A tuberculosis skin test can help a doctor determine if you need treatment. A positive test looks different for everyone, depending on your risk factors.
TB is a highly contagious disease that primarily affects your lungs and respiratory system. It is caused by an infection of bacteria called Mycobacterium tuberculosis (Mtb).
Exposure to Mtb can result in either active TB disease or a latent TB infection. Latent TB means you have an infection but no signs or symptoms. Latent infection can eventually reactivate as a secondary TB infection. If this happens, you may experience symptoms such as:
Doctors may treat latent TB to prevent future active disease. According to the Centers for Disease Control and Prevention (CDC),
A doctor may treat active TB with a combination of medications for about
A blood or skin test can help healthcare professionals diagnose TB. If your skin test is positive or if you may have a false negative, a doctor may administer a blood TB test.
The results of these tests can determine if you should receive treatment and with what type of medication.
A TB skin test, also called a Mantoux tuberculin skin test (TST), is usually well tolerated and rarely causes negative reactions, according to the
The test has two parts:
A doctor injects a tiny amount of tuberculin under the skin, usually in the forearm. Tuberculin is a sterile extract of a purified protein derivative (PPD) made from the bacteria that cause TB.
A small, pale bump will form at the injection site.
A doctor will check your skin
You must begin a new test if you wait more than 72 hours for a doctor to check your skin.
If your TB test is positive and you have symptoms or are considered at high risk of TB exposure, a doctor will likely prescribe medication to clear the infection and relieve symptoms.
If you’re at low risk of TB exposure but have a positive test, a doctor may recommend you take a TB blood test to confirm the diagnosis. The blood test is more accurate than the skin test, but like the skin test, it can’t differentiate between active TB disease and a latent TB infection.
The doctor may order a chest X-ray or CT scan to determine if you have active TB disease or latent TB, as well as a sputum test to identify the bacteria in your body and choose the most effective medication.
If you have an Mtb infection, the skin around the injection site should start to swell and harden within
This bump, or induration, may also change color. But the induration’s size, not the color, will determine your results.
A doctor or healthcare professional will measure the induration across your forearm, perpendicular to the axis between your hand and elbow. To determine whether your test is positive or negative, the doctor will consider
|Size of induration||Result|
|less than 5 mm||negative|
|at least 5 mm||positive if you:|
• have had recent contact with someone with TB
• are HIV-positive
• have had an organ transplant
• are taking immunosuppressant medication
• have previously had TB
|at least 10 mm||positive if you:|
• recently immigrated from a country with a high incidence of TB
• live in a high risk environment
• work in a hospital, medical laboratory, or other high risk setting
• are under age 4
• have used injected drugs
|15 mm or more||positive|
An induration of less than 5 millimeters (mm) is a negative result. If you have symptoms or know you’ve been exposed to someone with TB, a doctor may recommend you get another test.
If the induration is at least 5 mm, it is considered positive in people who:
- have had recent contact with a person with TB
- are HIV-positive
- have had an organ transplant
A doctor may interpret a 5 mm induration as a positive result if you are taking immunosuppressant medications or previously had TB.
An induration of at least 10 mm may be considered a positive result if you recently immigrated from a country with a high prevalence of TB.
The same is true if you live in a high risk environment such as a nursing home, or work in a high risk setting such as a hospital or medical laboratory. A 10 mm induration may also be considered positive in children under age 4 or people who use injected drugs.
An induration of 15 mm or more is considered positive in anyone, even people who don’t think they’ve been exposed to TB.
The TB skin test can’t differentiate between active TB disease and latent infection, but symptoms will only appear if you have active TB disease.
If your test is positive and you have symptoms or have a high risk of TB exposure, a doctor will likely have you take medication to clear the infection and relieve symptoms.
If you’re at low risk of TB exposure but have a positive test, the doctor may recommend additional testing, such as a TB blood test.
It’s possible to test positive without having TB. Reasons for a false-positive test include:
- You’ve received the bacillus Calmette-Guérin (BCG) vaccine. Some countries use the BCG vaccine to reduce a person’s risk for developing TB.
- A healthcare professional improperly administered your test.
- A healthcare professional incorrectly interpreted your test results.
- You have an infection with nontuberculous mycobacteria.
False negative result
You can also get a false-negative result. This means the test results are negative, but you have a TB infection. This could be because:
- A healthcare professional improperly administered your test.
- A healthcare professional inaccurately interpreted your test results.
- You’ve had an organ transplant.
- You’ve had a TB infection within the past
8 to 10 weeks.
- You’ve recently received a live-virus measles or smallpox vaccine.
Additionally, infants can sometimes have false-negative skin test results.
If you receive a negative result, but your symptoms or risk of TB exposure suggest you may have an infection, a doctor can immediately administer a second skin test. You can also have a blood test at any time.
Only active TB disease causes symptoms. Latent TB does not.
One of the most common TB symptoms is a cough that lasts longer than 3 weeks. You may also cough up blood. Other symptoms include:
These symptoms can occur with many conditions, so it’s important to get tested to determine whether you have TB.
A negative test result can be helpful, as it can rule out TB and help the doctor find other causes for your symptoms.
If you have a positive skin test, the doctor will likely
The doctor will look for white spots on your lungs that indicate areas where your immune system is responding to bacteria.
The doctor may use a CT scan instead of (or after) a chest X-ray because a CT scan provides more detailed images.
A doctor may also order a sputum test. Sputum is the mucus produced when you cough. A lab test can identify the type of TB bacteria causing the infection, which helps doctors decide which medication to prescribe.
During the intensive treatment phase, a doctor will likely prescribe a combination of four medications that you’ll typically take for 2 months. These medications may include:
During the continuation phase, a doctor will likely have you take a combination of isoniazid and rifampin for 4 months.
Short 3-to-4-month treatment courses are considered safe and effective for treating latent TB.
Longer treatment options that can last up to 9 months may be used if shorter ones are not feasible or not available.
TB is highly contagious but treatable.
A skin test can help determine whether you have TB, but it cannot differentiate between active TB disease and a latent TB infection.
The skin test involves receiving a small injection in your forearm and measuring the small bump, or induration, that appears after 48 to 72 hours. Depending on the size of the induration and your risk of TB exposure, the doctor will determine whether you tested positive or negative for TB.
If you test positive, the doctor will likely prescribe a chest X-ray or CT scan and a series of medication.