Tuberculosis (TB) is a highly contagious disease. It’s caused by an infection of bacteria called Mycobacterium tuberculosis (Mtb). Exposure to Mtb can result in latent TB, which means you’re infected but have no symptoms. Latent TB can eventually become active TB, which must be treated with medication.
There are two types of tests used to diagnose TB: a blood test and a skin test. Your results from either test will not reveal whether you have latent or active TB. Instead they are used to determine who should be treated and with what type of medication.
What happens during a TB skin test?
A TB skin test is also called a Mantoux tuberculin skin test (TST). The test is usually well tolerated and people rarely have negative reactions to it.
The TB skin test is done in two parts. During one visit to a doctor’s office or clinic, a tiny amount of tuberculin is injected into the skin, usually in the forearm. Tuberculin is a sterile extract purified protein derivative made from the bacteria that causes TB. After receiving the injection, a small, pale bump will form at the site of the injection.
The second phase of the test takes place 48 to 72 hours later. At that time, your doctor will look at your skin to see how it reacted to the tuberculin. Your skin’s reaction will help your doctor determine if you have been infected with TB. If you wait longer than 72 hours, you will have to start over with a new test and new injection.
Within several hours of the injection, your skin around the site of the injection will start to swell and harden. This bump, or induration as it’s referred to clinically, will also turn red. The size of the induration is used to determine your results. The induration should be measured across the forearm, perpendicular to the axis between your hand and elbow. Several factors affect how the test is interpreted.
|Size of induration||Result|
|less than 5 mm||negative for TB|
|at least 5 mm||positive if: |
• you’ve had recent contact with someone with TB
• you are HIV-positive
• you have had an organ transplant
• you are taking immunosuppressants
• you have previously had TB
|at least 10 mm||positive if: |
• you recently immigrated from a country with a high incidence of TB
• you live in a high-risk environment
• you work in a hospital, medical laboratory, or other high-risk setting
• you’re a child under the age of 4
• you’ve used injected drugs
|15 mm or more||positive|
An induration of less than 5 millimeters (mm) is considered a negative test result. If you have symptoms or you know you’ve been exposed to someone with TB, you may be advised to get another test later on.
If the induration is at least 5 mm, it will be considered positive in people who:
- have had recent contact with a person with TB
- are HIV-positive
- have had an organ transplant
If you are taking immunosuppressant medications or you previously had TB, a 5 mm induration may also be interpreted as a positive test.
An induration of at least 10 mm may be considered a positive test if you’re a recent immigrant from a country with a high prevalence of TB, or you live in a high-risk environment like a nursing home. The same is true if you work in a hospital, medical laboratory, or other high-risk setting, or if you’re a child under the age of 4. A 10 mm induration may also be considered positive in people who use injected drugs.
An induration of 15 mm or more is considered positive in anyone, even if you don’t think you’ve been exposed to anyone with TB.
What does a positive test mean?
If you have a positive test result and you have symptoms or are considered at high risk of TB exposure, you’ll likely be prescribed medications to clear up the infection and relieve symptoms.
If you are low-risk and have a positive test, your doctor may recommend a TB blood test to confirm the diagnosis. The TB skin test is less accurate than the blood test, so you could have a positive skin test and a negative blood test.
If you recently received a bacillus Calmette-Guerin (BCG) vaccine, which is used to reduce a person’s risk for developing TB, you may have a false-positive skin test result. Other reasons for a false-positive result include improper administration of the test, inaccurate interpretation of your test results, or infection with nontuberculous mycobacteria.
You can also get a false-negative result, meaning the test is negative but you are actually infected with TB. Again, incorrect administration of the test or interpretation of the result could lead to a false-negative test result. If you were exposed to TB in the past few weeks you may not yet test positive for TB. Infants, even if they have TB, may not always have a positive skin test.
If a false-negative result appears, but your risk of TB exposure or your symptoms suggest it’s likely that you have the infection, a second skin test can be done right away. A blood test can also be done any time.
You will only have symptoms if you have active TB disease. Having only the TB infection will not produce any noticeable symptoms.
One of the most common symptoms of TB is a cough that won’t go away. You may also cough up blood. Other symptoms include:
These symptoms can occur with many other conditions, so it’s important to get tested. Even a negative test is helpful because it can rule out TB and help your doctor find a cause for your symptoms.
A positive skin test will usually be followed by a chest X-ray. Your doctor will look for white spots that indicate areas where your immune system is responding to bacteria. There may be other changes in your lungs caused by TB disease. Your doctor may decide to use a computed tomography (CT) scan instead of a chest X-ray because a CT scan can provide greater detail of your lungs.
If the images indicate TB is present, your doctor may also order tests on your sputum, which is the mucus produced when you cough. A lab test can help identify the type of TB bacteria causing the infection. This helps doctors decide which medicine to prescribe.
TB is treatable. If you have TB, follow your doctor’s recommendations to improve your odds for a full recovery.