Why is a TB test done twice?
Asked by: Miss Marge Hickle | Last update: April 26, 2026Score: 4.8/5 (69 votes)
A two-step TB test (Tuberculin Skin Test) is required for baseline screening, especially for people tested periodically (like healthcare workers), to avoid confusing a "boosted reaction" with a new infection, as the first test can "wake up" an old, dormant immune response, making a later negative test look positive and falsely suggesting recent exposure when it's just a memory. It establishes a true baseline by checking for this booster effect, ensuring that any future positive tests truly signal new TB infection, not just an old one being recognized.
How often should you repeat a TB test?
How often you need a TB test depends on your risk factors, occupation, and local regulations; routine annual testing isn't usually needed unless you're a high-risk healthcare worker or have a known exposure, with testing often occurring upon hire and then based on risk assessment or symptoms, rather than a fixed schedule. The Centers for Disease Control and Prevention (CDC) recommends screening for healthcare personnel at hire and then focusing on symptom evaluation and risk assessment rather than annual tests unless there's documented exposure or transmission.
What if I missed my second TB test?
A patient who does not return within 72 hours will need to be rescheduled for another skin test.
How far apart does a 2 step TB test have to be?
Two-step testing
If the first TB skin test result is negative, a second TB skin test should be done 1 to 3 weeks later.
What happens if you get a TB test twice?
However, if they are tested a second time within up to one year of the first test, they may have a positive reaction. This positive reaction is due to a "boosted" ability to react to the tuberculin solution.
5 Things to Know About TB
How long to wait for the 2nd step in PPD?
Visit 3: 1-3 weeks after Visit 1, the second PPD is placed. Visit 4: 48-72 hours later, the PPD is read. In summary, 2-step PPD testing requires 2 PPD's to be placed, 1-3 weeks apart, with each of those tests read 48-72 hours after placement.
Is the second TB test the same as the first?
The two-step TST
The result of the second TST is the true baseline. Why two steps? The first TST may elicit little response yet stimulate an immune response. The second TST is given to elicit a greater immune response.
What can mess up a TB test?
Factors that can affect the results of a TB skin test include:
- having recent contact with another person with TB.
- working at a medical facility, such as a hospital, care center, or medical lab.
- having TB in the past.
- receiving an organ transplant.
- taking immunosuppressant drugs.
- being HIV positive.
When can you get the 2nd TB test?
The two-step TB skin test can lower the chance that a boosted reaction from an old TB infection will be misinterpreted as a recent infection. If the reaction to the first-step TB skin test is classified as negative, a second-step TB skin test is given one to three weeks after the first test is read.
How often does the CDC recommend TB testing?
Treatment for latent TB infection is highly recommended.
Perform annual screening for symptoms of TB disease. Annually reevaluate the risk and benefits of treatment for latent TB infection. Ensure health care personnel know the symptoms of TB disease that should prompt an immediate evaluation between screening.
How many months is a TB test valid?
Child care employees need to have a TB clearance within 12 months before hire. They need a new clearance if they move to a new center. School and college employees need a TB clearance within 12 months before hire or after their 16th birthday. This clearance does not expire for school employment.
What not to do after a TB test?
After the test, you should avoid using bandages, putting on lotion, or scratching the test area because it may affect the results. You can wash the area with water, but do not wipe or scrub. If the area itches, put an ice cube or cold cloth on it.
Can you get a false TB test?
TB can be wrongly diagnosed
In addition, different organisms share the same antigens, making tests results unreliable. These factors can result in TB disease not being identified or wrongly diagnosed.
How often do I need a 2 step TB test?
If the result is negative, they will repeat a second TST one to four weeks later (no sooner than one week and no later than four weeks). The result of the second TST is the true baseline. A two-step TST needs to be performed only once if properly administered and documented.
Will I lose my job if I have a positive TB test?
No, a positive TB test usually won't make you lose your job, but it requires follow-up (chest X-ray, doctor visit) to rule out active TB; if it's latent TB, you can often keep working, especially with proper treatment, but if you have active, contagious TB, you must stay home until cleared by a doctor to prevent spreading it, with employment status depending on your role (healthcare/high-risk jobs have stricter rules).
When to repeat PPD skin test?
Negative PPD Skin Test
If the patient is at a high risk of developing an active TB infection, a repeat test is recommended if the initial test is negative. Repeat testing should also be performed if the initial test was done within eight weeks of known exposure.
How common is tuberculosis?
About a quarter of the global population is estimated to have been infected with TB bacteria. In general, people with TB infection don't feel sick and are not contagious. About 5–10% of people infected with TB will eventually get symptoms and develop TB disease.
Can I take a shower if I got a TB test?
You can do all your usual activities right after the test, including taking a bath or a shower. After 48 hours, but no more than 72 hours* (2 to 3 days), you will return to your health care provider. They will need to touch your arm and check the test site.
What can mess up a TB skin test?
False positive or negative tuberculin skin test reaction may occur in some patients. This may occur if you have received a vaccine against tuberculosis (eg, BCG) or if you have been exposed to other mycobacteria.
Is it normal to feel sick after a TB test?
Side effects are uncommon from the Mantoux skin test. However, a person who has been exposed to TB germs may occasionally have a sizable reaction, which may cause some mild itching, swelling or irritation. These kinds of reactions should disappear in one to two weeks.
What does your arm look like if you fail a TB test?
Your health care provider MUST check your arm 2 or 3 days after the TB skin test, even if your arm looks OK to you. If you have a reaction to the test, it will look like a raised bump. Your health care provider will measure the size of the reaction. If there is a bump, it will go away in a few weeks.
Why didn't my TB skin test bubble?
If there is no reaction after 2-3 days, you most probably have not been exposed to the TB germ. If there is a reaction, some redness and a small bump will appear on the arm. A measurement of 10 millimetres or more is "positive", and means you may have the TB germ in your body.
What does a passing TB test look like?
What does a positive TB test look like? A positive TB skin test will look like a firm, red bump. Your provider will measure the size of the firmness of the bump. Your provider considers your particular risk factors to determine the results of your test.
What are the first signs of TB infection?
Symptoms of active tuberculosis (TB) disease depend on where the TB germs are growing in the body. Common symptoms of active TB disease include cough, pain in the chest, and coughing up blood or sputum (phlegm).