People who have been vaccinated with BCG should not be exempted from TB skin testing unless they have a documented positive result from a prior test. Two-step TSTs are not recommended for patients in other settings. The first "step" may stimulate or boost the immune system's ability to react to the test. If the second "step" is not performed as part of baseline screening, a subsequent positive TST reaction could be misinterpreted as a new infection.
BCG is used in many countries with a high prevalence of TB to prevent childhood tuberculous meningitis and miliary disease. However, BCG is not generally recommended for use in the United States because of the low risk of infection with Mycobacterium tuberculosis , the variable effectiveness of the vaccine against adult pulmonary TB, and the vaccine's potential interference with tuberculin skin test reactivity.
Severe reactions include anaphylaxis extremely rare , severe swelling, heavy blistering, or "weeping" of the skin. Incorrect interpretation of reaction, insufficient dose and inadvertent subcutaneous injection.
The absence of cell mediated immunity to tuberculin may be due to the lack of previous sensitization or due to a false-negative result for various reasons or due to anergy because of immune suppression. Most children with negative result have not been infected with M. A small proportion of otherwise normal children with M. From the time of infection to the development of CMI there is a window period of some two to six weeks, when the Mantoux test would be negative.
This is because the immune system needs to be functional to mount a response to the protein derivative injected under the skin. Negative tests can be interpreted to mean that the person has not been infected with the TB bacteria or that the person has been infected recently and not enough time has elapsed for the body to react to the skin test. A repeat test is not advocated before one week as the tuberculin injected for the first test has a booster effect on the subsequent dose.
Immunocompromised persons may be unable to react sufficiently to the Mantoux test, and either a chest X-ray or sputum sample may be required. Interpretation in children: A correctly applied Mantoux test can be invaluable in the assessment of a child with suspected TB. The interpretation of the result, however, is often difficult, with different workers using different induration sizes to indicate a positive reaction. Malnutrition has previously been shown to affect the results of tuberculin testing.
As in other studies, underweight children in this study were significantly more likely to have a negative Mantoux result [ Table 1 ]. As in many other studies, the majority of children did not have any reaction to tuberculin despite having received BCG immunization soon after birth. The reasons for this are not always clear but clearly whatever tuberculin sensitivity BCG might have caused could not have been significant or persistent.
This agrees with the current recommendation that for patients with a high risk for TB the history of BCG vaccination should not be a consideration in the interpretation of the tuberculin test.
In some persons who are infected with M. When given TST years after infection, these persons may have a false-negative reaction. However, the TST may stimulate the immune system, causing a positive or boosted reaction to subsequent tests.
When sensitization to mycobacteria has occurred many years earlier, an initial intradermal injection of tuberculin may produce a negative or weakly positive response due to there being too few sensitized lymphocytes in circulation to produce a significant local response. The second boosted reading is the correct one — that is, the result that should be used for decision-making or future comparison.
Boosting is maximal if the second test is placed between one and five weeks after the initial test, and it may continue to be observed for up to two years. Reversion is defined as the change to a negative Mantoux result following a previous positive result. Reversion is more common[ 13 ]. Whereas boosting is a recall of the hypersensitivity response in the absence of new Infection, conversion is the development of new or enhanced hypersensitivity due to infection with tuberculous or non-tuberculous mycobacteria, including BCG vaccination.
Mantoux conversion is defined[ 14 ] as a change within a two-year period of Mantoux reactivity which meets either of the following criteria:. There is debate about the time required for the immunological changes that produce Mantoux conversion following infection. After inadvertent vaccination with M.
Other studies have shown clinical illness, with a positive tuberculin test, from 19 to 57 days after exposure, with a mean of 37 days. Therefore, when testing TB contacts for conversion, the second tuberculin test is done eight weeks after the date of last contact with the source case. In the past, the traditional window period, or interval, of 12 weeks was used.
There is disagreement about the role of Mantoux testing in people who have been vaccinated. Previous TB disease: no useful diagnostic information will be gained and significant discomfort is likely. Infants under 12 weeks old: a positive reaction is very important, but a negative reaction may indicate that the child is too young to mount a response, and the test will need to be repeated if exposure has occurred. Pre-vaccination Mantoux testing before 12 weeks of age is not necessary unless the baby has been exposed to TB.
The Mantoux test is technically difficult to administer and read, so false readings may occur if the tester has insufficient skill. It may require four visits by the patient if a two-step test is performed, and compliance with this is sometimes difficult. A test that can be done on a single patient visit, such as a blood test, would be easier.
The blood test detects the presence of Mycobacterium tuberculosis TB infection by measuring interferon-gamma IFN-G harvested in plasma from whole blood incubated with the M.
This new immunodiagnostic test has been launched as an aid in the diagnosis of LTBI. It may be possible in future to replace the skin test with this, or an alternative in vitro assay. Source of Support: Nil.
Conflict of Interest: None declared. National Center for Biotechnology Information , U. Indian Dermatol Online J. Surajit Nayak and Basanti Acharjya. Author information Copyright and License information Disclaimer. Address for correspondence: Dr. E-mail: ni. This is an open-access article distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 3.
This article has been cited by other articles in PMC. Abstract The tuberculin skin test is one of the few investigations dating from the 19 th century that are still widely used as an important test for diagnosing tuberculosis. Keywords: Interpretation, Mantoux test, tuberculosis. End stage renal disease.
Several factors affect how the test is interpreted. An induration of less than 5 millimeters mm is considered a negative test result. The same is true if you live in a high-risk environment such as a nursing home or work in a high-risk setting such as a hospital or medical laboratory. A 10 mm induration may also be considered positive in children under the age of 4 or people who use injected drugs. The TB skin test is less accurate than the blood test, so you could have a positive skin test and a negative blood test.
Again, incorrect administration of the test or interpretation of the result could lead to a false-negative test result. Certain immune system conditions, especially an organ transplant , may also cause a false-negative skin test.
If you were exposed to TB in the past few weeks, you may not yet test positive for TB. Infants, even if they have TB, may not always have a positive skin test. A blood test can also be done any time. Having only the TB infection will not produce any noticeable symptoms. You may also cough up blood.
Other symptoms include:. Even a negative test is helpful because it can rule out TB and help your doctor find other causes for your symptoms. A positive skin test will usually be followed by a chest X-ray. This can help determine the difference between active TB disease and latent TB infection.
Your doctor will look for white spots that indicate areas where your immune system is responding to bacteria. There may be other changes in your lungs caused by TB disease.
Your doctor may decide to use a CT scan instead of or as a follow-up to a chest X-ray because a CT scan provides images of much greater detail. If the images indicate TB is present, your doctor may also order tests on your sputum. Sputum is the mucus produced when you cough. A lab test can identify the type of TB bacteria causing the infection. This helps doctors decide which medication to prescribe. Tuberculosis TB is a highly infectious disease that primarily affects the lungs.
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