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The tuberculosis test checks for exposure to the disease and is done using a purified protein derivative injected into the skin. Results can be false negative or positive, and a positive result does not necessarily mean the person has the disease. Further tests are needed to confirm the diagnosis.
There are a couple of versions of the tuberculosis (TB) test. One checks for exposure to tuberculosis and is much more commonly used to rule out exposure to tuberculosis, which could mean contagion with the disease. The other test is usually done only if TB is reasonably suspected, either because of a positive TB exposure test or other clear signs that the disease is present.
The first tuberculosis test is also called a purified protein derivative standard or PPD, and sometimes a tuberculosis skin test. This checkup is usually done in a doctor’s office or hospital. A doctor or nurse will clean the skin and often draw a circle on your arm, most typically the inside of the lower half of your arm. Once the circle has been drawn, a small amount of purified protein derivative is injected into the center of the circle. Some doctors and nurses don’t draw the circle, but it’s an easy way to measure test results.
Between two and four days later, the arm is checked again to see how it reacted to the injected PPD, and the size of the reaction in millimeters can determine whether a person has had previous exposure to TB. People usually need to go back to their doctor at a set time to interpret the test. Nurses or doctors may use a chart to check the size of the skin reaction to see if it is large enough to read as positive.
The test is imperfect and there can be about 20% false negative results. People with some autoimmune disorders can react in the opposite direction by showing a false positive. If there is any doubt about the results of the TB test, it may be done again to see if the same result occurs. If a person has knowingly been exposed to TB, doctors may want to retest in a few months if the first test is negative.
Once a person tests positive for TB, they likely always will in the future. Indeed, even after being successfully cured of the disease, most people will give a positive result in the future. A person with a previous positive test result or who has had tuberculosis should inform doctors that they wish to have this test done, as it is not particularly helpful.
It is important to remember that when an accurate positive test for TB occurs, it does not mean that a person has TB. It means they have been exposed to it and will not always develop tuberculosis symptoms. This is actually not an easy condition to get, although close exposure to others who have the disease can put people at greater risk.
When you first test positive for tuberculosis, doctors need to do other tests to determine if the positive test indicates the presence of the disease. The second round involves the collection of sputum samples which can be evaluated to look for evidence of active disease. Doctors may also order chest x-rays to help confirm the diagnosis. If tuberculosis is present, it is treated with several drugs that are highly successful in curing the disease.
