If there is a reaction, some redness and a small bump will appear on the arm. A measurement of 10 millimetres or more is "positive", and means you may have the TB germ in your body. At that point, the nurse will request additional information and tests, including chest x-ray and the collection of sputum samples. When results are abnormal, the nurse will then consult with a medical health officer who may then recommend a further course of action.
There may also be a small amount of blood at the needle site. A small lump may form at the injection site over the next few days and you will need to return to your doctor or nurse 3 days later to have this assessed. The size of your lump — measured in millimetres — will indicate whether or not you have a latent TB infection. If the lump is below a certain size, the result of the test is negative.
Usually this means that you have not been infected with TB bacteria. However, if your contact with TB was very recent, you may need to have the test repeated in 8 to 12 weeks. A false negative can occur if you are unwell, have a weak immune system, or have had a recent live vaccination.
If the lump is above a certain size, the result of the test is positive. It means that you may have been infected with the TB bacteria. It does not mean you have TB disease active TB. When you are infected with TB bacteria you are not infectious and you cannot pass the bacteria onto other people. You are not sick but, at some stage in the future, you may develop TB disease. If you are told your TST is positive you may be asked to see a doctor for review.
You will be given your result when you have the reading done. If the result is negative, you may be advised that no further follow up is required. You may be recommended to have a repeat test in the future depending on why the test was initially performed. If the result is positive, you will require a chest X-ray and review by a medical officer.
Some people with a latent TB will be offered medication to prevent them developing active TB. If your test is positive you may be advised not to have the test repeated. The TST is a simple and safe test and side effects are uncommon. The side effects are mainly related to the size of the reaction and include swelling, redness and itchiness that may take a few weeks to clear. This may cause the wheal to grow to over 15 mm in diameter. This indicates a positive outcome no matter what other circumstances there may be.
In many cases, doctors will use additional methods to be sure the results are as accurate as possible. There is room for error with TB skin tests. Doctors use them as one part of a more detailed diagnosis, as opposed to stand-alone tests.
If someone has TB, they may be started on medication immediately. If the diagnosis is unclear, the doctor will use other methods to reach a correct diagnosis. One of the next steps will involve looking for signs of TB in the lungs, by using either an X-ray or CT scan. TB causes changes in the lungs. Most commonly, little white spots will be visible, which means the body is fighting bacteria. X-rays are usually accurate enough, but a CT scan may also be used to give a closer look.
CT scans provide a more detailed picture, which helps a doctor decide what action to take. This is the mixture of saliva and mucus that is coughed up due to an infection. A sputum test is used to determine what type of TB bacteria is attacking the body. This also helps decide how best to treat it. Some people may have a bad reaction to the TB skin test.
Although this test suits some people, it is not right for everyone. As such, people should always talk to a doctor to determine which test is best for them. It is common for a person with the TB infection to experience no symptoms initially, and to only develop these once the bacteria has become active in the body.
A positive TB skin test is an indicator that the body is infected with TB.
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