What is Mantoux test

The Mantoux skin test is also known as Mantoux tuberculin skin test, PPD (purified protein derivative) skin test or tuberculin skin test, is a skin test to detect if you have been infected with tuberculosis bacteria (Mycobacterium tuberculosis). Mantoux skin test or tuberculin skin test is used to screen for tuberculosis infection when someone has potentially been exposed to tuberculosis. The Mantoux skin test or PPD tuberculin skin test is also used as a diagnostic tool when someone is showing symptoms of tuberculosis disease. It is important to detect tuberculosis early so that treatment can be started as soon as possible. Mantoux skin test is conducted by injecting a small amount of testing fluid called tuberculin into the inside of your forearm and measuring the resulting swelling several days later.

The Mantoux skin test is used to detect exposure to tuberculosis bacteria by measuring your immune response to an inactivated or killed version of Mycobacterium tuberculosis.

If you have had prior exposure to the tuberculosis bacteria (Mycobacterium tuberculosis), antibodies are formed and remain in your body. During the Mantoux skin test, the tuberculosis antigen (inactivated or killed version of Mycobacterium tuberculosis) is injected under your skin and if antibodies are present, your body will have an immune response. There will be an area of inflammation at the site of the injection. Reliable administration and reading of the Mantoux skin test requires standardization of procedures, training, supervision, and practice.

A Mantoux skin test detects if a patient has ever been infected with tuberculosis bacteria (Mycobacterium tuberculosis) but does not determine if a patient currently has tuberculosis infection or tuberculosis disease. Further testing is required to confirm or rule-out a diagnosis of tuberculosis disease.

There are two tuberculosis-related conditions.

1. The first is called tuberculosis infection or latent tuberculosis infection. Tuberculosis infection describes a stage of tuberculosis in which a person’s immune system is able to control the infection. Patients with tuberculosis infection or latent tuberculosis infection don’t become ill or spread tuberculosis to others, but may develop tuberculosis disease if left untreated.
2. The second is referred to as tuberculosis disease, active tuberculosis, or reactivation tuberculosis. Tuberculosis disease occurs in 5 to 10% of people with tuberculosis infection. Patients with tuberculosis disease usually develop symptoms of tuberculosis and can spread tuberculosis to others.

A Mantoux skin test (tuberculin skin test) may be performed to screen for tuberculosis infections or to assist in diagnosing tuberculosis disease:

1. Screening for tuberculosis infection: Screening for tuberculosis infection means testing for TB in a patient without symptoms. Screening is often conducted when someone has a high risk of having tuberculosis and would benefit from treatment if a tuberculosis infection is diagnosed.
2. Diagnosing tuberculosis disease: Diagnostic tests are used when a patient has symptoms of tuberculosis. A positive Mantoux skin test (tuberculin skin test) supports a diagnosis of tuberculosis disease. In addition to the results of a Mantoux skin test (tuberculin skin test), doctors consider a patient’s medical history and the results of a physical exam, imaging, and other lab tests to diagnose tuberculosis disease.

A Mantoux skin test (tuberculin skin test) requires 2 visits with a health care provider. On the first visit the tuberculin purified protein derivative (PPD) skin test is placed; on the second visit the health care provider reads the test result. A person given the tuberculin skin test must return within 48 to 72 hours to have a trained health care worker look for a reaction on the arm.

The Mantoux skin test (tuberculin skin test) is performed by injecting 0.1 ml of tuberculin purified protein derivative (PPD) into the inner surface of the forearm. The injection should be made with a tuberculin syringe, with the needle bevel facing upward. The tuberculin skin test is an intradermal injection. When placed correctly, the injection should produce a pale elevation of the skin (a wheal) 6 to 10 mm in diameter.

The Mantoux skin test (tuberculin skin test) reaction should be read between 48 and 72 hours after administration. A patient who does not return within 72 hours will need to be rescheduled for another tuberculin skin test as soon as possible.

The reaction should be measured in millimeters of the induration (palpable, raised, hardened area or swelling). The reader should not measure erythema (redness). The diameter of the indurated area should be measured across the forearm (perpendicular to the long axis).

The result of the tuberculin skin test depends on the size of the raised, hard area or swelling.

• Positive Mantoux skin test (tuberculin skin test): This means the person’s body was infected with tuberculosis bacteria. Additional tests are needed to determine if the person has latent tuberculosis infection or tuberculosis disease. Other tests, such as a chest x-ray and a sample of sputum, are needed to see whether the person has tuberculosis disease.
• Negative Mantoux skin test (tuberculin skin test): This means the person’s body did not react to the test, and that latent tuberculosis infection or tuberculosis disease is not likely.

There is no problem in repeating a tuberculosis skin test. If repeated, the additional test should be placed in a different location on the body (e.g., other arm).

The Mantoux skin test (tuberculin skin test) is the preferred tuberculosis test for children under the age of five.

You may have a positive Mantoux skin test reaction if you:

• Have had tuberculosis before and have been cured.
• Have been exposed to the tuberculosis bacteria and are well or have been immunized for tuberculosis with BCG (Bacille Calmette-Guérin).
• Have tuberculosis.

A negative Mantoux skin test result may actually be incorrect (false negative) if you:

• Are taking medicine to lower their immunity e.g. steroids or chemotherapy drugs.
• Have a viral illness e.g. measles
• Have been vaccinated for measles within the last month.
• Are very sick.

If you have a Mantoux test, you will need to return to your doctor or clinic in two or three days to have the result of the Mantoux tuberculin skin test checked. The reaction can get itchy but scratching the test site may cause an infection, so it is best not to touch it. Bandaids, bandages and ointments can affect the test results, so it is important to keep the skin clear and uncovered (long sleeves and jumpers can be worn). If you get blisters around the spot where the injection was given, do not break them.

You can do all your normal daily activities e.g. playing sports, having a shower, going to work or school. If you know you have had a positive Mantoux test before or has been immunized for tuberculosis (TB) please let the person doing the Mantoux skin test (tuberculin skin test) know. If you have any questions ask the person who is doing the test.

If the Mantoux skin test (tuberculin skin test) is positive then you may need to have a chest X-ray and see a doctor.

Figure 1. Mantoux tuberculin skin test

Figure 2. Tuberculosis test pictures

Figure 3.  Positive tuberculin skin test

What is Latent Tuberculosis Infection and Tuberculosis Disease?

Not everyone infected with tuberculosis bacteria (Mycobacterium tuberculosis) becomes sick. As a result, two TB-related conditions exist: latent tuberculosis infection (LTBI) and tuberculosis disease ¹.

• Latent Tuberculosis Infection (LTBI). Tuberculosis bacteria (Mycobacterium tuberculosis) can live in the body without making you sick. This is called latent tuberculosis infection (LTBI). In most people who breathe in TB bacteria and become infected, the body is able to fight the bacteria to stop them from growing. People with latent tuberculosis infection (LTBI):
  • Have no symptoms
  • Don’t feel sick
  • Can’t spread TB bacteria to others
  • Usually have a positive tuberculin skin test reaction or positive TB blood test
  • May develop tuberculosis disease if they do not receive treatment for latent tuberculosis infection (LTBI)
  • Many people who have latent tuberculosis infection (LTBI) never develop tuberculosis disease. In these people, the tuberculosis bacteria (Mycobacterium tuberculosis) remain inactive for a lifetime without causing disease. But in other people, especially people who have a weak immune system, the bacteria become active, multiply, and cause tuberculosis disease.
• Tuberculosis Disease (active tuberculosis). Tuberculosis bacteria (Mycobacterium tuberculosis) become active if your immune system can’t stop them from growing. When TB bacteria are active (multiplying in your body), this is called tuberculosis disease. People with tuberculosis disease are sick. They may also be able to spread the bacteria to people they spend time with every day.

Many people who have latent tuberculosis infection (LTBI) never develop tuberculosis disease. Some people develop tuberculosis disease soon after becoming infected (within weeks) before their immune system can fight the tuberculosis bacteria (Mycobacterium tuberculosis). Other people may get sick years later when their immune system becomes weak for another reason.

For people whose immune systems are weak, especially those with HIV infection, the risk of developing tuberculosis disease is much higher than for people with normal immune systems.

Table 1. Difference between Latent Tuberculosis Infection (LTBI) and Tuberculosis Disease

A Person with Latent Tuberculosis Infection (LTBI)	A Person with Tuberculosis Disease
Has NO symptoms	Has symptoms that may include: a bad cough that lasts 3 weeks or longer pain in the chest coughing up blood or sputum weakness or fatigue weight loss no appetite chills fever sweating at night
Does not feel sick	Usually feels sick
Cannot spread TB bacteria to others	May spread TB bacteria to others
Usually has a skin test or blood test result indicating TB infection	Usually has a skin test or blood test result indicating TB infection
Has a normal chest x-ray and a negative sputum smear	May have an abnormal chest x-ray, or positive sputum smear or culture
Needs treatment for latent TB infection to prevent TB disease	Needs treatment to treat TB disease

[Source ¹ ]

What does the Mantoux tuberculin skin test measure?

A Mantoux tuberculin skin test measures a person’s immune response to a testing solution that is made from Mycobacterium tuberculosis antigens. Antigens are protein markers that exist on the surface of the bacteria and trigger an immune response.

During a Mantoux tuberculin skin test, the testing solution is injected under the skin of the forearm, which creates an elevated, swollen spot on the surface of the skin. After 48 to 72 hours, a health professional reads the results of a Mantoux tuberculin skin test by measuring the size of the elevated spot. To interpret this test, a doctor considers a person’s risk of tuberculosis infection and the diameter of the swelling, measured in millimeters.

The Mantoux tuberculin skin test is one of two types of tests used to detect TB. The other type of test is a TB blood test called an IGRA TB Test (interferon-gamma release assays). The decision of which type of TB test to use for an individual patient depends on several factors including where the test is conducted, availability, and the cost of each test.

An IGRA (interferon gamma release assay) TB test may be used instead of a Mantoux tuberculin skin test for a number of reasons, including the test setting, the cost of testing, and test availability. Doctors may also recommend an IGRA TB test because this test only requires one visit. However, both tests are acceptable for detecting TB infections.

Who should be tested for tuberculosis infection?

Certain people should be tested for tuberculosis infection because they are at higher risk for being infected with tuberculosis bacteria, including:

• People who have spent time with someone who has tuberculosis disease
• People from a country where tuberculosis disease is common (most countries in Latin America, the Caribbean, Africa, Asia, Eastern Europe, and Russia)
• People who live or work in high-risk settings (for example: correctional facilities, long-term care facilities or nursing homes, and homeless shelters)
• Health-care workers who care for patients at increased risk for tuberculosis disease
• Infants, children and adolescents exposed to adults who are at increased risk for latent tuberculosis infection or tuberculosis disease

Many people who have latent tuberculosis infection never develop tuberculosis disease. But some people who have latent tuberculosis infection are more likely to develop tuberculosis disease than others. Those at high risk for developing tuberculosis disease include:

• People with HIV infection
• People who became infected with tuberculosis bacteria in the last 2 years
• Babies and young children
• People who inject illegal drugs
• People who are sick with other diseases that weaken the immune system
• Elderly people
• People who were not treated correctly for tuberculosis in the past

Tuberculosis tests are generally not needed for people with a low risk of infection with tuberculosis bacteria.

When should I get a Mantoux test?

A Mantoux tuberculin skin test may be recommended to screen a person who is at an increased risk of TB infection. People whose job or living condition puts them at an increased risk of TB infection include those who live or work in group settings where tuberculosis is more common, such as:

• Health care settings
• Correctional facilities
• Homeless shelters
• Nursing homes
• Countries where TB infection is common, including Mexico, India, and China

If a patient is showing symptoms of TB disease, a Mantoux tuberculin skin test may be ordered to assist in making a diagnosis. Symptoms of TB disease include:

• A bad cough that lasts longer than 3 weeks
• Coughing up blood and mucus
• Chest pain
• Fatigue
• Lack of appetite or weight loss
• Fever, chills, or night sweats

Who can receive a Mantoux test?

Most persons can receive a Mantoux test. Mantoux tuberculin skin test is contraindicated only for persons who have had a severe reaction (e.g., necrosis, blistering, anaphylactic shock, or ulcerations) to a previous tuberculin skin test. It is not contraindicated for any other persons, including infants, children, pregnant women, persons who are HIV-infected, or persons who have been vaccinated with Bacille Calmette-Guérin (BCG) vaccine.

BCG (Bacillus Calmette-Guérin) vaccine is a vaccine for tuberculosis (TB) disease. Bacillus Calmette-Guérin (BCG) vaccine contains a live attenuated strain of Mycobacterium bovis. BCG vaccine is not widely used in the United States, but it is often given to infants and small children in other countries where tuberculosis (TB) is common. BCG vaccine does not always protect people from getting tuberculosis (TB).

Are Mantoux test results accurate?

The Mantoux tuberculin skin test is a widely used test. There are known circumstances that can lead to false negative and false positive test results.

There are several factors that can contribute to false positive test results, in which a person has a positive test result despite not having an infection.

Known causes for False Positive test results include:

• Previous vaccination with the bacille Calmette-Guérin (BCG) TB vaccine
• An infection from another type of bacteria from the same family as Mycobacterium tuberculosis

False Negative results, in which a person has a negative test result despite having a TB infection, can occur for several reasons, including:

• Recent vaccination using the live-virus measles or smallpox vaccine
• A recent TB infection acquired within 8 to 10 weeks before a TB skin test
• Testing on infants
• A phenomenon known as anergy, which describes a lack of normal immune response to the test fluid.

Do I need follow-up tests?

In the event of a positive Mantoux tuberculin skin test result, follow-up tests are used to rule out TB disease. Tests used to rule out TB disease include a physical exam, chest x-rays and sputum culture. In some cases, doctors may suggest an IGRA TB test to confirm a positive tuberculin skin test result.

If the result of a Mantoux tuberculin skin test is negative, follow-up testing depends on the patient’s circumstances. For example, if a patient with a compromised immune system is exposed to someone with TB disease, they may be treated for TB even after a negative Mantoux tuberculin skin test. TB testing is then repeated eight weeks after the patient begins treatment. In older adults who were previously infected with TB bacteria, a second tuberculin skin test may be administered after an initial negative test result, known as two-step testing.

If Mantoux tuberculin skin test results are not read by a trained provider within the 48 to 72 hour window, another tuberculin skin test may be ordered.

What are the possible side effects of Mantoux test?

The Mantoux tuberculin skin test is a simple and safe test and side effects are uncommon. The side effects are mainly related to the size of the reaction and include swelling, redness and itchiness that may take a few weeks to clear.

Very strong reactions are uncommon but may result in a painful swelling of several centimeters in size, blistering or ulceration. This will heal with almost no scarring. The effects of the Mantoux tuberculin skin test are localized to the site and more general reactions are extremely rare.

Should I get a Mantoux test if I am pregnant?

You may be tested under your healthcare practitioner’s supervision if there is a need to do so. Since tuberculosis can be passed from mother to child during pregnancy, if you are at an increased risk of contracting tuberculosis, your healthcare practitioner may want you to have a Mantoux tuberculin skin test or interferon gamma release assay (IGRA) blood test done. Either of these is considered safe during pregnancy.

Testing for tuberculosis in BCG-vaccinated people

Many people born outside of the United States have been BCG-vaccinated.

People who were previously vaccinated with BCG may receive a tuberculosis skin test to test for tuberculosis infection. Vaccination with BCG may cause a positive reaction to a tuberculosis skin test. A positive reaction to a tuberculosis skin test may be due to the BCG vaccine itself or due to infection with tuberculosis bacteria.

Tuberculosis blood tests (IGRAs or interferon-gamma release assays), unlike the tuberculosis skin test, are not affected by prior BCG vaccination and are not expected to give a false-positive result in people who have received BCG. Two tuberculosis blood tests are approved by the U.S. Food and Drug Administration (FDA) and are available in the United States: the QuantiFERON®–tuberculosis Gold In-Tube test (QFT-GIT) and the T-SPOT®.tuberculosis test (T-Spot). A health care provider will draw a patient’s blood and send it to a laboratory for analysis and results.

Tuberculosis blood tests are the preferred tuberculosis test for:

• People who have received the tuberculosis vaccine bacille Calmette–Guérin (BCG).
• People who have a difficult time returning for a second appointment to look for a reaction to the tuberculin skin test.

For children under the age of five, the tuberculosis skin test is preferred over tuberculosis blood tests.

A positive tuberculosis skin test or tuberculosis blood test only tells that a person has been infected with tuberculosis bacteria. It does not tell whether the person has latent tuberculosis infection or has progressed to tuberculosis disease. Other tests, such as a chest x-ray and a sample of sputum, are needed to see whether the person has tuberculosis disease.

Can Mantoux tuberculin skin test be given to persons receiving vaccinations?

Vaccination with live viruses may interfere with Mantoux tuberculin skin test reactions. For persons scheduled to receive a tuberculin skin test, testing should be done as follows:

• Either on the same day as vaccination with live-virus vaccine or 4-6 weeks after the administration of the live-virus vaccine
• At least one month after smallpox vaccination

How often can Mantoux test be repeated?

In general, there is no risk associated with repeated Mantoux tuberculin skin test placements. If a person does not return within 48 to 72 hours for a tuberculin skin test reading, a second test can be placed as soon as possible. There is no contraindication to repeating the tuberculin skin test, unless a previous tuberculin skin test was associated with a severe reaction.

What is a boosted reaction?

In some persons who are infected with Mycobacterium tuberculosis, the ability to react to tuberculin may wane over time. When given a tuberculin skin test years after TB infection, these persons may have a false-negative reaction. However, the tuberculin skin test may stimulate the immune system, causing a positive, or boosted reaction to subsequent tests. Giving a second tuberculin skin test after an initial negative tuberculin skin test reaction is called two-step testing.

Why is two-step testing conducted?

Two-step testing is useful for the initial skin testing of adults who are going to be retested periodically, such as health care workers or nursing home residents. This two-step approach can reduce the likelihood that a boosted reaction to a subsequent tuberculin skin test will be misinterpreted as a recent TB infection.

Why is the mantoux test performed?

The mantoux tuberculin skin test is done:

• to find latent tuberculosis in a person who may have been exposed to someone diagnosed with active tuberculosis
• to check if a person has latent tuberculosis infection before they start work at a health care facility
• to check if a person has latent tuberculosis before they travel to a region where there are high rates of tuberculosis
• before the BCG (Bacille Calmette-Guérin) vaccination is given to babies over 6 months of age.

How is the mantoux test performed?

No sample is required. The mantoux tuberculin skin test is performed on a person’s skin. A small amount of Tuberculin or purified protein derivative (PPD) solution that contains Mycobacterium tuberculosis antigens, but not live bacteria, is used to provoke a hypersensitivity skin reaction (a red, raised bump) in those who have been infected by tuberculosis (TB).

A healthcare practitioner will wipe your inner forearm with alcohol and let the skin dry. Using a 1 mL syringe and a tiny needle, the healthcare practitioner will then inject a small amount of Tuberculin (purified protein derivative) solution just under the first layer of the skin, usually on the inside of the left forearm. When done correctly, the injection forms a small bubble of fluid that looks like a blister. The site should be left uncovered and undisturbed. The site must be examined by a healthcare practitioner at 48 and/or 72 hours to see if a local skin reaction has occurred.

How do I take care of the injection site?

• Do not scratch or rub the injection site.
• Leave the area open: do not cover with any dressing, cream or ointment.
• If blisters develop and it causes discomfort, apply a cool compress to the area.
• Continue your normal activities after having the Mantoux tuberculin skin test including showering, swimming and sport.

What if I have a mantoux tuberculin skin test and it is more than 72 hours before I go back to have it evaluated?

In most cases, if you do not return within the designated 48-72 hours, then your test cannot be adequately evaluated and would need to be redone.

How is the mantoux skin test used?

Mantoux tuberculin skin tests are not used as general population screens but are used to screen people who are at high risk for tuberculosis exposure, such as:

• People with diseases or conditions that weaken the immune system, such as HIV or AIDS, which makes them more vulnerable to a tuberculosis infection
• Those who are in confined living conditions such as homeless shelters, migrant farm camps, nursing homes, schools, and correctional facilities
• Healthcare workers and others whose occupations bring them in close contact with those who may have active tuberculosis
• People who have signs and symptoms consistent with active tuberculosis
• Those who come from or have lived for a period of time in a foreign country where tuberculosis may be more common
• Those who inject illegal drugs

Either a mantoux tuberculin skin test or a blood test called an interferon gamma release assay (IGRA) may be performed to screen for tuberculosis, but in most cases the interferon gamma release assay is now preferred over the mantoux tuberculin skin test. The interferon gamma release assay test measures the release of a substance called gamma interferon by white blood cells in a sample of blood when the cells are exposed to specific tuberculosis antigens.

Recommendations from the Centers for Disease Control and Prevention (CDC), the Infectious Diseases Society, of America, and the American Thoracic Society list a preference for an interferon gamma release assay (IGRA) blood test when:

• Someone is unlikely to return to have their Mantoux tuberculin skin test evaluated
• The person is 5 years or older
• Is likely to be infected with tuberculosis
• Has a low to intermediate risk of progressing to active tuberculosis
• A test for latent tuberculosis infection is warranted
• And/or if the person being tested has received the BCG (Bacille Calmette-Guérin) vaccine that might interfere with the interpretation of a mantoux tuberculin skin test. Bacille Calmette-Guérin (BCG) is not used as a vaccine in the United States, but it is often routinely administered in countries with a higher incidence of tuberculosis and it is used in the U.S. as a treatment for some cancers.

The Mantoux tuberculin skin test test is considered an acceptable alternative to the interferon gamma release assay if the interferon gamma release assay is not available or is considered too costly or burdensome.

These same agencies do NOT recommend testing people who are not likely to be infected with tuberculosis, or those who are considered at a low risk for tuberculosis infection and disease progression.

However, when testing for latent tuberculosis infection is required, such as for employment or a legal requirement, they suggest:

• An interferon gamma release assay rather than a Mantoux tuberculin skin test when someone is 5 years or older
• A second test, either an interferon gamma release assay or mantoux tuberculin skin test, when the initial test is positive and to only consider the person positive if both tests are positive
• A Mantoux tuberculin skin test rather than an interferon gamma release assay when a person is less than 5 years old, healthy, and the tuberculosis screening is warranted

If there is a risk that the first Mantoux tuberculin skin test is a false-negative reaction, a second skin test can be given so that the mantoux tuberculin skin test can have the chance to stimulate the immune system, causing a positive or boosted reaction in the second test.

When is mantoux tuberculin skin test ordered?

Mantoux tuberculin skin tests are ordered less frequently than interferon gamma release assay (IGRA) tests for tuberculosis screening but may still be ordered:

• On a yearly basis for those who are part of a high-risk group, either because they have a disease that weakens their immune system or because they work or live around others in high-risk groups
• Prior to a person joining an at-risk population, such as healthcare workers
• When someone has been in close contact with someone who has an active case of tuberculosis; this would be done a few weeks after a suspected exposure as it usually takes about 6 weeks after contact and initial infection before a positive result would be detected
• When an individual has lived for an extended time in a country where tuberculosis is common
• When a person has signs and symptoms of tuberculosis, such as a chronic cough that produces phlegm or sputum, sometimes with bloody streaks, fever, chills, night sweats, and unexplained weight loss

A tuberculosis skin test should not be done when a person has had a previous positive reaction. Once positive, a Mantoux tuberculin skin test reaction will usually remain positive for life and the skin reaction to subsequent tuberculin skin tests may become increasingly severe.

What does Mantoux test positive means?

Mantoux tuberculin skin test interpretation depends on two factors:

1. The measurement of the diameter of the swelling in millimeters.
2. Person’s risk of being infected with TB (tuberculosis) and of progression to disease if infected.

These criteria mean that the size of an induration required for a positive test result depends on other aspects of a patient’s risk profile. For example, an induration of 5 or more millimeters is considered positive in people living with HIV or who have imaging test results suggestive of TB disease. An induration of 15 or more millimeters is required for a positive diagnosis in patients with no known risk factors for TB.

A healthcare practitioner will interpret a tuberculin skin test result by looking at the injection site on the person’s forearm at 48 or 72 hours (in most cases). A positive result will form a red and swollen raised circle at the site of the injection. The size (diameter) of the swollen raised circle determines whether exposure to tuberculosis has occurred. The size that is considered positive varies with the health status and age of the individual. Even when infected, children, the elderly, and people who are severely immune compromised (such as those with AIDS) may have smaller, delayed, or even negative reactions to the Mantoux tuberculin skin test.

A positive Mantoux tuberculin skin test or interferon gamma release assay (IGRA) test result means that the person is likely to have been exposed to tuberculosis and the person may have a latent or active tuberculosis infection. If a healthcare practitioner suspects that someone has active tuberculosis, a history and physical examination and other tests, such as chest X-rays and acid-fast bacillus (AFB) laboratory testing, are used to confirm the diagnosis. Some people with a latent tuberculosis will be offered medication to prevent them developing active tuberculosis. If your test is positive you may be advised not to have the test repeated.

Positive Mantoux tuberculin skin test results are also commonly seen in those who have received a BCG (Bacille Calmette-Guérin) vaccination. Interferon gamma release assay (IGRA) results are not affected by BCG. A person should generally wait 4-6 weeks to do a Mantoux skin test after having had a vaccination with a live-virus vaccine.

Occasionally, a person infected with or exposed to other Mycobacterium species, for example Mycobacterium kansasii, will give a false-positive Mantoux tuberculin skin test result for tuberculosis. Positive results must be followed up by other tests such as chest X-rays to look for signs of active tuberculosis disease. If active tuberculosis disease is suspected, acid-fast bacillus (AFB) testing including smears and cultures and sensitivity testing, may be used to confirm the diagnosis and determine the drug susceptibility for the Mycobacterium tuberculosis infecting the person.

Mantoux skin test positive

An induration of 5 or more millimeters is considered positive in:

• HIV-infected persons
• A recent contact of a person with tuberculosis disease
• Persons with fibrotic changes on chest radiograph consistent with prior tuberculosis
• Patients with organ transplants
• Persons who are immunosuppressed for other reasons (e.g., taking the equivalent of >15 mg/day of prednisone for 1 month or longer, taking TNF-a antagonists)

An induration of 10 or more millimeters is considered positive in:

• Recent immigrants (< 5 years) from high-prevalence countries
• Injection drug users
• Residents and employees of high-risk congregate settings
• Mycobacteriology laboratory personnel
• Persons with clinical conditions that place them at high risk
• Children < 4 years of age
• Infants, children, and adolescents exposed to adults in high-risk categories

An induration of 15 or more millimeters is considered positive in any person, including persons with no known risk factors for tuberculosis (TB). However, targeted skin testing programs should only be conducted among high-risk groups.

False-Positive reactions

Some persons may react to the tuberculin skin test even though they are not infected with Mycobacterium tuberculosis (TB). The causes of these false-positive reactions may include, but are not limited to, the following:

• Infection with nontuberculosis mycobacteria
• Previous BCG (Bacille Calmette-Guérin) vaccination
• Incorrect method of tuberculin skin test administration
• Incorrect interpretation of reaction
• Incorrect bottle of antigen used

False-Negative reactions

Some persons may not react to the tuberculin skin test even though they are infected with Mycobacterium tuberculosis. The reasons for these false-negative reactions may include, but are not limited to, the following:

• Cutaneous anergy (anergy is the inability to react to skin tests because of a weakened immune system)
• Recent TB infection (within 8-10 weeks of exposure)
• Very old TB infection (many years)
• Very young age (less than 6 months old)
• Recent live-virus vaccination (e.g., measles and smallpox)
• Overwhelming TB disease
• Some viral illnesses (e.g., measles and chicken pox)
• Incorrect method of tuberculin skin test administration
• Incorrect interpretation of reaction

What does Mantoux test negative result means?

A negative result means that it is likely that the person tested does not have a tuberculosis infection. However, it does not entirely rule out tuberculosis. It may mean that the person’s immune system has not responded to the antigen in the Mantoux tuberculin skin test or that it is too early to detect exposure. It takes about 6 weeks after infection before a person demonstrates a positive reaction to tuberculosis screening tests. If suspicion of tuberculosis remains high and a healthcare practitioner wants to confirm a negative or indeterminate result, the practitioner may repeat the Mantoux tuberculin skin test in 8 to 12 weeks or do an interferon gamma release assay (IGRA) as an alternate follow-up test.

A negative mantoux tuberculin skin test may cause mild itching or discomfort at the injection site.

A person may not respond to a mantoux tuberculin skin test (even with tuberculosis exposure) if the person has had a recent viral infection, a “live” vaccine (such as measles, mumps, chickenpox, influenza), or has overwhelming tuberculosis, another bacterial infection, or is taking immune suppressive drugs such as corticosteroids.

What is acid-fast bacillus (AFB) testing?

Most samples that are submitted for acid-fast bacilli (AFB) testing are collected because the healthcare practitioner suspects that a person has tuberculosis (tuberculosis), a lung infection caused by Mycobacterium tuberculosis. Mycobacteria are called acid-fast bacilli because they are a group of rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure where the bacteria retain the color of the stain after an acid wash (acid-fast). acid-fast bacillus laboratory tests detect the bacteria in a person’s sample and help identify an infection caused by acid-fast bacillus.

There are several types of acid-fast bacillus that may be detected with this testing; however, the most common and medically important ones are members of the genus Mycobacterium. Mycobacterium tuberculosis is one of the most prevalent and infectious species of mycobacteria.

Another group of mycobacteria referred to as nontuberculous mycobacteria can also cause infections. However, only a few of the more than 60 species of mycobacteria that have been identified cause infections in humans. Some examples include Mycobacterium avium-intracellulare complex, which can cause lung infection and disseminated disease in people with weakened immune systems. In addition to tuberculosis, acid-fast bacillus testing can help identify infections caused by these nontuberculous mycobacteria.

How is the acid-fast bacilli (AFB) test sample collected for testing?

Sputum is the most commonly tested sample. Sputum is phlegm, thick mucus that is coughed up from the lungs. Preferably, three early morning samples obtained by deep cough are collected on consecutive days in individual sterile cups to increase the likelihood of detecting the bacteria.

If a person is unable to produce sputum, a healthcare practitioner may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows the healthcare practitioner to look at and collect samples from the bronchi and bronchioles. Once a local anesthetic has been sprayed onto the patient’s upper airway, the practitioner can insert a tube into the bronchi and smaller bronchioles and aspirate fluid samples for testing. Sometimes, the healthcare practitioner will introduce a small amount of saline through the tubing and into the bronchi and then aspirate it to collect a bronchial washing.

Since young children cannot produce a sputum sample, gastric washings/aspirates may be collected. This involves introducing saline into the stomach through a tube, followed by fluid aspiration.

If the healthcare practitioners suspect tuberculosis is present outside of the lungs (extrapulmonary), they may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if the practitioner suspects tuberculosis has infected the kidneys. A needle may be used to collect fluid from joints or from other body cavities, such as the pericardium or abdomen. Occasionally, the practitioner may collect a sample of cerebrospinal fluid (CSF) or perform a minor surgical procedure to obtain a tissue biopsy.

1. Latent TB Infection and TB Disease. https://www.cdc.gov/tb/topic/basics/tbinfectiondisease.htm[↩][↩]