as few as 10 bacteria/ml of material (94); (2) growth
of the organisms is necessary for precise species identification;
(3) drug susceptibility testing requires culture of the organisms;
and (4) genotyping of cultured organisms may be useful
to identify epidemiological links between patients or to detect
laboratory cross-contamination. In general, the sensitivity
of culture is 80–85% with a specificity of approximately 98%
Liquid
systems allow for rapid growth [detection of mycobacteria
growth within 1–3 wk compared with solid media, where
growth takes 3–8 wk (104)], whereas agar media provide an
opportunity to examine colony morphology and detect mixed
cultures
PCR guidelines MMWWr (CDC)
Conventional tests for laboratory confirmation of TB include acid-fast bacilli (AFB) smear microscopy, which can produce results in 24 hours, and culture, which requires 2–6 weeks to produce results (5,6). Although rapid and inexpensive, AFB smear microscopy is limited by its poor sensitivity (45%–80% with culture-confirmed pulmonary TB cases) and its poor positive predictive value (50%–80%) for TB in settings in which nontuberculous mycobacteria are commonly isolated
Compared with AFB smear microscopy, the added value of NAA testing lies in its 1) greater positive predictive value (>95%) with AFB smear-positive specimens in settings in which nontuberculous mycobacteria are common and 2) ability to confirm rapidly the presence of M. tuberculosis in 50%–80% of AFB smear-negative, culture-positive specimens (3,7–9). Compared with culture, NAA tests can detect the presence of M. tuberculosis bacteria in a specimen weeks earlier than culture for 80%–90% of patients suspected to have pulmonary TB whose TB is ultimately confirmed by culture
CDC guideline
Routinely collect respiratory specimens (e.g., sputum), process (liquefy, decontaminate, and concentrate), and test by AFB smear microscopy and culture as previously recommended (6). Specimen collection and microbiologic testing should not be delayed to await NAA test results.
2. At least one specimen, preferably the first diagnostic specimen, from each patient to be tested by NAA should be processed, suspended in a sufficient volume of buffer to ensure adequate sample volume for all planned tests (e.g., microscopy, culture, and NAA), and tested using an NAA test for TB. NAA testing should be performed in accordance with the manufacturer’s instructions or a validated standard operating procedure.
3. Interpret NAA test results in correlation with the AFB smear results.
a. If the NAA result is positive and the AFB smear result is positive, presume the patient has TB and begin anti-TB treatment while awaiting culture results.
b. If the NAA result is positive and the AFB smear result is negative, use clinical judgment whether to begin anti-TB treatment while awaiting culture results and determine if additional diagnostic testing is needed. Consider testing an additional specimen using NAA to confirm the NAA result. A patient can be presumed to have TB, pending culture results, if two or more specimens are NAA positive.
c. If the NAA result is negative and the AFB smear result is positive, a test for inhibitors should be performed and an additional specimen should be tested with NAA. Sputum specimens (3%–7%) might contain inhibitors that prevent or reduce amplification and cause false-negative NAA results (8,9).
i. If inhibitors are detected, the NAA test is of no diagnostic help for this specimen. Use clinical judgment to determine whether to begin anti-TB treatment while awaiting results of culture and additional diagnostic testing.
ii. If inhibitors are not detected, use clinical judgment to determine whether to begin anti-TB treatment while awaiting culture results and determine if additional diagnostic testing is needed. A patient can be presumed to have an infection with nontuberculous mycobacteria if a second specimen is smear positive and NAA negative and has no inhibitors detected.
d. If the NAA result is negative and the AFB smear result is negative, use clinical judgment to determine whether to begin anti-TB treatment while awaiting results of culture and additional diagnostic tests. Currently available NAA tests are not sufficiently sensitive (detecting 50%–80% of AFB smear-negative, culture-positive pulmonary TB cases) to exclude the diagnosis of TB in AFB smear-negative patients suspected to have TB
Culture remains the gold standard for laboratory confirmation of TB and is required for isolating bacteria for drug-susceptibility testing and genotyping
TST
The tuberculin test is based on the fact that infection with M.
tuberculosis produces a delayed-type hypersensitivity reaction
to certain antigenic components of the organism that are contained
in extracts of culture filtrates called “tuberculins
Most of the constituents of PPD are small proteins with
molecular masses of approximately 10,000 Da, but there are
also polysaccharides and some lipids present (121). The relatively
small size of the protein constituents in PPD is the reason
that PPD does not sensitize individuals who have not been
exposed to mycobacteria
A batch of PPD (lot 49608) called PPD-S, which was produced
by Seibert and Glenn in 1939, has continued to serve as
the international standard as well as the standard reference
material in the United States (122). All PPD lots must be bioassayed
to demonstrate equal potency to PPD-S (123). “Tuberculins”
and “PPDs” have been prepared from other mycobacterial
species, but these materials are less sensitive and
specific for diagnosis of nontuberculous mycobacterial infections
than is PPD for M. tuberculosis infections. These preparations
are occasionally used for epidemiologic purposes (124),
but have little clinical utility.
The standard 5-tuberculin unit (TU) dose of PPD-S is defined
as the delayed skin test activity contained in a PPD-S dose
of 0.1 mg/0.1 ml. The standard test dose of a commercial PPD
preparation is defined as the dose of the product that is biologically equivalent to that contained in 5 TU of PPD-S.
The reaction to intracutaneously injected tuberculin is the
classic example of a delayed (cellular) hypersensitivity reaction.
T cells sensitized by prior infection are recruited to the
skin site where they release lymphokines (127). These lymphokines
induce induration through local vasodilatation,
edema, fibrin deposition, and recruitment of other inflammatory
cells to the area (128). Features of the reaction include (1)
its delayed course, reaching a peak more than 24 h after injection
of the antigen; (2) its indurated character; and (3) its
occasional vesiculation and necrosis. Reactivity of the PPD
provides a general measure of a person’s cellular immune responsiveness
(121).
Typically, the reaction to tuberculin begins 5 to 6 h after injection,
causes maximal induration at 48 to 72 h, and subsides
over a period of days. In a few individuals (the elderly and
those who are being tested for the first time), the reaction may
not peak until after 72 h
The tuberculin test, like all medical tests, is subject to variability,
but many of the inherent variations in administration and
reading of tests can be avoided by careful attention to details.
The test is administered by injecting 0.1 ml of 5-TU PPD intradermally
(Mantoux method) into the volar or dorsal surface of
the forearm. Other areas may be used, but the forearm is preferred.
The use of a skin area free of lesions and away from
veins is recommended. The injection is made using a onequarter-
to one-half-inch, 27-gauge needle and a tuberculin syringe.
The tuberculin should be injected just beneath the surface
of the skin, with the needle bevel upward or downward
(130). A discrete, pale elevation of the skin (a wheal) 6 to 10
mm in diameter should be produced when the injection is
done correctly. If it is recognized that the first test was improperly
administered, another test dose can be given at once,
selecting a site several centimeters away from the original injection
Tests should be read between 48 and 72 h after injection,
when the induration is maximum. Tests read after 72 h tend to
underestimate the true size of induration
Reading
To interpret the tuberculin skin test appropriately, one must
understand the sensitivity and specificity of the test as well as
the positive and negative predictive value of the test. The sensitivity
of a test is the percentage of people with the condition
who have a positive test. If false-negative results are uncommon,
the sensitivity is high. The PPD skin test has a reported
false-negative rate of 25% during the initial evaluation of persons
with active tuberculosis (134). This high false-negative
rate appears to be due to poor nutrition and general health,
overwhelming acute illness, or immunosuppression
Because
of the low sensitivity of the test, especially in acutely ill
patients and those who are infected with HIV, the tuberculin
test cannot be used to eliminate the possibility of active tuberculosis
(136). Other factors that may result in a false-negative
test are shown in Table 5
Vaccination with live-attenuated virus can cause suppression
of the PPD response in patients known to be infected
with M. tuberculosis. Live-attenuated vaccines that may cause
false-negative PPD results are measles, mumps, rubella, oral
polio, varicella, yellow fever, BCG, and oral typhoid (TY21a).
This suppression does not appear within the first 48 h after
measles vaccination, so the Advisory Committee on Immunization
Practices recommends that tuberculin testing be done
either on the same day as vaccination with live virus or 4–6 wk
later (137–140).
The specificity of the test is the percentage of people without
the condition who have a negative test. False-positive results
decrease the specificity of a test. mycobacteria, including vaccination with BCG. Some antigens
in PPD are shared with the other mycobacteria (141, 142) and
thus can elicit a skin test response
In any population, the likelihood that a positive test represents
a true infection is influenced by the prevalence of infection
with M. tuberculosis. Table 6 shows how the prevalence of
infection influences the predictive value of a positive tuberculin
test (positive predictive value). The tuberculin skin test has
a specificity of approximately 99% in populations that have no
other mycobacterial exposures or BCG vaccination, but the
specificity decreases to 95% in populations where cross-reactivity
with other mycobacteria is common.
In contrast, among persons who have been in close contact
to individuals with infectious tuberculosis, there is a 25–50%
chance of being infected with M. tuberculosis. Likewise, in
high-prevalence countries, adults have a similarly high prevalence
of infection. In such individuals or populations, the tuberculin
skin test is highly specific and a positive test is highly
likely to indicate tuberculosis infection.
For individuals who are at great risk of developing tuberculosis
disease if they become infected with M. tuberculosis (143),
a cut point of > 5 mm is recommended. Reactions in persons
who have had recent close contact with tuberculosis and in
persons with abnormal chest radiographs consistent with tuberculosis
are more likely to represent infection with M. tuberculosis
than cross-reactions. Persons who are immunosuppressed
because of disease (e.g., HIV infection) or drugs (e.g.,
corticosteroids) are more likely to progress to tuberculosis disease
if they are infected with M. tuberculosis. Therefore, using
a lower cut point (e.g., 5 mm) for separating positive from negative
reactions is appropriate in these groups
A cut point of > 10 mm is suggested for individuals who
have normal or mildly impaired immunity and a high likelihood
of being infected with M. tuberculosis but are without
other risk factors that would increase their likelihood of developing
active disease
Persons who are not likely to be infected with M. tuberculosis
should generally not be tuberculin tested since the predictive
value of a positive test in low-prevalence populations is poor. However, if a skin test is done, e.g., at entry into a work site where some risk of exposure to tuberculosis is anticipated
and a longitudinal tuberculin testing program is in place, a
higher cut point of > 15 mm is suggested in order to improve
the specificity of the test
If PPD is administered to infected
individuals whose skin tests have waned, the reaction of the
initial test may be small or absent; however, there may be an
accentuation of response on repeated testing. This is called the
“booster effect” and can be misinterpreted as a skin test conversion.
Boosted reactions also are particularly common in individuals
exposed to other mycobacteria or who have been
vaccinated with BCG. If repeated tuberculin testing is anticipated,
as in health care workers, for example, a two-step
method is recommended. In this method, persons who have a
negative initial PPD skin test undergo a second tuberculin test
1–3 wk after the first. The results from the second test should
be considered to be the “correct”.
It is usually prudent to consider
“positive” reactions to 5 TU of PPD tuberculin in BCGvaccinated
persons as indicating infection with M. tuberculosis,
especially among persons from countries with a high prevalence
of tuberculosis. There are several reasons for not assuming
that a large reaction to tuberculin is due to BCG vaccination:
(1) tuberculin test conversion rates after vaccination may
be much less than 100%; (2) the mean reaction size among persons
who have received BCG is often , 10 mm; and (3) tuberculin
sensitivity tends to wane after vaccination. Although a
positive tuberculin skin test due to BCG vaccination can wane
over time, it can be “boosted” by serial testing. Because most
persons who have received BCG are from high-prevalence areas
of the world, it is important that vaccinated persons who
have a positive reaction to a tuberculin skin test be evaluated
for tuberculosis and treated accordingly
For persons with negative tuberculin
skin test reactions who undergo repeat skin testing (e.g.,
health care workers), an increase in reaction size of 10 mm or
more within a period of 2 yr should be considered a skin test
conversion indicative of recent infection with M. tuberculosis.
Skin tests to identify anergy are placed by intradermal injection,
using the Mantoux method, but there is no standard
convention for classifying a positive response. Individuals who
mount a response to any antigen are considered to have relatively
intact cellular immunity, whereas those who cannot
mount any responses are considered “anergic.
Because of these findings, a positive DTH response
to antigens other than PPD is not proof that a negative PPD
Treatment CDC guidelines
