methods of diagnosis

Tests and diagnosis

If your doctor suspects TB, you will need a complete medical evaluation and tests for TB infection.

Skin test

The most commonly used diagnostic tool for TB is a simple skin test. Although there are two methods, the Mantoux test is preferred because it's more accurate.

For the Mantoux test, a small amount of a substance called PPD tuberculin is injected just below the skin of your inside forearm. You should feel only a slight needle prick. Within 48 to 72 hours, a health care professional will check your arm for swelling at the injection site, indicating a reaction to the injected material. A hard, raised red bump (induration) means you're likely to have TB infection. The size of the bump determines whether the test results are significant, based on your risk factors for TB.

The Mantoux test isn't perfect. A false-positive test suggests that you have TB when you really don't. This is most likely to occur if you're infected with a different type of mycobacterium other than the one that causes tuberculosis, or if you've recently been vaccinated with the bacillus Calmette-Guerin (BCG) vaccine. This TB vaccine is seldom used in the United States, but widely used in countries with high TB infection rates.

On the other hand, some people who are infected with TB — including children, older people and people with AIDS — may have a delayed or no response to the Mantoux test.

Blood tests
Blood tests may be used to confirm or rule out latent or active TB. These tests use sophisticated technology to measure the immune system's reaction to Mycobacterium tuberculosis. These tests are quicker and more accurate than is the traditional skin test. They may be useful if you're at high risk of TB infection but have a negative response to the Mantoux test, or if you received the BCG vaccine.

Further testing
If the results of a TB test are positive (referred to as "significant"), you may have further tests to help determine whether you have active TB disease and whether it is a drug-resistant strain.

These tests may include:

• Chest X-ray or CT scan. If you've had a positive skin test, your doctor is likely to order a chest X-ray. In some cases, this may show white spots in your lungs where your immune system has walled off TB bacteria. In others, it may reveal a nodule or cavities in your lungs caused by active TB. A computerized tomography (CT) scan, which uses cross-sectional X-ray images, may show more subtle signs of disease.

• Culture tests. If your chest X-ray shows signs of TB, your doctor may take a sample of your stomach secretions or sputum — the mucus that comes up when you cough. The samples are tested for TB bacteria, and your doctor can have the results of special smears in a matter of hours.

  Samples may also be sent to a laboratory where they're examined under a microscope as well as placed on a special medium that encourages the growth of bacteria (culture). The bacteria that appear are then tested to see if they respond to the medications commonly used to treat TB. Your doctor uses the results of the culture tests to prescribe the most effective medications for you. Because TB bacteria grow very slowly, traditional culture tests can take four to eight weeks.

• Other tests. Testing called nuclear acid amplification (NAA) can detect genes associated with drug resistance in Mycobacterium tuberculosis. This test is generally available only in developed countries.

  A test used primarily in developing countries is called the microscopic-observation drug-susceptibility (MODS) assay. It can detect the presence of TB bacteria in sputum in as little as seven days. Additionally, the test can identify drug-resistant strains of the TB bacteria.

What if my test is negative?
Having little or no reaction to the Mantoux test usually means that you're not infected with TB bacteria. But in some cases it's possible to have TB infection in spite of a negative test. Reasons for a false-negative test include:

• Recent TB infection. It can take eight to 10 weeks after you've been infected for your body to react to a skin test. If your doctor suspects that you've been tested too soon, you may need to repeat the test in a few months.
• Severely weakened immune system. If your immune system is compromised by an illness, such as AIDS, or by corticosteroid or chemotherapy drugs, you may not respond to the Mantoux test, even though you're infected with TB. Diagnosing TB in HIV-positive people is further complicated because many symptoms of AIDS are similar to TB symptoms.
• Vaccination with a live virus. Vaccines that contain a live virus, such as the measles or smallpox vaccine, can interfere with a TB skin test.
• Overwhelming TB disease. If your body has been overwhelmed with TB bacteria, it may not be able to mount enough of a defense to respond to the skin test.
• Improper testing. Sometimes the PPD tuberculin may be injected too deeply below the surface of your skin. In that case, any reaction you have may not be visible. Be sure that you're tested by someone skilled in administering TB tests.

Diagnosing TB in children
It's harder to diagnose TB in children than in adults. Children may swallow sputum, rather than coughing it out, making it harder to take culture samples. And infants and young children may not react to the skin test. For these reasons, tests from an adult who is likely to have been the cause of the infection may be used to help diagnose TB in a child.