A negative TB test means there is no bump or only a very small bump (less than 5mm) at the spot where the test fluid was put into your skin. This good news usually shows you don’t have TB germs in your body.
Getting a TB test can feel a little scary. You might wonder what the results will show and what they mean for your health. This guide will help you know exactly what to look for after your TB test, understand what negative results really mean, and learn when you might need more testing.
We’ll walk you through what a negative test looks like, how doctors read your results, and why some people get false results. You’ll also find out what to do next and when you should talk to your doctor.
What Does the TB Test Check For?
TB stands for tuberculosis. It’s a sickness caused by tiny germs called bacteria. These germs can live in your body without making you feel sick right away. That’s called latent TB infection. Or the germs can make you very sick, which is called active TB disease.
The TB skin test checks if you have TB germs in your body. It doesn’t tell you if the germs are making you sick today. It just shows if the germs are there at all.
When a doctor gives you a TB test, they put a tiny bit of testing fluid under your skin. This fluid contains special protein from TB bacteria. If your body has seen TB bacteria before, your immune system reacts by creating a firm bump at the test spot.
The test works because your body remembers things it has fought before. Think of it like this: if you’ve ever had chicken pox, your body remembers it. The TB test works the same way.
How to Read Your TB Test Results
You must go back to your doctor 48 to 72 hours after getting the TB test for an official reading. This timing is very important. If you wait too long, the test won’t work right and you’ll need to start over.
What Your Doctor Looks For
Your doctor will measure any firm, raised area at the test spot. This raised area is called induration. They don’t measure the red area around it – just the hard bump itself.
Healthcare providers measure the induration across your forearm, not along the length of your arm. They use a special ruler to get the exact size in millimeters.
Don’t try to read the test yourself. Even if you think you know what you’re looking at, only trained healthcare workers can give you the right answer. They know exactly what to measure and how to measure it.
The 48 to 72 Hour Window
This time window isn’t random. It takes 2 to 8 weeks after you first get infected with TB bacteria for your immune system to react to the test. The bump shows up fastest between 48 and 72 hours after the test is given.
If you miss your appointment and come back later, the bump might have already gone down. Then the doctor can’t read your test. You’ll have to get a new test and start over.
What a Negative TB Test Looks Like
When you have a negative TB skin test, there is no bump or only a very small bump at the spot where the fluid was put in. Your arm looks almost normal, or there’s just a tiny raised spot that measures less than 5mm.
The Visual Check
Look at the injection site on your inner forearm. You might see a little redness. That’s okay – redness doesn’t matter for the test results. What matters is whether there’s a hard, raised bump you can feel.
If your test is negative, you won’t feel much of anything. Maybe a tiny raised spot, but nothing big or obvious. The skin should be mostly flat and soft.
Size Matters
For most people with no TB risk factors, any induration less than 15mm is considered negative. But the exact cutoff depends on your health history.
People with certain health problems need different measurements:
- High-risk people (like those with HIV or who live with someone who has TB): A bump of 5mm or more is positive
- Medium-risk people (like healthcare workers or people from countries where TB is common): A bump of 10mm or more is positive
- Low-risk people (healthy adults with no TB risk factors): A bump needs to be 15mm or more to be positive
Your doctor knows which group you’re in based on your health history. They’ll tell you if your test is negative or positive.
What Does a Negative Result Mean?
A negative skin test result usually means inactive TB or active TB disease is unlikely. This is good news for most people. It means:
- You probably don’t have TB germs in your body
- You can’t spread TB to other people
- You likely don’t need treatment right now
- You can move forward with work, school, or whatever you needed the test for
But remember – “probably” is the key word here. Tests aren’t perfect. We’ll talk more about false results in a minute.
When a Negative Result Still Needs More Testing
Sometimes a negative test isn’t the end of the story. Your healthcare provider may do more tests if you have symptoms of active TB disease like coughing, chest pain, fever, weight loss, or tiredness, if you have HIV, or if you were recently exposed to TB germs.
Your doctor might also want to do more testing if:
- You spent time with someone who has active TB in the past few weeks
- You’re from a country where TB is very common
- You work in a hospital, prison, homeless shelter, or nursing home
- You have a weak immune system from illness or medicine
In these cases, the doctor might suggest a blood test for TB. This test is more accurate than the skin test for some people. Or they might want to test you again in a few weeks.
How Doctors Measure Your TB Test
The measurement process is very specific. Healthcare workers measure the induration in millimeters across the forearm, perpendicular to the long axis of the arm.
Here’s what happens during your reading appointment:
- The doctor or nurse finds the test spot on your arm
- They feel the area to find any hard, raised bump
- They measure only the firm bump – not the red area
- They write down the size in millimeters
- They compare the size to your risk factors
- They tell you if your test is positive or negative
Why Professional Reading Matters
The Mantoux test is technically difficult to read, so false readings can happen if the person reading it doesn’t have enough skill. That’s why you need a trained professional to read your test.
Some people try to read their own test at home. This is a bad idea. You might:
- Measure the wrong part of your arm
- Include redness instead of just the hard bump
- Get the size wrong
- Not know your risk factors well enough
- Worry about nothing or miss something important
Always let your doctor or nurse read your test.
False Negative TB Test Results
Sometimes people have TB germs in their body but still get a negative test result. This is called a false negative. It doesn’t happen often, but it can happen.
Common Causes of False Negatives
False-negative results may occur because of anergy (inability to react to a TB skin test because of a weakened immune system), recent TB infection within the past 8 to 10 weeks, or certain life-threatening illnesses.
Let’s break down these causes:
Recent Infection: If you were just exposed to TB in the past few weeks, your body might not react to the test yet. Your immune system needs time to “learn” about the TB bacteria before it can respond.
Weak Immune System: If your immune system isn’t working well, it can’t create the bump that shows a positive test. This happens with:
- HIV infection
- Cancer treatment
- Organ transplants
- Medicines that suppress your immune system
- Very young age (babies)
- Very old age
Serious Illnesses: Many life-threatening illnesses are linked with false-negative TB skin test results, such as protein-calorie malnutrition, advanced cancer, and TB disease itself, notably miliary TB and TB meningitis.
Other Factors: Live virus vaccines (like MMR or Sabin vaccine) should not be given within 3 weeks of a TB skin test. These vaccines can affect your test results.
What Happens If You Have a False Negative
If your doctor thinks you might have a false negative, they won’t just send you home. They’ll probably:
- Ask about your symptoms
- Do a chest X-ray
- Order a TB blood test
- Test you again in 8 to 10 weeks
The blood test for TB is called an IGRA (interferon-gamma release assay). False-negative TB blood test results may occur if the TB infection occurred within 8 weeks of testing because it can take 2 to 8 weeks after being infected with TB bacteria for the body’s immune system to mount a response detectable by the test.
Taking Care of Your Arm After the Test
After you get your TB test, you need to take care of the test spot until your reading appointment. Here’s what to do:
Do’s and Don’ts
DON’T:
- Cover the spot with a bandage or tape
- Rub or scratch the area
- Put lotion or cream on it
- Worry too much if it itches a little
DO:
- Wash your arm and dry it gently, put a cold cloth on it if the spot itches, and be careful not to rub or scratch it
- Keep the area clean
- Watch for any severe reactions (very rare)
- Remember your follow-up appointment
Most people don’t have any problems after a TB test. The injection site might feel a little sore or look a bit red. This is normal and will go away.
When to Worry About Your TB Test
Most TB tests go smoothly. But sometimes you should call your doctor before your scheduled reading appointment.
Call your doctor right away if you have:
- Very bad swelling at the test spot
- Heavy blistering or “weeping” skin
- Signs of an allergic reaction (trouble breathing, hives all over your body, or your face swelling up)
- A fever after the test
- Severe pain at the test spot
Severe reactions include anaphylaxis (extremely rare), severe swelling, heavy blistering, or “weeping” of the skin. These reactions are very uncommon. Most people just have a little redness or soreness.
If you’ve ever had a bad reaction to a TB test before, tell your doctor before getting another one. You might need the blood test instead.
TB Blood Tests as an Alternative
Some people get a TB blood test instead of a skin test. The blood test is also called an IGRA. TB blood tests are the preferred test for people who have received the BCG vaccine, including children.
Who Should Get a Blood Test?
Blood tests work better for:
- People who got the BCG vaccine (common in other countries)
- People who need to get tested often
- People who might not come back for the reading appointment
- Children (in some cases)
- People with certain skin conditions
How Blood Tests Work
TB blood tests measure the immune response to TB proteins in whole blood. A nurse draws a small amount of blood and sends it to a lab. You get your results in one to three days.
The big advantage of blood tests? You only need one visit to the doctor. You don’t have to go back 2 to 3 days later for someone to read your test.
Blood tests give results as positive, negative, or indeterminate (can’t tell). A negative blood test means the same thing as a negative skin test – you probably don’t have TB germs in your body.
Two-Step Testing for Healthcare Workers
Some people need special testing called two-step testing. 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.
Why Two-Step Testing Matters
Here’s the problem it solves: Some people got infected with TB many years ago. Their body fought it off, but they still have the germs (latent TB). Over time, their immune system “forgets” a little bit about the TB.
When they get a TB test, the first test might be negative. But that first test “wakes up” their immune system. If they get tested again a few weeks later, the test might be positive – not because they got newly infected, but because their immune system remembered.
Two-step testing is a strategy used to reduce the likelihood that a boosted reaction will be misinterpreted as a recent infection if the person has to be tested again.
How Two-Step Testing Works
If you need two-step testing:
- You get your first TB skin test
- You go back 48 to 72 hours later for a reading
- If it’s negative, you get a second test 1 to 3 weeks after the first test
- You go back 48 to 72 hours later to get the second test read
- The second result becomes your “baseline” for future testing
This process is common for:
- Healthcare workers
- Nursing home employees
- Prison or jail staff
- Anyone who will get tested regularly
Understanding Your Risk Factors
Not everyone has the same chance of getting TB. Your doctor thinks about your personal risk when they read your test.
High-Risk Groups
You have a higher risk of being exposed to TB germs if you were born in or frequently travel to countries where TB is common, including some countries in Asia, Africa, and Latin America, live or used to live in large group settings where TB is more common such as homeless shelters, prisons, or jails, recently spent time with someone who has active TB disease, or work in places where TB is more likely to spread such as hospitals, homeless shelters, correctional facilities, and nursing homes.
Risk Factors for Developing TB Disease
Even if you have TB germs in your body, you might never get sick. But some things make it more likely that the germs will become active. You have a higher risk of developing TB disease once infected if you have a weaker immune system because of certain medications or health conditions such as diabetes, cancer, or HIV, or became infected with TB germs in the last two years.
Other risk factors include:
- Being very young or very old
- Poor nutrition
- Smoking
- Drinking too much alcohol
- Kidney disease
- Taking steroids for a long time
If you have any of these risk factors, your doctor will be extra careful about reading your test and might want to do more follow-up.
What Happens After a Negative Test
For most people with a negative TB test, that’s the end of the story. You don’t need treatment. You can do whatever you needed the test for – start a new job, go to school, volunteer, or whatever it was.
Do You Need Another Test Later?
That depends on your situation. Some people need regular TB testing:
- Healthcare workers might get tested every year
- Nursing home staff need regular testing
- Prison employees get tested regularly
- People who work with TB patients need frequent testing
If you don’t work in these types of jobs and you’re healthy, you probably won’t need another test unless:
- You’re exposed to someone with active TB
- You develop symptoms of TB
- A new job or school requires it
Long-Term Health
In 2024, the United States reported 10,347 TB cases with a rate of 3.0 cases per 100,000 people. Most people in the U.S. have a very low risk of getting TB.
Keep yourself healthy by:
- Eating well
- Getting enough sleep
- Managing stress
- Staying active
- Not smoking
- Seeing your doctor for regular checkups
If you spend time with someone who has active TB, tell your doctor right away. You’ll need testing and might need medicine to prevent TB.
Special Situations and Concerns
Pregnancy and TB Testing
Pregnant women can safely get TB tests. According to the Minnesota Department of Health, TB tests are safe during pregnancy. If you’re pregnant and need a TB test, don’t worry. It won’t hurt your baby.
If you’re pregnant and have a positive TB test or active TB disease, you’ll need special care. TB can be serious during pregnancy. But with the right treatment, most pregnant women with TB have healthy babies.
Children and TB Testing
Kids can get TB tests just like adults. The reading process is the same. But children under 5 years old often get blood tests instead of skin tests when possible.
Very young children can have false negative results more often than adults. Their immune systems are still developing and might not react to the test correctly.
If your child was exposed to TB, your doctor will watch them closely even if the first test is negative. They might do another test in a few weeks.
BCG Vaccine and TB Testing
The BCG vaccine is a vaccine given to prevent TB in some countries. It’s not used much in the United States, but it’s common in other parts of the world.
A meta-analysis found that the BCG vaccine reduced infections by 19-27% and reduced progression to active tuberculosis by 71%. But here’s the tricky part: the BCG vaccine can make TB skin tests come back positive even if you don’t have TB.
If you got the BCG vaccine:
- Tell your doctor before getting a TB skin test
- You might get a blood test instead (blood tests aren’t affected by BCG)
- BCG vaccination does not induce positive results when TB blood tests are used
Common Questions About Negative TB Tests
Can a negative test turn positive?
Yes, if you get exposed to TB after your test. That’s why some people need regular testing. A negative test today doesn’t protect you from getting TB in the future.
Should I get tested again if I feel fine?
Not usually. If you had a negative test and you feel healthy, you probably don’t need another test unless:
- Your job requires it
- You were exposed to TB
- You develop symptoms
- Your doctor recommends it
What if I forgot my reading appointment?
Call your doctor right away. If it’s been more than 72 hours since your test, you’ll probably need a new test. A patient who does not return within 72 hours will probably need to be rescheduled for a new test.
Can I exercise after a TB test?
Yes. You can do everything you normally do after a TB test. Just don’t rub or scratch the test spot. Exercise, showering, and normal activities are all fine.
When to Contact Yorktown Health
If you need a TB test or have questions about your results, Yorktown Health in Lisle can help. We provide:
- Professional TB test administration
- Expert reading of test results
- Clear explanations in simple language
- Follow-up care if needed
- Help understanding your risk factors
Our team takes the time to answer all your questions. We know getting a TB test can feel stressful. That’s why we make sure you understand every step of the process.
We also offer other health services to keep you and your family healthy:
You can reach us through our contact page or stop by our office at 2009 Warrenville Road Suite C, Lisle, IL 60532.
Final Thoughts
A negative TB test is usually great news. It means you probably don’t have TB germs in your body and you’re not at risk of spreading TB to others. The test looks for a firm bump at the injection spot. If there’s no bump or just a very tiny one (less than 5mm), your test is negative.
Remember these key points:
- Only trained healthcare workers should read your TB test
- Go back 48 to 72 hours after your test for the official reading
- Don’t cover, rub, or scratch the test spot before your reading
- A negative result usually means you don’t have TB
- Sometimes more testing is needed even with a negative result
- False negatives can happen but aren’t common
If you have any doubts or questions about your TB test results, talk to your doctor. They can explain your specific situation and help you understand what your results mean for your health.
Getting tested is the first step in protecting yourself and your community from TB. With proper testing and follow-up care, TB can be caught early and treated successfully. Don’t hesitate to reach out to Yorktown Health for professional TB testing and caring, patient-focused service.