TB test: What positive PPD and negative PPD results mean
The TB skin test (TST), which is performed using the Mantoux method, is the most commonly used test in the United States for diagnosing Tuberculosis. There are two types of TB disease: latent and active.
Interferon-gamma release assays (IGRA) or TB blood tests specifically check if you have TB bacteria in your body, but won’t determine if you’re an active or latent type. It’s important to remember that not all latent TB infections will become active.
It’s good to think of Latent TB as being a sleeper cell of a terrorist bacteria called Mycobacterium Tuberculosis. Tuberculosis bacteria get inside your body and immediately go into hiding, living inside the very cells that fight infections – the white cells. They can go undetected for years and they multiply ever so slowly, thus, evading detection from your body’s troops.
This latent TB phase can last days, weeks, months, or years. Latent TB means that you have a TB infection but the bacteria aren’t active and causing symptoms yet. According to the Mayo Clinic, about 2 billion people in the world have latent TB. Although it isn’t an infectious disease, it can turn into active TB at any time, so it’s still important to keep an eye on it.
Active TB means that the bacteria started dividing and growing inside your tissues. Typically, TB bacteria need a lot of oxygen, so they prefer living inside lung tissue where fresh oxygen is always available. Active TB means you’re contagious. Some symptoms of active TB include:
- Coughing that brings up phlegm
- Cough that lasts three or more weeks
- Coughing up blood or blood-tinged sputum
- Chest pain, especially while breathing or coughing
- Unintentional weight loss
- Fever, especially in the evening
- Night sweats
- Loss of appetite
Unchecked, TB can also affect other parts of your body including the kidneys, spine, and brain. It’s important to talk to your doctor immediately after a positive PPD skin test result to get a treatment plan in place.
What does a positive TB test look like?
Indurations (size of skin reaction) that indicate a positive Purified Protein Derivative test looks different for different kinds of people. Your previous health-related issues and your lifestyle can both affect your positive reading. For a healthy person who has a normal immune system, a hard pea-sized swelling develops under the skin. If this induration is greater than or equal to 10 mm, the testing result is positive.
False-negative and false-positive TB skin test
If there’s no induration present at the site of the test, you most likely don’t have TB. However, factors, such as viral infections, vaccinations, certain diseases, or medications, could generate a false-negative or false-positive result for a Tuberculin skin test.
People undergoing chemotherapy treatments, who have the AIDS virus, or who use steroids can experience a false-negative result. Be sure to answer all your doctor’s questions honestly so they can read your test results accurately.
Testing for TB infection may generate a false-positive result in people who got the BCG vaccine when they were younger. Sometimes the PPD is read as positive because of skin inflammation. If blisters are present on that person’s arm, the medical professional needs to determine if it’s an allergic reaction to the protein (hypersensitivity) or a true positive result.
It’s quite common to have redness (Erythema) around the PPD injection site or to have an allergic reaction to the glue in the tape. This glue upon the skin causes inflammation of the skin (contact dermatitis), which may have nothing to do with the underlying PPD injection. A health care worker must have medical knowledge and the right clinical experience to diagnose TB.
Problems with tuberculin testing
Not everyone has a typical immune response. The test gets a little more specific in unique situations. For example, if an induration measures 4 millimeters or more, the USCIS considers it a positive test for purposes of immigration forms. Only an induration is measured for the size, not the redness.
Guide to skin reaction size
An induration of 5 mm can be indicative of TB for people who:
- Have suppressed immune systems
- Are HIV positive
- Have seen changes on chest x-rays that are consistent with the previous TB
- Are recipients of organ transplants or are on steroids, chemotherapy drugs, or TNF-Alpha-blockers
If an induration measures 10 mm or more, it’s positive if:
- You’re a recent immigrant (within 5 years) from a place with a high prevalence of TB
- You have recently spent more than a month in areas with a high prevalence of TB
- You’re a resident or an employee working in a high-risk area (such as a hospital and urgent care staff)
- You use IV drugs
- You had stomach surgery
- You have kidney disease, diabetes, certain cancers, a Body Mass Index of less than 18.5%, and silicosis
- You’re under the age of 5
An induration of 15 mm or more is positive in a low-risk person who would otherwise not need PPD testing to screen for TB.
An important note on positive test results: It’s normal if your induration won’t go away for 3-4 weeks. If it becomes red and starts creating ulcers, consult your doctor.
What is a 2-step PPD test?
A 2-Step PPD test helps eliminate false-negative results. It’s simply two tests done within two months of each other. Repeating the test after a 7-day interval boosts the immune response of an otherwise negative PPD. Most healthcare jobs require a 2-step test as it’s more accurate than a single PPD.
Follow-up tests for TB
Your care doesn’t stop once you receive your PPD test results. If you test positive, there’s no need to take the test again. Once you’ve been exposed to Tuberculosis, the TB skin tests will always come back positive for the rest of your lifetime.
The follow-up checkups you’ll have with your doctor will include medication and symptoms management. TB responds differently to different medications, so getting treatment as soon as possible will help you maintain a healthy lifestyle.
You’ll need to schedule two chest x-rays, one year apart, to monitor your lung function after your initial diagnosis. If the chest x-ray is negative, you may still qualify to get treated for Latent TB.
If your chest x-ray is positive, you need a work-up and treatment for Tuberculosis. This includes seeing a chest specialist and getting three samples of your coughed-up sputum for a test called Sputum for AFBs, which tests for Acid Fast Bacilli. Based on those results, you may need medical treatment as per WHO or CDC (Centers for Disease Control and Prevention) guidelines.
Pregnant women will also need special care, so keep in touch with your doctor as you manage your TB.
On the other hand, a negative result doesn’t mean that you’ll remain negative forever. It’s important to stay away from risk factors, such as alcohol and tobacco smoke, which can speed up the development of tuberculosis. Also, everyone should consider taking an annual PPD test.