![]() ![]() In a negative test the lymphocytes will not move to the injection site as quickly and so the raised area will not be very big, or diagnostically very big. ![]() The same method of testing is done in which a small amount of tuberculin protein is injected into the skin and then the raised area measured after a 48 to 72 hour time period. What is Negative TB skin test? Definition:Ī negative TB skin test is when the Mantoux tuberculin skin test (TST) indicates that a person does not have the tuberculosis bacterium in their body based on the appearance of the skin prick test after 48 to 72 hours. They are also given a course of antibiotics to take which needs to be completed. Patients need to avoid contact with others and may need to be isolated. The condition is contagious especially in a person who is actively coughing. If the diagnosis of TB is confirmed, then the person needs to receive treatment as soon as possible. In addition, a person who has recently had the Bacillus Calmette-Guérin (BCG) vaccine may also show a positive result on this test. A further limitation is that occasionally there are false positive test results due to a person having been exposed to a different type of bacteria that is closely related to M. Further testing such as chest X-rays or sputum analysis needs to be done to determine this. ![]() Limitations:Ī positive test cannot be used to determine if a person has an active infection or a dormant infection. A positive result for a person who does not have any risk factors is a swelling of 15 mm in diameter. Individuals recently arrived from a country where the disease is very prevalent or people working with the bacterium in the lab may show a raised area of more than 10 mm in diameter. Patients who have a suppressed immune system or who have been in contact with a person who is ill with TB can show a swelling of more than 5 mm in diameter. Appearance:Ī positive result from the test is a hardened swollen raised area on the skin of a certain size depending on the history of the person being tested. It is the raised part that is measured, not the extent of the redness that is present. The measurement needs to be taken across the arm at right angles to the long axis of the arm, and the measurement is done in millimeters. After 48 to 72 hours the patient needs to return to have the raised area that occurs measured. Here the white blood cells, the T cells, will arrive and release chemicals called lymphokines that attract more immune system cells and cause swelling of the area. The idea is that if you have been exposed to a TB infection that your immune system will quickly rush to the site of the injection. This is actually a protein that has been extracted from the bacterium and carefully modified and purified. Method:Ī special syringe is used to inject 0.1 ml of tuberculin into the skin. What is Positive TB skin test? Definition:Ī positive TB skin test is when the Mantoux tuberculin skin test (TST) indicates that a person has the tuberculosis bacterium, Mycobacterium tuberculosis in their body based on the appearance of the skin prick test after 48 to 72 hours. A negative TB skin test is when the result indicates that bacteria that cause tuberculosis are absent. ![]() A positive TB skin test is when the result indicates that bacteria that cause tuberculosis are present. ![]()
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