Tuberculosis Tests and diagnosis

The most commonly used diagnostic tool for TB is a simple skin test. Although there are two methods, doctors consider the Mantoux test the more accurate.

For the Mantoux test, a small amount of a substance called PPD tuberculin is injected within 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 a local reaction to the injected material. Depending on your response, the test is diagnosed as positive or negative. A positive response — usually shown by a hard, raised bump at the injection site — means you're likely to have TB infection.

The Mantoux test isn't perfect — it's possible to have either a false-positive or false-negative test. 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 mycobacterium other than the one that causes TB or if you've ever been vaccinated with bacillus Calmette-Guerin, also known as BCG, a TB vaccine that's seldom used in the United States, but widely used in countries with high TB infection rates.

A blood test that detects the presence of TB bacteria has been approved by the Food and Drug Administration. Called QuantiFERON-TB Gold (QFT) , results may be available in as soon as one day. The test is not yet widely available, however.

Researchers in October 2006 also reported encouraging results from another test under investigation for use primarily in developing countries. It's called the microscopic-observation drug-susceptibility (MODS) assay and relies on sputum samples to detect the presence of TB bacteria. MODS produces very accurate results in as little as seven days. Additionally, the test can identify drug-resistant strains of the TB bacteria.

Further testing
If the results of a TB test are positive, you may have further tests to help determine whether you have active TB disease:

These tests may include:

• Chest X-ray. 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 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.
• Culture tests. If your chest X-ray shows signs of TB or a urine sample indicates infection, 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.

Although it takes longer, 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.

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 HIV, 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 — they're far less likely than adults to have signs and symptoms of the disease, even when they're quite sick. Children also 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.

Diagnosing TB in people with HIV/AIDS
Diagnosing TB in HIV-positive people can be challenging, in part because signs and symptoms of HIV/AIDS are often similar to those of TB. What's more, people with HIV may not react to a standard TB skin test, and X-rays, sputum tests and other exams may fail to show evidence of early TB infection.