How Tuberculosis (TB) Is Diagnosed

Types of Diagnostic Tests and What They Can Tell You

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There are two different types of tests used to detect the presence of tuberculosis (TB) bacteria—the Mantoux skin test and interferon gamma release assays, a type of blood test. If your skin test is positive, it means you are infected with the TB bacteria but do not necessarily have an active and contagious form of the disease; this is called latent TB. Your doctor will pursue additional testing, including a chest X-ray and sputum culture to find out whether you have active TB.

Your healthcare provider will recommend the TB test that is best for you based on the reason for testing, test availability, and cost. Generally, it is not recommended to get both a skin test and a blood test. 

In addition, your doctor will perform a physical exam and go over your medical history. Certain illnesses, such as HIV and diabetes, can put you at higher risk for TB.

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Skin Testing

The Mantoux skin test uses tuberculin purified protein derivative (PPD), a solution that contains certain components of the TB bacterium. A syringe with a small gauge needle is filled with the PPD solution, which is then injected under the skin on your forearm.

The PPD causes specialized immune system cells, known as T-cells, to recognize the injected derivative as a threatening invader, causing a skin reaction. The degree of the response is interpreted to assess whether someone is negative or positive for TB.

Unlike a vaccination that is given intramuscularly, the PPD injection is so superficial that it leaves a small, pale, raised area of skin called a “wheal” at the injection site. The wheal will eventually disappear over the course of several hours, but if it never appears, the test must be repeated. 

Results

The skin reaction reaches its peak about 48 to 72 hours after the PPD is injected. You must go back to your healthcare provider within that time period to have your reaction interpreted properly. Waiting too long or not long enough may cause inaccurate results.

Your doctor will look for a raised, hardened area of skin called an induration. Its size will determine if you are positive or negative for TB, and what size indicates TB for you depends on other risk factors you may have.

For example, otherwise healthy individuals must have a larger (15 mm) induration that those who are immune compromised, say due to HIV, or who have been in recent contact with someone with an active TB infection (5 mm). An induration smaller than that indicates no TB infection.

A positive test will also result in redness and itching.

False-negatives and false-positives are more common with the skin test than blood tests. These results are often due to improper administration of the skin test or misinterpretation of its results. A false-negative result can also occur if you were only recently exposed to TB; a detectable immune response may not develop for eight to 10 weeks after contact with the bacteria.

Labs and Blood Tests

A blood test that can identify TB bacteria is an alternative to a skin test—one that is often preferred for several reasons. If your blood test is positive, your doctor may also collect a sputum sample and have it cultured to determine if you have TB disease.

Interferon Gamma Release Assays (IGRAs) 

Blood tests used to identify the presence of TB bacteria are called interferon-gamma release assays (IGRAs). With these, blood is collected into special tubes using a needle and then sent to a laboratory for testing. There is no need for patient follow-up and results are available in around 24 hours.

There are two IGRA tests approved by the Food and Drug Administration (FDA):

  • QuantiFERON-TB Gold In-Tube test (QFT-GIT)
  • T-SPOT.TB test (T-Spot)

These blood tests are often preferred because they tend to be more accurate than the skin test. That is, in part, due to the fact that it's fairly common for people to miss the required 48- to 72-hour timeframe for proper evaluation of their skin test results. 

The blood test is also more accurate than the skin test for people who have received bacille Calmette-Guerin (BCG), a vaccine for tuberculosis disease that is used in many countries with a high prevalence of TB.

A positive TB test means that, at some point in your life, you were exposed to and infected with TB. In this case, other tests are necessary to determine whether you have latent TB, which causes no symptoms and is not contagious, or active TB. 

Sputum Culture

If your skin or blood test is positive for TB bacteria, your doctor will do follow-up tests, including culture of a sputum (phlegm) sample. Sputum is a thick fluid produced in the lungs as a result of illness. The sample will be placed in a test tube or petri dish to see if the bacteria grows. Mycobacterium tuberculosis tends to grow slowly, so it can take up to 21 days for accurate results. 

Imaging

Testing positive for tuberculosis bacteria will also prompt the ordering a chest X-ray. If you have active TB, the result of this test will usually be abnormal, possibly showing a shadowy area.

Occasionally, TB appears in organs outside the lungs; in those cases, CT scans, and MRIs may be used as well.

Differential Diagnoses

Some of the symptoms of (TB)—coughing, fever, loss of appetite, weight loss, and night sweats—are common signs of numerous other illnesses, including those that affect the lungs. These include:

The full battery of TB tests, along with specific tests for the above diseases, will lead to an accurate diagnosis.

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