A positive TB test looks like a raised, firm bump at the place where you got the shot. The size of this bump tells your doctor if you have TB germs in your body. A positive TB skin test will look like a firm, red bump that your provider will measure for size. This bump is called “induration” and it feels hard when you touch it.
This article will help you know what to look for after your TB test. We’ll cover how big the bump needs to be, what it means, and what happens next if your test comes back positive.
Understanding TB Test Results
What Makes a TB Test Positive
The reaction should be measured in millimeters of induration (firm swelling). The reader should not measure erythema (redness). Your doctor only cares about the hard, raised part – not any red color around it.
The bump size that counts as positive depends on your risk level. According to the Centers for Disease Control and Prevention, people with higher TB risk need smaller bumps to be called positive. People with lower risk need bigger bumps.
How TB Tests Work
When you get a TB skin test, a tiny bit of TB protein goes under your skin. If you have TB germs in your body, your immune system will react. This creates a bump within 2-3 days.
This response is a classic example of ‘delayed-type hypersensitivity reaction’. Your body’s defense system sends cells to fight what it thinks is TB, making the area swell up.
What Positive TB Test Results Look Like
Visual Appearance
A positive TB test creates these signs:
- Raised bump: The area will be higher than the skin around it
- Hard feeling: When you touch it, the bump feels firm or hard
- Size matters: The bump is usually round or oval shaped
- Color changes: The area might look red, but this doesn’t count for the test
If you have an Mtb infection, the skin around the injection site should start to swell and harden. The hardness is what your doctor measures, not the redness.
Different Bump Sizes Mean Different Things
The size of your bump tells your doctor about your TB risk:
5mm or bigger is positive if you:
- Have HIV or AIDS
- Take medicines that weaken your immune system
- Were recently around someone with active TB
- Have chest X-ray changes that look like old TB
- Had an organ transplant
10mm or bigger is positive if you:
- Came from a country where TB is common
- Live in high-risk places like nursing homes
- Work in hospitals or labs
- Use injection drugs
- Have health problems like diabetes
15mm or bigger is positive for:
- People with no known TB risk factors
- Healthy adults who haven’t been exposed to TB
In people with no known risks of TB, 15 mm or more of firm swelling at the site indicates a positive reaction.
TB Blood Test vs Skin Test Results
Blood Test Advantages
TB blood tests work differently than skin tests. According to the National Institutes of Health, TB blood tests measure the immune response to TB proteins in whole blood. These tests give faster results and don’t need a second visit to your doctor.
Blood tests are better for people who:
- Got the BCG vaccine (common in other countries)
- Have trouble coming back for a second appointment
- Had skin test problems before
When Each Test is Used
Your doctor at Yorktown Health will pick the best test for you. Current CDC guidelines recommend the TB skin test as the method of testing for children younger than 5 years of age.
For adults, both tests work well. Blood tests cost more but give quicker results.
Common Questions About Positive TB Tests
Can TB Tests Give Wrong Results?
Yes, TB tests can sometimes be wrong. This happens in two ways:
False Positive Results:
- You got the BCG vaccine in another country
- You have other types of bacteria that aren’t TB
- The test wasn’t done right
- You have certain health conditions
False Negative Results:
- Your immune system is too weak to react
- You got infected very recently (within 8-10 weeks)
- You’re very sick with other illnesses
- You take certain medicines
The effect on TST of BCG received in infancy is minimal, especially > or =10 years after vaccination, according to research published in the International Journal of Tuberculosis and Lung Disease.
What If You Had the BCG Vaccine?
Many people from other countries got the BCG vaccine as children. This vaccine can make your TB skin test positive even if you don’t have TB germs.
TB blood test is not affected by the BCG vaccine. This means that your TB blood test will be “positive” only if you have TB germs in your body.
If you had the BCG vaccine, your doctor might want you to get a blood test instead of a skin test.
What Happens After a Positive TB Test
Next Steps Your Doctor Will Take
If your TB test is positive, don’t panic. A positive skin test result for TB infection means you have TB germs in your body. Your health care provider will do other tests to determine if you have inactive TB or active TB disease.
Your doctor will:
- Ask about your symptoms
- Do a chest X-ray
- Maybe test your spit (sputum)
- Check your overall health
Two Types of TB Infection
Latent TB (Sleeping TB):
- You have TB germs but feel fine
- You can’t spread TB to others
- Medicine can prevent it from becoming active
- Most people with positive tests have this type
Active TB Disease:
- You have symptoms like cough, fever, weight loss
- You can spread TB to other people
- You need treatment right away
- This is more serious but still treatable
Getting tested and treated for TB can protect yourself, your family and friends, and your community.
TB Testing at Yorktown Health
Professional TB Testing Services
At Yorktown Health, our medical team provides TB test administration for patients in Vernon Hills and surrounding areas. We understand that getting tested for TB can feel scary, but our caring staff will help you through each step.
Our services include:
- Both skin and blood TB testing
- Expert reading of test results
- Follow-up care if needed
- Connection to treatment resources
When to Get Tested
You should get a TB test if you:
- Work in healthcare, schools, or other high-risk jobs
- Traveled to countries where TB is common
- Live with someone who has TB
- Have health problems that increase your TB risk
- Need testing for work or school
Understanding Your Risk Factors
High-Risk Groups for TB
Some people are more likely to get TB or have it become active:
Health Conditions:
- HIV/AIDS
- Diabetes
- Kidney disease
- Cancer or cancer treatment
- Taking immune-suppressing medicines
Life Situations:
- Living in crowded conditions
- Being homeless or in shelters
- Being in jail or prison
- Coming from countries with lots of TB
Age Groups:
- Babies and young children
- Adults over 65 years old
Workplace Requirements
Many jobs require TB testing, including:
- Healthcare workers
- Teachers and school staff
- Food service workers
- Childcare providers
- Prison or jail staff
On May 7, 2019, the Centers for Disease Control and Prevention updated guidelines for Tuberculosis (TB) testing of healthcare workers. The new rules don’t require yearly testing for most healthcare workers unless there’s been an exposure.
Timeline and Follow-Up Care
When Results Are Available
Skin Test Timeline:
- Get the test on day 1
- Come back in 2-3 days to have it read
- Can’t wait longer than 72 hours or you need a new test
Blood Test Timeline:
- Give blood sample
- Results usually ready in 1-2 days
- Some labs may take longer
Follow-Up Testing
If your first test is positive, you might need:
- Chest X-rays every year
- Sputum tests if you have symptoms
- More blood work
- Regular check-ups with your doctor
Once you have a positive TB blood test or TB skin test result, you may still have a positive test result on future TB tests. This means you usually don’t need testing again unless you have new symptoms.
Treatment Options for Positive Tests
Latent TB Treatment
If you have latent TB, treatment can prevent it from becoming active disease. According to the World Health Organization, treatment for latent TB infection is 90% effective for preventing the development of TB disease.
Treatment usually involves:
- Taking one or two medicines
- Treatment lasts 3-9 months
- Regular doctor visits to check progress
- Blood tests to watch for side effects
Active TB Treatment
Active TB needs stronger treatment with multiple medicines. This usually means taking 4 different medicines for 2 months, then 2 medicines for 4 more months.
The good news is that TB treatment works very well when you take all your medicines as directed.
Prevention and Next Steps
Protecting Others
If you have active TB, you can spread it to others through coughing or sneezing. Important steps include:
- Stay home until your doctor says it’s safe
- Cover your mouth when coughing
- Take all your medicines as prescribed
- Get follow-up testing as needed
Family and Close Contacts
If you have active TB, people close to you may need testing too. This includes:
- Family members living in your home
- Close friends you spend time with
- Coworkers in your office or workplace
Your primary care doctor can help coordinate testing for your family and provide guidance on next steps.
Staying Healthy
Whether you have latent or active TB, taking care of your overall health helps your body fight infection:
- Eat healthy foods
- Get enough sleep
- Exercise regularly
- Don’t smoke or use drugs
- Take medicines as prescribed
- Keep all doctor appointments
Final Thoughts
A positive TB test might feel scary, but remember that millions of people have TB germs in their bodies and never get sick. The test helps find TB early so it can be treated before it becomes a problem.
TB blood tests are the preferred tests for people who have received the BCG TB vaccine, so talk to your doctor about which test is right for you.
If you need TB testing or have questions about your results, the medical team at Yorktown Health is here to help. We provide complete family health services including TB testing, treatment, and follow-up care for patients throughout Vernon Hills and the surrounding area.
