CONTENTS




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Tuberculin Skin Test (TST)
A positive TST reaction is a sign of primary infection with M.tuberculosis (M.T). In most children, tuberculin reactivity becomes apparent within 3-6 weeks, but occasionally may take up to 12 weeks after initial infection. Tuberculin reactivity due to M.T. infection usually remains positive for the lifetime of the individual, even after treatment. The standard method used in many countries for detecting infection by M.T. is the Mantoux test, with 5 TU/ 0,1ml . The reaction is measured as millimeters of induration (not erythema) after 48 to 72 hours. Several multi-puncture methods, such as the heaf and tine test, are available and used widely (especially in the UK.) because of the speed and ease with which they can be administrered. Up to 10 % of otherwise normal children with culture-proven tuberculosis do not react to tuberculin initially. Most of these children will become reactive during treatment, suggesting that tuberculosis disease may itself contribute to immunosuppression. In Greece this percentage is less than 1% and this is attributed to the early diagnosis of tuberculous disease thanks to the screening program and the good physical condition of greek children.

The rate of false-negative TST in children with tuberculosis infected with HIV is unknown, but it is certainly higher than 10% and is dependent on the degree of immunosuppression. HIV-positive children usually have an attenuated and not a negative reaction, consequently an induration of 5-9 mm is considered positive in such patients.

There has been much uncertainty regarding the effect of BCG vaccination on TST results. BCG vaccination may cause a transiently reactive TST, but most children who received BCG as infants, have a negative TST at 5 years of age. Among older children or adolescents who receive BCG, most develop a reactive TST initially; however, by 10-15 years post-vaccination, the majority will have lost tuberculin reactivity. Studies have shown that BCG vaccination had little impact in the interpretation of TST in children being tested as part of contact investigation.

False-positive TST is often attributed to asymptomatic or symptomatic infection by environmental non-tuberculous mycobacteria. Skin reactivity can also be boosted, probably through antigenic stimulation, by serial TSTesting in many children and adults who received BCG.

TST results should be interpreted taking into consideration a lot of factors such as the specific tuberculin, host and enviromental factors. We have recorded for a nine-year period the verified tuberculosis cases of our clinic that had been tested with the Pasteur-Merieux Lot 5180A tuberculin. If interpretation of TST had been based on the American Academy of Pediatrics criteria, 27% of verified positive patients would have been considered as uncertain since they had an induration of 10-14 mm. This shows that each geographic area has different disease and population characteristics and that the local public health authority should determine the appropriate cutoff values for the community. Tuberculin RT23 skin testing has been studied in our patients from 1987 until 1995 and the results are presented in the following table.

Table. Distribution of TST reaction size with tuberculin RT23 in children with confirmed infection or disease.
The numbers within brackets are the percentages
Age (in years) <10mm10-15mm16-20mm>20mmΣύνολο
0-33 (2.2)83 (60)33 (24)19 (14)138 (40)
4-61 (0.8)64 (54.7)30 (25.5)22 (19)117 (26)
7-141 (0.5)94 (48)61 (31.2)39 (20)195 (44)
Σύνολο5 (1)241 (53)124 (28)80 (18)450 (100)


The information is based on the greek population and is not applicable in countries where the climate, the genetic characteristics of the population, the incidence of enviromental mycobacterial infections, the living conditions, the population nutrition, etc differ.


Radiology
Radiological evidence of pulmonary tuberculosis usually includes lymphadenopathy (hilar, paratracheal or mediastinal) and/or lung parenchymal changes. The most common parenchymal changes are segmental hyperinflation, atelectasis, alveolar infiltration, pleural effusion and rarely, a focal mass. Cavitation is rare in young children but is more common in adolescents, who may develop adult-type post-primary tuberculosis. Miliary tuberculosis is characterized by fine bilateral reticular shadowing, sometimes called a “snowstorm” appearance.

Computer tomography (CT) imaging may be helpful in demonstrating pulmonary disease such as endobronchial disease, early cavitation, bronchiectasis or mediastinal lymphadenopathy. Especially in the latter case chest x-ray is normal or unhelpful. In the case of negative x-ray, the significance of these CT findings in asymptomatic children is not clear. Some experts even suggest that a two drug regimen would be more appropriate than isoniazid alone. CT imaging is also useful in investigating CNS disease such as meningitis or tuberculomas.


Microscopy and culture
Early and timely diagnosis of tuberculosis relies on microscopic examination of clinical samples for acid-fast bacilli using the Ziehl-Neelsen (ZN) stain. Microscopy can detect 66-70% of culture-positive samples with revealing mycobacteria when the specimen contains at least 5000 organisms/ml. Less than 20% of children with proven TB will have a gastric aspirate sample that is positive on ZN stain compared with 75% in adults. Newer fluorochrome stains, such as auramine and rhodamine, are superior to the ZN Stain. These tests are easy to perform and are inexpensive and rapid.

Sputum is usually unavailable in young children and three early morning gastric aspirates have been demonstrated to be an acceptable alternative because bronchoscopy specimens offer little extra yield. The rate of positivity of ZN Staining from other body fluids and tissues in children, especially those with extra-pulmonary TB, is even lower, with the exception of superficial lymph nodes with tuberculosis from MT or environmental mycobacteria.

Mycobacterial culture of three consecutive morning gastric aspirates yield MT in 30-40% of cases and may be as high as 70% in infants. Because of the paucibacillary nature of the extra-pulmonary tuberculosis in children, the culture yield from other body fluids or tissues is usually less than 50%.


Polymerase Chain Reaction (PCR)
Although the specificity of a well-developed PCR can be high the sensitivity is significantly less than that of the use of culture. The sensitivity of a good quality PCR would be expected to be 90-100% on smear-positive patients and 60-70% on culture-positive smear-negative patients.

However, there are several problems with applying this technique to routine clinical care, including variation in methodology, high cost and high risk of contamination resulting in false positive results. Studies in children have found the PCR test on clinical samples to have a sensitivity of 40-60% when compared to a clinical diagnosis. This compares favorably to standard cultures, which have a sensitivity of 30-40%.

The specificity of PCR ranges from 80-95% but is dependent on the type of assay used. Furthermore, up to 39% of children with no radiographical or clinical evidence of tuberculous disease also had positive PCR results.

Due to the existing limitations, PCR alone is inadequate to diagnose tuberculosis in children. PCR detection in other body fluids or tissues, such as cerebrospinal fluid appears to be even less successful.

In view of the problems highlighted above, PCR methods have a limited role in the diagnosis of tuberculosis in children. However, they may be useful where the diagnosis is not easily established using standard clinical, microbiological and epidemiological methods. PCR may also have a future role in the diagnosis of tuberculosis in immunocompromised children, or these with extra-pulmonary tuberculosis.


Immunodiagnosis
TST suffers from poor specificity due to variability of the host response and broad antigenic cross-reactivity, that makes M.tuberculosis infection difficult to differentiate from the effects of BCG immunization and environmental mycobacteria infection. A way to develop better diagnostic tests is to identify specific immune responses, especially cell-mediated ones, to antigens that are specific to M. tuberculosis and different from other mycobacteria. QuantiFeron-TB Gold is an in vitro diagnostic test intended as an aid in the detection of infection with M.tuberculosis. The TB-specific antigens used in QuantiFeron-TB Gold are ESAT-6 and CFP 10. Both of these antigens are absent in all BCG strains and most environmental mycobacteria, with the exception of M.kansasii, M.szulgai and M.marinum.

Quanti Feron-TB is recommended by the FDA for detection of TB infection, irrespective of whether it is active or latent.

QuantiFeron-TB has been evaluated for use with immunocompetent healthy adults with or without identified risk factors for TB infection.

QuantiFeron-TB Gold has not been assessed in individuals with lymphocyte counts out of the normal range and the effect of lymphocyte count on reliability is unknown.

It has not evaluated in those with HIV infection, AIDS, transplant recipients, diabetes, silicosis, chronic renal failure, leukemia, lymphomas and other specific malignancies such as carcinoma of the head or neck and lung; immunosupressed patients as those taking corticosteroids, methotrexate, azathoprine or chemotherapy; individuals under the age of 17 years and pregnant women.


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