sábado, 28 de agosto de 2010

PULMONAR TUBERCULOSIS



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

applied at the same time indicates absence of TB infection

Treatment CDC guidelines