Tuberculosis (TB) is a deadly bacterial illness that kills more than a million people every year. It is one of the leading causes of death from infection globally. TB is responsible for more than $22 billion annually worldwide, and multidrug-resistant TB is a significant public health threat.
Because of the significant health burden associated with TB, it is important to screen high-risk populations for this contagious illness. The sooner it is identified, the sooner it can be treated for the health of the infected person and the prevention of spread.
Fortunately, there are easy testing options for TB. This article will guide you through TB testing options, help you interpret your results, and help you know what to do next.
Your health could hinge on the outcome of a simple bump. How can you interpret it with confidence?
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TB Test Basics
TB testing is part of a global effort to combat this deadly bacterial illness and has been estimated to have saved 79 million lives since 2000.
What Is a TB Test?
Not everyone with TB has symptoms. Certain groups of people should be screened for TB periodically to detect inactive or latent TB. Both active and latent TB are treatable. Treating latent TB may help prevent you from developing active disease.
Early detection of TB is important for preventing the spread of disease. Systematic screening with TB testing benefits both the individual and the larger community and helps to decrease the burden of this disease.
Why Are TB Tests Necessary?
The World Health Organization (WHO) recommends TB testing to improve treatment outcomes and prevent transmission of Mycobacterium tuberculosis, the bacteria responsible for TB infections.
Systematic screening of high-risk populations reduces the prevalence of TB and helps prevent widespread disease.
High-risk populations who should be screened for TB include:
- Anyone with symptoms of TB, including coughing up blood, unexplained weight loss, or persistent cough
- People who have spent time with someone who has active TB
- People who are from or regularly travel to countries where TB rates are higher.
- People who do or have lived in communal settings with higher risk, such as homeless shelters or jails
- People who work in higher-risk environments such as hospitals, correctional facilities, nursing homes, or homeless shelters
- People who have conditions that make them at higher risk for developing active TB infection if exposed
TB testing is also required for some employment, school, or travel circumstances and frequently for immigration health screening.
Types of TB Tests
Two primary forms of TB testing are available, as well as supplementary tests used in specific situations.
Tuberculin Skin Test (TST)
The tuberculin skin test (TST) is also known as a PPD (purified protein derivative) test.
The PPD test involves injecting a small amount of diluted tuberculin-purified protein under the skin and then examining the injection 48-72 hours later to determine if there is a reaction.
Immune systems exposed to Mycobacterium tuberculosis will react to the protein, causing a raised red skin reaction. The PPD is positive 2 to 8 weeks after infection. It can be positive in the case of both latent and active infection.
The PPD test is inexpensive, doesn't require a blood draw, and is easy to perform. Years of clinical research have established clear guidelines for what constitutes a positive PPD test.
On the other hand, the PPD requires training to administer and read. You have to go in at least twice to complete the test (some people have to do the test twice to get cleared). The PPD solution requires refrigeration, and multi-dose vials expire 30 days after initial use.
Because it requires an immune system reaction, other infections, and immunosuppressive medications can cause false negative results. The Bacille Calmette-Guerin (BCG) vaccination or infection with outer forms of mycobacteria (not tuberculosis) can cause false positives.
Interferon-Gamma Release Assays (IGRAs)
For people who can't get the PPD skin test, another form of test can be done through blood work.
TB blood tests are interferon-gamma release assays (IGRAs). Two forms of this test are approved for use in the United States: QuantiFERON®-TB Gold Plus and T-SPOT®.
TB blood tests can be used for people who are likely to have a false positive or false negative PPD, such as those who received the BCG vaccine or who have abnormal immune systems.
People who don't have time or cannot reliably return for the second appointment to read the PPD test may also benefit from getting an IGRA.
Supplementary Diagnostic Tests
For people with symptoms of TB, such as a cough or bloody sputum, or those with a positive PPD or IGRA test, a chest X-ray may be necessary.
People who are known to have been exposed to TB may also benefit from a chest X-ray. A chest X-ray cannot diagnose TB alone but can confirm a diagnosis if specific abnormalities are present. Chest X-rays may be falsely normal in cases of a suppressed immune system.
In some cases, a sputum culture may be necessary. Sputum cultures can be grown on a petri dish or can be performed through a process called nucleic acid amplification tests.
Sputum cultures can take one to eight weeks to return, while rapid sputum tests can produce results in 24 hours. Three samples are generally required to rule out TB.
The TB Test Process
TB testing can be a simple process.
Preparing for the Test
There is no special preparation for a TB test. However, your healthcare provider should know about any medications or supplements you are taking and any health conditions you have.
In addition, you should tell your healthcare provider if you have had the BCG vaccine.
Step-by-Step Process
The TST is performed by injecting a small amount of solution under the skin. A small blister forms under the skin that will then absorb.
Leave the bump uncovered and avoid touching it or putting any lotions or cream on it. You can shower normally.
Return to the clinic after 48 to 72 hours to have the test site evaluated. You may feel a pinch when the purified protein is injected under your skin.
The IGRA test is performed by a blood draw. A phlebotomist or other healthcare professional applies a tourniquet and draws blood from a vein in the arm.
This may cause a small amount of pain when the needle enters the vein. Generally, no repeat testing is needed.
Interpreting TB Test Results
Once you have your testing, you'll need to interpret the results.
Positive vs Negative Results: Key Indicators
The amount of redness and swelling at the injection site is measured for the TST. Depending on health conditions, what is considered a positive test is classified differently.
Induration of 5 mm or more is positive for patients who have:
- HIV infection
- Recent contact with someone with infectious TB
- Chest x-ray findings suggestive of previous TB
- Organ transplants
- Other immunosuppressive conditions
Induration of 10 mm or more is positive for people who:
- Were born in countries where TB is more common
- Use drugs and alcohol
- Live or work in high-risk settings
- Work in a lab that regularly encounters mycobacteria
- People with medical conditions that put them at increased risk
- People <90% of ideal body weight
- Kids under age 5
- Children exposed to adults who are in a high-risk category
Induration of 15 mm or more is considered positive for people who don't meet the other criteria.
A value above 0.35 IU/mL is a positive result for the IGRA test. If the level is close to 0.35 IU/mL, repeat testing may be required at the guidance of your healthcare provider.
Factors That Influence Results
Several factors can influence the results of testing. Certain immunosuppressive medical conditions may cause false negative testing, while the BCG vaccine can cause a false positive test. Recent exposure to TB may also influence results.
What Happens After a Positive TB Test?
So, what do you do if your PPD or IGRA test is positive? Often, your healthcare provider will start with confirmatory testing with a chest x-ray or sputum culture may be necessary.
If TB bacteria are detected, the laboratory will frequently test for antibiotic resistance.
Latent TB is generally treated with antibiotics. Several medication regimens, varying in length from three to nine months, can be used.
Some medications can interfere with other drugs, so your healthcare provider needs to know all medications you may be taking. Alcohol may also interfere with medications, so don't drink while on medications.
Active TB is treated with antibiotic regimens, which vary based on the bacteria's antibiotic resistance. It is vital to take all prescribed antibiotics to avoid developing drug-resistant bacteria.
Like with latent TB, the antibiotics can interfere with other medications or alcohol. People with active TB often need to be in a negative pressure isolation room if they are hospitalized.
Allergic Reactions and False Positives
While rare, some people may react to the protein used to test for TB.
Allergic Reactions
The rate of severe allergic reactions to PPD is about 1 in 1 million. Reactions may vary from local skin reactions to anaphylaxis, which is a medical emergency. Anaphylactic reactions can be treated the same as other types of anaphylactic reactions.
Understanding False Positives and Negatives
False positives and false negatives can occur. The most common reasons for false-positive TST are the BCG vaccine or an infection with mycobacteria other than M. tuberculosis.
Other common reasons for a false positive are incorrect interpretation or administration of the test.
On the other hand, false-negative results can result from several factors. Some of these factors include:
- Weakened immune system, resulting in the inability of the skin to react
- TB infection within 8-10 weeks of exposure
- TB infection that was many years ago
- Age less than six months
- Recent vaccination with a live virus vaccines such as smallpox or measles
- Widespread TB infection
- Certain viral illnesses
- Incorrect administration or interpretation of the test
Occasionally, the results can be inconclusive (neither positive nor negative). Inconclusive tests are generally repeated or retested with an alternative test when appropriate.
If a definitive answer cannot be determined, your healthcare provider may decide to treat you based on your symptoms and other risk factors.
Free or Affordable TB Testing Options
There are multiple options for free or low-cost TB testing.
- Public and Community Health Clinics: Many public and community health clinics offer low-cost screening for TB. A search engine may help you locate a clinic in your area.
- Government and Nonprofit Programs: The Centers for Disease Control and Prevention (CDC) and other healthcare organizations, such as the WHO, have launched initiatives to eliminate TB. The CDC funds testing and treatment options managed through health departments as part of these initiatives.
Tips for Finding Free Testing Services
To find free or low-cost testing options near you, visit the website of your local health department or use a search engine. Many health departments also have hotline phone numbers for more information.
Post-Test Actions and Prevention Tips
After your test, your healthcare provider is the best resource for next steps.
Interpreting Results with Your Doctor
Your doctor will review the test results. If negative, ask how often you should be screened or whether you have risk factors for false negative results.
If your testing is positive, your healthcare provider will likely recommend additional testing. TST and IGRA testing are screening tests that cannot determine whether the disease is active or latent.
Additional testing, such as sputum cultures or chest X-rays, will often help clarify the diagnosis.
Preventing TB Transmission
If you've been diagnosed with TB, you are probably wondering how to avoid spreading the infection to your loved ones. The first step in preventing the spread of TB is to take all prescribed medications as directed.
Follow up regularly with your healthcare provider to ensure no changes are necessary. Your healthcare provider will direct you regarding any isolation requirements. You should not go out until your healthcare provider says it is safe.
You should also regularly air out your room and use precautions such as wearing a mask and covering your mouth when you cough.
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Key Takeaways
- TB is an infectious disease that is easily spread.
- TB can be active or latent.
- Regular TB testing is recommended for anyone in a high-risk population.
- Skin or blood testing for TB can be done for screening.
- If you have a positive screening test, additional testing may be recommended.
- Regular screening tests for TB can help prevent the spread of this highly infectious illness.
- Understanding your TB test is your first step toward safeguarding your health and the health of your community.
- Talk to your healthcare provider for more information about your testing options.
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