Tuberculosis/Tuberculin Skin Test

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Introduction

Tuberculosis is an airborne disease that is contagious.  Infectious people may propel tuberculosis pathogens referred to as Mycobacterium tuberculosis into the air through sneezing, coughing, spiting or talking. It is only a matter of inhaling a small number of bacilli pathogens for a person to be infected with tuberculosis. Mycobacterium tuberculosis may remain inactive for a given number of years because it is protected by a thick, waxy lining initiated by the immune system. The tuberculin skin test which is also referred to as the tuberculosis skin test is used to find out whether an individuals body has formed an immune response to bacilli pathogens that transmit tuberculosis.


Discussion

Natural History of Tuberculosis

Almost all Mycobacterium tuberculosis transmission occur from one person to the other through respiratory droplets that turn into airborne when an infected person sneezes, coughs, sings, laughs and breathes. The degree of infectiousness depends on environmental factors like poor air circulation as well as some given characteristics of the individuals infected with tuberculosis (Gorbach, Bartlett and Blacklow, 2004). Contrary to adults, many children infected with tuberculosis are non-infectious. Studies conducted from European orphanages in early 1900s indicated that many children contracted tuberculosis in cases of an adult infected with tuberculosis within the orphanage. However, in cases where only a child had tuberculosis, other children did not contract the disease (Madkour, 2004).


It is important to note that children who are infectious with a type of tuberculosis associated with adults and showing positive results for acid-fast sputum smear can be infectious. Hence, the type of the relevancy relies on the type of disease that a child develops and not the age of the child. Children who are below the age of ten years seldom develop the adult tuberculosis type. It is vital to isolate children portraying symptoms of extensive sputum production, infiltrates and cavity on chest x-ray so that it can be determined whether they are infectious or not (Gorbach, et al, 2004).


Multiplication of tuberculosis bacilli takes place in the alveolar ducts and alveoli at the beginning. Macrophages that transport the pathogens to the regional lymph nodes through the lymphatic ducts do not kill some of the ingested tuberculosis bacilli (Kaufmann, 2003). There are some key groups of lymph nodes that are associated with children and some of them include paratracheal, hilar region and the subcarinal lymph nodes. In adults, the incubation period for tuberculosis bacilli ranges from three to twelve weeks. A fever that may last for a period of one to three weeks may be experienced in infected children to mark the beginning of a hypersensitive reaction (Madkour, 2004).


Tuberculosis Prevalence

As per the year 2008, statistics from the World Health organization indicated that the South Eastern Asia region had the highest number of novel tuberculosis cases. The cases accounted for approximately 35% of the total cases in the world. It is however important to note that Sub-Saharan Africa region is estimated to have two times as many cases as those of South Eastern Asia (World Health Organization, 2010). In 2009, an estimate of 1.7 million tuberculosis mortality cases was reported, with the highest number of deaths occurring in Africa. The number of new TB cases emerging annually is continually rising in the World Health organization Mediterranean regions, Africa and the South Eastern Asia regions.


In the United States, a total of approximately 13, 290 tuberculosis cases were reported in the year 2007. A burden of TB that is disproportionately distributed is continually borne by racial and ethic minorities as well as foreign-born people in the United States (World Health Organization, 2010).According to the 2007 statistics, the tuberculosis rate was about 9.6 times higher in foreign-born individuals compared to the United States-born individuals. The highest rates were recorded among the Asians followed by the blacks and Hispanics.


The slow reduction in the incidence of tuberculosis as well as the continual disparities between foreign-born and U.S.-born persons and between minorities and the whites are a threat to the pursuit of eliminating tuberculosis in the U.S (Madkour, 2004). There are several strategies pursued by the CDC in addressing the increased rate of tuberculosis in foreign-born individuals in U.S. as well as the rising proportion of cases represented by these individuals. An example of a strategy implemented is medical screening of that is a must be done to all oversees applicants (World Health Organization, 2010).


Diagnostic test

The basis of a tuberculin test is the fact that there is production of a delayed-type hypersensitivity reaction following infection with Mycobacterium tuberculosis. The elements of the purified protein derivatives (PPD) of tuberculin are made of the tuberculosis bacilli components. The material contained in PPD is the one required for tuberculosis skin testing (Kaufmann, 2003).The onset of reaction to tuberculin PPD is marked by recruiting of sensitized T-cells to the skin site by the immune system. The sensitized T-cells then produce chemical messengers known as lymphokines, which induce swelling of the injection site. In order to obtain a positive PPD test, it is essential to have a 12 weeks incubation period following exposure to the tuberculosis bacilli (Puisis, 2006).


Administration of the standard recommended tuberculin test, also referred to as the Mantoux test is through an intra-dermal injection of 0.1 milliliters volume that contains five tuberculin units. It is recommendable to use a skin part that lacks any abnormalities and that is located far away from the veins. A quarter to half an inch and 27-gauge needle as well as a tuberculin syringe is used to administer the injection. A correct injection is characterized by a pale and discrete skin elevation that has a diameter ranging from six to ten millimeters. In cases of incorrect administration of the initial test, it is vital to administer an immediate second test. The site for the second test should be located many centimeters from the first injection (Gorbach, et al, 2004).


Two-step testing is the other form of testing that is used in adults who would require periodical retesting for instance the heath care workers. The first step is to perform the initial test and read the results within 48 to 72 hours after administering the injection. The person is considered infected if the first test turns out to be positive. However, if the test is negative, a second test should be given after a period of one to three weeks. A second positive test is an indication that the person was previously infected while a second negative test means the person is not infected. A person diagnosed as infected with tuberculosis following a two-step testing is known as a tuberculin converter (Puisis, 2006).


Detecting of a raised and thickened area of the skin reaction known as induration is referred to as reading the skin test. Hence induration is the key detection item rather than bruising or redness. It is recommendable to read skin tests within 48 to 72 following the injection. There may be underestimation of indurations for tests read after a time period of 72 hours. The skin test is read on the basis of the absence or presence and the degree of induration. A transverse diameter of the induration should be determined and noted down in millimeters. The induration area represents tuberculin reaction (Kaufmann, 2003).


Sensitivity, specificity, false negative, false positives and interpretation of results

A positive tuberculin reaction should be classified on the basis of the induration’s diameter. A healthy person with a normal immune system and an induration diameter that is equal or greater than 15 millimeters is said to have a skin test that is positive. The presence of blisters is also an indication of a positive test. An induration that is 5mm is considered a positive test in immuno-compromised people like those with Crohn’s disease or rheumatoid arthritis. A 10 mm induration is said to be a positive skin test in persons with a history or diabetes of kidney disease as well as personal contact to healthcare workers for people with active tuberculosis. A negative skin test on the other hand is characterized by an induration that is has a diameter of less than 2 millimeters with the absence of blisters (Gorbach, et al, 2004).


One feature of tuberculosis skin test is that it has a low specificity. Due to the low specificity nature, majority of positive reactions among the low-risk individuals are actually false-positives. The two common causes of false positive results are previous administration of BCG vaccine and nontuberculous mycobacterium. It is also possible to get false positive results following touching of the injected area, which leads to itching and swelling. A false negative result on the other hand is obtained following tests done to immuno-compromised individuals, mostly the ones infected with HIV and having low counts of CD4 T cells. The weak immune system of such individuals fails to respond to the PPD administered beneath the skin (Madkour, 2004).


Pros and Cons of the Test and Future Directions

The benefit of having a tuberculosis skin test is that it enables one to know whether there has been development of an immune response to a pathogen that transmits tuberculosis. The other benefit is that there is no chance of contracting tuberculosis from the test because live bacteria are not use (Kaufmann, 2003).However, there are some risks associated with the test. There is a mild risk of developing an extreme reaction to the tuberculin test, resulting to redness of the arm and swelling especially for individuals with a history of tuberculosis and those who have had a BCG vaccine earlier. There are also rare cases of allergic reactions and complications associated with the PPD test (Puisis, 2006).


Conclusion

The tuberculin skin test is also referred to as PPD test or tuberculosis test, which is used to find out whether a person has developed an immune system that is responsive to Mycobacterium tuberculosis, the causative agent of tuberculosis. The standard test that is recommendable is known as the Mantoux test. Reading of skin tests should be done within 48 to 72 hours after injection on the basis amount of induration as well as the absence or presence of an induration. Due to the low sensitivity nature of the test, it is possible to obtain false positives and false negatives. Hence, a negative test is not always an indication that a person is not infected with tuberculosis.


 References

Gorbach, S. L., Bartlett, J. G., & Blacklow, N. R. (2004). Infectious diseases (3rd Ed). Lippincott Williams and Wilkins, Page 151

Kaufmann, S. H. (2003). Mycobacteria and TB. Karger Publishers

Madkour, M. M. (2004). Tuberculosis. Birkhauser

Puisis, M. (2006). Clinical practice in correctional medicine (2nd Ed). Elsevier health Sciences

World Health Organization (2010). Global Tuberculosis Control 2010. World Health Organization

 


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