How is tuberculosis diagnosed?
Diagnosing tuberculosis (TB) is sometimes straightforward, but the diagnosis may be more difficult for some people. In general, the diagnosis is made by looking at the clinical picture (your symptoms and a doctor's examination), combined with the results of certain tests. To start with, you will usually have a chest X-ray and/or a tuberculin skin test, followed by phlegm (sputum) tests.
A chest X-ray usually shows any active lung TB. It may also show healed or inactive TB.
Tuberculin skin testing (Mantoux test)
This test shows whether you have been in contact with TB germs (bacteria) at some point in your life. However, it cannot prove you have a current active infection. The tuberculin is made from part of the TB bacterium. It is injected into the skin. The injection site is examined a few days later.
A positive reaction is a red inflamed area of the skin. This means that you have an active infection, or have had a previous infection, or you have been immunised in the past with BCG. (BCG is the vaccine used to prevent TB.) A negative skin reaction tends to rule out TB. However, the result may be falsely negative for some people with TB infection - for example, if you have severe TB, if you have AIDS or a poor immune system, or in young children in the early stages of infection.
If the chest X-ray or tuberculin test results suggest that TB is possible, the next test will be to look for TB bacteria from the lung. This is done by sending samples of sputum to the laboratory.
A smear of the sputum is examined in a laboratory, under a microscope using a special dye (stain) to show the TB bacteria. The results are obtained quite quickly, usually within a few days.
Another test for the sputum samples is a culture test. This involves growing (culturing) the TB bacteria in the laboratory. This can take several weeks because TB bacteria grow slowly. There are two important reasons for doing this test. First, to detect TB bacteria that may not be found on the smear test. Second, the culture test can check whether the TB bacteria are resistant to any antibiotic medicines. (Antibiotic resistance is explained below.)
It's considered that sputum results should be awaited before treatment is started, unless the illness is thought to be life-threatening.
It is sometimes difficult to get sputum for the test (for example, with children). A sample of fluid from the stomach (gastric washings) may then be used instead.
Other possible tests for suspected TB are:
- A blood test called an interferon gamma test. This can be helpful if the tuberculin skin test result was unclear. The advantage of this test is that the result is not affected by the BCG vaccine.
- An HIV test should be offered. This is because TB is more common in people who have HIV, and treatment may be needed for both conditions.
- A computerised tomography (CT) scan or a magnetic resonance imaging (MRI) scan may be used to look for TB in internal organs. For example, a brain scan is useful if TB meningitis or TB infection in the brain is suspected.
Samples from other parts of the body: TB may be suspected in organs other than the lung. It may then be helpful take a sample of tissue or fluid from the affected part of the body. This sample can then be tested in the laboratory by the same methods used for sputum samples (above). For example, samples can be taken from urine, from lymph glands near the skin, or from the lung. A test called a lumbar puncture samples fluid near the spine, if meningitis is suspected.
New tests are being developed. Some are similar to the culture test above, but give faster results. Other tests help identify bacteria which are resistant to antibiotics.
Further reading and references
Tuberculosis; NICE Guideline (January 2016)
Tuberculosis (TB); World Health Organization
Tuberculosis; NICE CKS, January 2015 (UK access only)
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