A positive TB test looks like a raised, firm bump at the place where you got the shot. This bump is called “induration” and it feels hard when you touch it. Your doctor will measure this bump to see if you have TB germs in your body. The size matters more than any red color around it.
This complete guide will walk you through what to look for after your TB test, how doctors decide if it’s positive, what different bump sizes mean, and what happens next if your test comes back positive.
Understanding Your TB Skin Test Results
What the Bump Really Means
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 bump shows that your body remembers TB bacteria and is trying to fight it off.
The bump you see is your body’s way of saying “I know this germ!” But here’s the key thing: Your doctor only cares about the hard, raised part – not any red color around it. Think of it like a mosquito bite – the hard part in the middle is what counts, not the red skin around it.
How Doctors Check Your Test
Your healthcare provider needs to check your arm 48 to 72 hours after getting the test. They’ll feel the spot with their fingers to find the edges of the hard bump. The reaction should be measured in millimeters of induration (firm swelling). The reader should not measure erythema (redness).
The way they measure is very specific. They measure across your forearm – like drawing a line from side to side, not up and down your arm. Only trained healthcare workers should read your test. You can’t check it yourself at home.
Why Timing Matters
You must return to get your test checked within that 48 to 72 hour window. If you miss this time, you’ll need to start over with a new test. It’s important to go to your second appointment to get an official result. If you don’t return within 72 hours, you’ll need to take the test again.
What Size Bump Makes a Test Positive?
The 5mm Rule
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, or had an organ transplant.
This smaller size counts as positive for people at highest risk because their bodies might not make a big reaction even when infected. It’s like having a quiet smoke alarm – even a small signal needs attention.
The 10mm Rule
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, or have health problems like diabetes.
This group includes people who face more TB exposure than average folks. Think of healthcare workers who might breathe in TB germs at work, or people from countries where TB spreads more easily.
The 15mm Rule
15mm or bigger is positive for people with no known TB risk factors, or healthy adults who haven’t been exposed to TB.
For healthy people with no special risks, the bump needs to be bigger before doctors call it positive. This helps avoid false alarms in people who probably don’t have TB.
What Happens After a Positive Test?
First Steps After Testing Positive
Don’t panic if your test is positive. A positive skin test result for TB infection means you have TB germs in your body. But this doesn’t mean you’re sick or can spread TB to others. Most people with positive tests have what’s called “latent TB” – sleeping germs that aren’t causing problems.
Any individual with a newly identified positive test for TB infection should be evaluated for TB disease with a medical examination and a chest x-ray. Your doctor needs to figure out if you have latent TB (not sick) or active TB disease (sick and possibly contagious).
Getting a Chest X-Ray
The doctor will look for white spots on your lungs that indicate areas where your immune system is responding to bacteria. A normal chest X-ray usually means you have latent TB – the germs are sleeping and not hurting your lungs.
If the X-ray shows problems, your doctor might order more tests. Your healthcare professional may take a sample of the mucus that comes up when you cough, also called sputum. If you have active TB disease in your lungs or voice box, lab tests can detect the bacteria.
Blood Tests for Confirmation
Sometimes doctors want a blood test to double-check your skin test result. TB blood tests measure the immune response to TB proteins in whole blood. These tests give results in 1-2 days and work differently than skin tests.
Blood tests are especially helpful if you had the BCG vaccine (common in other countries) or if your skin test result isn’t clear. The blood test won’t turn positive just because you had the vaccine.
Understanding False Positive Results
The BCG Vaccine Factor
The BCG vaccine may cause a false-positive TB skin test reaction. There is no reliable way to distinguish a positive TB skin test reaction caused by BCG vaccination from a reaction caused by true TB infection. Many people who were born outside the U.S. got this vaccine as babies.
If you had the BCG vaccine, your doctor might want you to get a blood test instead of a skin test. The good news is that TB blood tests (interferon-gamma release assay or IGRA) are the preferred test for people who have received the BCG vaccine, including children. BCG vaccination does not induce positive results when TB blood tests are used.
How Long BCG Affects Tests
The effect on TST of BCG received in infancy is minimal, especially 10 years or more after vaccination. BCG received after infancy produces more frequent, more persistent and larger TST reactions.
This means if you got the vaccine as a baby, it probably won’t cause false positives years later. But if you got it as an older child or adult, it might still affect your skin test results.
Other Reasons for False Positives
Besides the BCG vaccine, some persons may have a positive result from a TB skin test even though they are not infected with TB bacteria. The causes of these false-positive results may include infection with nontuberculous mycobacteria (mycobacteria other than M. tuberculosis) or incorrect measurement or interpretation of reaction.
Think of these other bacteria as TB’s cousins – they’re similar enough to confuse the test but don’t cause the same disease.
Treatment Options After a Positive Test
Treating Latent TB
If you have latent TB (positive test but not sick), treatment can stop it from waking up later. According to the World Health Organization, treatment for latent TB infection is 90% effective for preventing the development of TB disease.
Treatment usually means taking pills for 3 to 9 months. Most latent TB infections are treated for three or four months. Your doctor picks the best medicine plan based on your health and other factors.
Treating Active TB Disease
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.
It’s really important to take every single dose of medicine. Skipping doses can make the TB germs stronger and harder to kill. Your public health department may use a program called directly observed therapy (DOT). With DOT, a healthcare worker visits you at home to watch you take your dose of medicines.
Why Treatment Matters
With a normal immune system, you have a 10% lifetime chance of developing TB disease; half of the risk of progressing from latent TB infection to active TB disease occurs in the first 2 years after encountering the TB bacteria.
This means the highest risk comes right after infection. Getting treated protects you and keeps you from spreading TB to family and friends if it becomes active.
Special Considerations for Healthcare Workers
Updated Testing Guidelines
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.
Healthcare workers still need baseline testing when they start work. But they don’t need regular retesting unless they’re exposed to someone with TB or work in high-risk areas.
What Positive Tests Mean for Work
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.
If you work in healthcare with a positive test, you’ll need a chest X-ray to prove you don’t have active TB. As long as you’re not sick, you can keep working. Your employer might ask for yearly symptom checks instead of new tests.
Living With a Positive TB Test
Long-Term Monitoring
Once your test is positive, it stays positive for life. This doesn’t mean you’re always infected – it means your body remembers TB. Think of it like a scar that shows you once had a cut.
You don’t need regular chest X-rays if you stay healthy. But tell every new doctor about your positive test. They need this information to take good care of you.
When to Seek Medical Care
See your doctor right away if you develop:
- A cough lasting more than 3 weeks
- Coughing up blood
- Chest pain
- Fever and night sweats
- Losing weight without trying
- Feeling very tired all the time
These symptoms might mean latent TB has become active. Quick treatment protects your health and stops spread to others.
Protecting Your Family
It is important to understand that if you have latent TB infection (positive TB blood test or TB skin test, normal chest x-ray, no symptoms of TB), you cannot give anyone else TB.
Only people with active TB disease in their lungs or throat can spread it. Latent TB is like having a seed that hasn’t grown – it can’t jump to other people.
Getting Your TB Test at Yorktown Health
Why Choose Professional Testing
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.
Professional testing ensures accurate results. Our trained staff knows exactly how to give the test, read results, and explain what they mean for your health. We also provide all the paperwork you need for work or school.
Our Testing Process
At Yorktown Health in Lisle, we make TB testing simple. First, we give you the skin test and explain how to care for the spot. We’ll remind you to come back in 48-72 hours for your reading.
When you return, we carefully check and measure any reaction. If your test is positive, we guide you through next steps, including arranging for a chest X-ray if needed. We handle all the documentation for your employer or school.
Convenient Local Care
Our Lisle location at 2009 Warrenville Road offers quick appointments that fit your schedule. No long waits or confusion – just clear, professional care when you need it.
We know you’re busy, so we make the process as smooth as possible. From scheduling your annual physical to getting your TB test for work, we respect your time while giving thorough care.
Final Thoughts
A positive TB test shows as a firm bump at your injection site, but what it means depends on the size and your risk factors. Remember that most positive tests indicate latent TB – you have the germs but aren’t sick or contagious. With proper medical evaluation including a chest X-ray and possible blood tests, your doctor will determine if you need treatment.
Getting tested and properly evaluated protects both your health and your community. Whether you need testing for work, school, or health monitoring, understanding what to look for and what happens next takes away the worry and confusion.
Ready to get your TB test done right? Contact Yorktown Health in Lisle today to schedule your appointment. Our experienced team provides professional TB testing with clear results and all required documentation. Don’t wait – protect your health and meet your testing requirements with confidence.