The results of this test must be interpreted carefully. The person's medical risk factors determine the size of induration the result is positive 5 mm, 10 mm, or 15 mm. A record should also be made of formation of vesicles, bullae, lymphangitis, ulceration and necrosis at the test site.
The formation of vesicles, bullae or necrosis at the test site indicates high degree of tuberculin sensitivity and thus presence of infection with tubercle bacilli. Organ transplant recipients and other immunosuppressed patients who are on cytotoxic immune-suppressive agents such as cyclophosphamide or methotrexate. Residents and employees of high-risk congregate settings e. Persons with clinical conditions that place them at high risk e.
Children less than four years of age, or children and adolescents exposed to adults in high-risk categories. Persons with no known risk factors for TB. Reactions larger than 15 mm are unlikely to be due to previous BCG vaccination or exposure to environmental mycobacteria. Some persons may react to the TST even though they are not infected with M.
The causes of these false-positive reactions may include, but are not limited to, the following:. Due to the test's low specificity, most positive reactions in low-risk individuals are false-positives. Prior vaccination with BCG may result in a false-positive result for many years later. A negative Mantoux result usually signifies that the individual has never been exposed to M. However, there are factors that may cause a false-negative result or diminished ability to respond to tuberculin.
Cutaneous anergy anergy is the inability to react to skin tests because of a weakened immune system. 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 ]. More commonly known as TB, tuberculosis is a bacterial infection that is spread through the air. Most TB infections start in the lungs, so when someone with the disease coughs or speaks, the bacteria can enter the air and be inhaled by people nearby. It settles in the lungs, where it can grow and move through the blood to other parts of the body. Without treatment, TB can be fatal. What is it? How is the TST Administered?
How is the TST Read? What Are False-Positive Reactions? What Are False-Negative Reactions? The reasons for these false-negative reactions may include, but are not limited to, the following: Anergy Recent TB infection within the past 8 to 10 weeks Very young age younger than 6 months Recent live-virus measles or smallpox vaccination Incorrect method of giving the TST Incorrect measuring or interpretation of TST reaction.
What is a Boosted Reaction? Why is Two-Step Testing Conducted? For persons scheduled to receive a TST, testing should be done as follows: Either on the same day as vaccination with live-virus vaccine or At least 1 month after the administration of the live-virus vaccine. Are there alternative tests to the TST? Diagnosis of TB Disease TB disease is diagnosed by medical history, physical examination, chest x-ray, and other laboratory tests. What are treatment options for latent TB infection?
Additional Information. Guidelines for preventing the transmission of Mycobacterium tuberculosis in health-care settings, MMWR ; 54 No. Targeted tuberculin testing and treatment of latent tuberculosis infection pdf icon.
MMWR ; 49 No. Lewinsohn et al. You could have latent TB infection if you have ever spent time close to someone with active TB disease even if you didn't know they were sick. Your health care provider will use a small needle to inject some harmless testing fluid called "tuberculin" under the skin on your arm. If you have a reaction to the test, it will look like a raised bump.
Your health care provider will measure the size of the reaction. If there is a bump, it will go away in a few weeks. The test is "negative" if there is no bump or only a very small bump at the spot where the fluid was injected. A negative TB skin test usually means that you don't have TB.
The test is "positive" if there is a bump of a certain size where the fluid was injected. This means you probably have TB germs in your body. Most people with a positive TB skin test have latent TB infection. To be sure, your doctor will examine you and give you a chest x-ray. You may need other tests to see if you have active TB disease.
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