A tuberculosis (TB) skin test, formally called the tuberculin skin test or TST, produces a visible and measurable reaction on the inner forearm that a healthcare provider reads 48 to 72 hours after injection. A negative result looks like little more than a tiny dot at the injection site, with no raised area or only minimal swelling. A positive result shows a firm, raised bump called an induration, and the size of that bump determines the interpretation, with different thresholds depending on your health and risk factors. The distinction sounds simple, but several factors can make the reading tricky.
What Happens During the Test
A TB skin test begins with a small injection of a substance called tuberculin (purified protein derivative, or PPD) just under the surface of the skin on the inside of your forearm. The injection is shallow enough that it should immediately produce a small, pale bump known as a wheal, typically 6 to 10 millimeters wide. That wheal confirms the dose landed in the right layer of skin. If the solution leaks out significantly or goes too deep, the test needs to be repeated right away at a different spot on the arm.1PubMed. Tuberculin testing: placement and interpretation
The wheal itself is not part of the result. It fades within minutes to an hour and has nothing to do with whether the test is positive or negative. The reaction that matters develops over the next two to three days as your immune system responds, or doesn’t respond, to the tuberculin proteins.
How the Immune Response Creates the Result
The TST works by triggering a delayed-type hypersensitivity reaction. If your immune system has previously encountered TB bacteria (or something similar), specialized immune cells recognize the injected proteins and migrate to the site. This creates a visible infiltrate in the skin, with immune cells flooding the area and causing it to swell and harden. That response begins roughly 6 to 12 hours after injection, peaks around 48 to 72 hours, and can linger for up to a week.2PubMed Central. Tuberculin Test Measurement at 48 and 72 hours: mismatch and clinical significance
If your immune system has never encountered TB, there is nothing to recognize, and the site stays flat. That is the basic biology behind what you see on your arm: a positive test means your body already has immune memory of TB proteins, while a negative test means it does not.
What a Negative Result Looks Like
A negative TB skin test is visually uneventful. When you return to have your arm checked, you might see a small mark or faint redness at the injection site, but there is no raised, firm bump to measure. The induration, the hard area that healthcare providers palpate with their fingers, is either completely absent (0 mm) or too small to cross the threshold for your risk category.
Slight redness alone does not count. This is a point that confuses many people: your skin can look a bit pink or irritated at the injection site and still be read as negative. Providers measure only the induration, the hardened lump you can feel when you press on it, not the surrounding redness or discoloration. In a study of calibrated models, all patients correctly identified a 0 mm site with no induration, and trained professionals detected every induration present, confirming that a truly flat site is reliably distinguishable from one with a bump.3PubMed. Assessing the validity of tuberculin skin test readings by trained professionals and patients
What a Positive Result Looks Like
A positive result shows a distinct, firm, raised area at the injection site. It can range from a modest bump to a large, clearly visible swelling. Some positive reactions also include redness, warmth, and mild itching, but the defining feature is that hard, palpable lump. In rare cases, blistering or breakdown of the skin can occur at the site, which is considered a strongly positive reaction.4Indian Journal of Public Health. Tuberculin Skin Testing
The provider measures the induration across its widest point using a ruler, pressing gently to find the edges of the firm area. This is where interpretation gets more nuanced than just “bump versus no bump,” because the size that counts as positive depends on who you are.
The Three Cutoff Thresholds
There is no single number that separates positive from negative for everyone. Instead, three induration thresholds are used, calibrated to a person’s risk of having been exposed to TB or of becoming seriously ill if infected:
- 5 mm or more: Considered positive for people at highest risk, including those with HIV, organ transplant recipients on immunosuppressive drugs, people with recent close contact with someone who has active TB, and anyone whose chest X-ray shows old, healed TB lesions.
- 10 mm or more: Considered positive for people with moderate risk factors, such as healthcare workers, recent immigrants from countries where TB is common, people who inject drugs, and residents or employees of high-risk congregate settings like prisons or shelters.5Journal of Occupational and Environmental Medicine. Tuberculosis Screening, Testing, and Treatment of US Health Care Personnel
- 15 mm or more: Considered positive for people with no known risk factors, meaning otherwise healthy adults who have no particular reason to have been exposed to TB.
This tiered system exists because the consequences of missing a true infection are much worse in someone who is immunocompromised or who works in healthcare than in a healthy person with no known exposures. A 7 mm induration on the arm of a person living with HIV is a positive result that triggers treatment. The same 7 mm on a healthy, low-risk adult with no TB contacts is read as negative.
Why Measurement Is Harder Than It Sounds
Reading a TB skin test is a hands-on skill, and even trained professionals do not always agree on the exact size. A study of measurement reliability found that the same observer’s second reading of the same induration could differ by roughly 3 mm in either direction, and two different observers could disagree by even more. That variability is enough to reclassify a borderline result from positive to negative or vice versa.6PubMed. Reliability of tuberculin skin test measurement
The ballpoint-pen technique, where the reader slides a pen toward the induration from each side and marks where it stops, reduces this imprecision by about a third compared to feeling the edges by hand alone. Even so, if your result falls within a few millimeters of a cutoff, there is real uncertainty about which side of the line you are on. This is one reason clinicians consider the whole clinical picture rather than treating the number as an absolute verdict.
When a Positive Test Does Not Mean Active TB
A positive TST tells you that your immune system has been sensitized to TB proteins. It does not tell you whether you have active, contagious TB disease or latent TB infection, a state where the bacteria are present but dormant and you feel fine. Most people who test positive have latent infection, not active disease. The test also cannot distinguish between infection with TB bacteria and immune memory from the BCG vaccine, which is widely used outside the United States.
A 55-year follow-up study found that BCG vaccination received after infancy roughly doubled the risk of testing positive on a TST in the first 15 years after vaccination, and the effect persisted even decades later, though it weakened over time.7PubMed. The Long-term Effect of Bacille Calmette-Guérin Vaccination on Tuberculin Skin Testing: A 55-Year Follow-Up Study In children under five, BCG vaccination had a clear impact on TST responses, while in older children the effect was less pronounced. For BCG-vaccinated children, raising the cutoff to 10 mm helped maintain a high negative predictive value while improving specificity.8Thorax. The impact of BCG vaccination on tuberculin skin test responses in children is age dependent: evidence to be considered when screening children for tuberculosis infection
This is a real-world problem for millions of people. If you were vaccinated with BCG as a child, which is routine in much of Asia, Africa, Latin America, and parts of Europe, your skin test may come back positive even if you have never been infected with TB. That false positive then triggers further testing, and sometimes unnecessary treatment.
When a Negative Test Might Be Wrong
False negatives happen too. Several conditions can suppress the immune response needed to produce an induration, including HIV infection, malnutrition, certain medications (especially immunosuppressants and corticosteroids), very recent TB infection (the immune response takes two to eight weeks to develop after exposure), and advanced age. Severe active TB itself can paradoxically suppress the skin test reaction. In any of these situations, a flat reading does not reliably rule out infection.
There are also technical reasons a test might come back falsely negative: the tuberculin was stored improperly, the injection was given too deep, or the reading was done too early or too late. The 48-to-72-hour reading window exists because the immune reaction peaks in that window. Reading it at 24 hours could catch a response before it has fully developed.
Two-Step Testing and the Booster Effect
Some workplaces, especially hospitals and long-term care facilities, use a two-step TB testing process for new employees. This involves getting a second TST one to three weeks after the first.9Journal of Pharmacy Technology. Effectiveness of Two-Step Tuberculin Skin Testing in Hospital Employees The purpose is to account for the booster phenomenon: in people who were infected with TB long ago or who received BCG vaccination, immune memory can fade to the point where the first test comes back negative. But that first injection “reminds” the immune system about TB proteins, so a second test a few weeks later produces the positive reaction that was always latent.
Without two-step testing, a person with faded immunity could test negative on their initial screening and then positive on an annual follow-up, creating the false impression of a new infection (a “conversion”) when in fact the immune memory was old.10PubMed. The booster effect in two-step tuberculin testing among young adults in Montreal Two-step testing helps sort out who has a genuinely new positive result from who simply needed a booster to reveal a longstanding one.
Blood Tests as an Alternative
Interferon-gamma release assays (IGRAs) are blood tests that can be used instead of or alongside the skin test. The two main versions are QuantiFERON and T-SPOT. Like the TST, they detect immune memory of TB, but they do it by measuring immune cell responses in a blood sample rather than on the skin. A key advantage is that IGRAs are not thrown off by prior BCG vaccination, since the antigens they use are specific to TB bacteria and are not present in the BCG vaccine strain.11PubMed Central. Interferon-Gamma Release Assays versus Tuberculin Skin Testing for the Diagnosis of Latent Tuberculosis Infection: An Overview of the Evidence
A 2024 analysis found that both the T-SPOT and QuantiFERON tests were significantly better than the TST at predicting which individuals would go on to develop active TB disease.12JAMA Network Open. Comparison of Tuberculin Skin Testing and Interferon-γ Release Assays in Predicting Tuberculosis Disease That said, IGRAs have their own limitations. Results can fluctuate when the same person is tested repeatedly, and there is no consensus on exactly how to interpret a test that flips between positive and negative on successive draws. Neither test, skin or blood, does a good job predicting which latently infected individuals will eventually get sick.
For people who received BCG as children, IGRAs offer a cleaner answer because they sidestep the vaccination confounding entirely. In countries where BCG is routine, many TB programs have shifted toward IGRAs for exactly this reason.
What Happens After a Positive Result
A positive skin test or blood test is not a diagnosis on its own. The next step is almost always a chest X-ray, which looks for signs of active or past TB in the lungs. In people with latent TB, the X-ray is usually normal, though a pooled analysis of studies found that about 15% of individuals with latent infection had subtle radiological findings, most commonly calcified nodules or enlarged lymph nodes.13Open Forum Infectious Diseases. Radiological Signs of Latent Tuberculosis on Chest Radiography: A Systematic Review and Meta-Analysis
Among asymptomatic children who tested positive on a TST or IGRA, a retrospective study found that roughly 2.6% had chest X-ray findings suggestive of active TB.14BMJ Journals. To x-ray or not to x-ray? Screening asymptomatic children for pulmonary TB: a retrospective audit That low percentage underscores the point that most positive skin tests reflect latent infection, not disease. When the chest X-ray does show something concerning, especially enlarged lymph nodes near the airways, CT imaging can provide a more detailed look.15PubMed. Evaluation of Chest X-ray and Thoracic Computed Tomography in Patients with Suspected Tuberculosis
If the X-ray is normal and you have no symptoms, the working diagnosis is latent TB infection. Treatment with antibiotics for several months can dramatically reduce the chance of latent infection progressing to active disease. If the X-ray or your symptoms suggest active TB, sputum samples are collected to look for TB bacteria under a microscope and in culture. Microscopy can identify bacteria relatively quickly but catches only about 60% of culture-confirmed cases; culture remains the gold standard for confirming active disease.16PubMed Central. Sputum microscopy for the diagnosis of HIV-associated pulmonary tuberculosis in Tanzania
Uncommon but Alarming Skin Reactions
Most TB skin tests produce either nothing visible or a modest raised bump. Occasionally, though, the local reaction can be intense enough to alarm people who are not expecting it. A small study documented blistering and skin breakdown at the injection site in about 3.5% of patients tested, along with scattered cases of hives and swelling.4Indian Journal of Public Health. Tuberculin Skin Testing These severe local reactions are generally considered strongly positive, but they are not dangerous in themselves. The blistering reflects an intense immune response and heals on its own, though it can leave a small scar.
Truly systemic allergic reactions to tuberculin are rare. If your test site blisters or becomes very swollen, it does not mean something went wrong with the injection. It means your immune system reacted vigorously, and your provider will factor that into the interpretation.
A Next-Generation Skin Test
One of the biggest frustrations with the traditional TST is its inability to distinguish TB infection from BCG vaccination. A newer skin test called C-Tb, developed using two proteins specific to TB bacteria (ESAT-6 and CFP-10), aims to fix that problem. In a clinical trial, C-Tb achieved a specificity of about 99% at a 5 mm cutoff, compared to just 63% for the standard tuberculin test at the same cutoff in BCG-vaccinated individuals.17PLoS ONE. Randomised Clinical Trial Investigating the Specificity of a Novel Skin Test (C-Tb) for Diagnosis of M. tuberculosis Infection Induration sizes looked similar to the traditional test, so reading the result uses the same physical examination technique. The key difference is that the C-Tb result is not affected by BCG vaccination status.18The Lancet Respiratory Medicine. Safety and diagnostic performance of C-Tb, a novel ESAT-6 and CFP-10 skin test for tuberculosis infection
C-Tb essentially combines the practical simplicity of a skin test with the specificity advantages of a blood test. It requires no laboratory equipment, no blood draw, and no cold-chain transport of blood samples, which matters enormously in lower-resource settings where TB is most common. It has not replaced the traditional TST everywhere, but it represents where TB skin testing is heading: same bump-on-the-arm format, much cleaner interpretation for the billions of people who have been vaccinated with BCG.