A TB skin test works by injecting a tiny amount of protein derived from tuberculosis bacteria just under the skin of your forearm, then checking 48 to 72 hours later for a raised, firm bump that signals your immune system recognizes TB. The test doesn’t check whether you currently have active tuberculosis. It detects whether your body has ever mounted an immune response to TB bacteria.
What Gets Injected and Why
The substance used is called purified protein derivative, or PPD. It’s a cell-free protein fraction extracted from a strain of the TB bacterium grown in a lab on synthetic medium and then inactivated. There are no live or dead bacteria in the injection. The protein itself cannot cause TB infection.
A healthcare worker injects 0.1 ml of PPD into the top layers of skin on the inner surface of your forearm, using a very small needle angled almost flat against the skin. When done correctly, it creates a small, pale bump (sometimes called a wheal) that looks like a tiny blister. This bump usually disappears within minutes and is not part of the test result.
The Immune Reaction Behind the Test
If you’ve been exposed to TB bacteria at some point in your life, your body went through a process that makes this test possible. During that original infection, immune cells called T lymphocytes learned to recognize TB proteins inside your lymph nodes. Over the course of three to eight weeks, those specialized cells multiplied and entered your bloodstream, where they can circulate for years.
When PPD is injected under your skin, those memory T cells recognize the familiar protein and rush to the injection site. They release chemical signals that draw more immune cells into the area, causing the tissue to swell and harden. This is a delayed-type hypersensitivity reaction. It’s the same basic process behind other immune responses like contact dermatitis from poison ivy, just triggered by a different substance. The reaction typically begins five to six hours after injection, peaks between 48 and 72 hours, and fades over several days.
What Happens at Your Reading Appointment
You need to return to your healthcare provider within that 48-to-72-hour window so they can evaluate your arm. Results read outside this window may not be accurate.
The provider doesn’t look at redness. Redness around the injection site is common and doesn’t mean much on its own. What matters is induration: a raised, firm area of skin you can feel with your fingertips. The provider runs a finger across the site to locate the edges of any hardened tissue, then measures the diameter in millimeters. That measurement determines whether your result counts as positive, and the threshold depends on your personal risk factors.
What Counts as a Positive Result
A positive TB skin test doesn’t use a single cutoff for everyone. The threshold is lower for people at higher risk of serious consequences from TB, and higher for people with no known risk factors. Three tiers are used:
- 5 mm or more is positive for people living with HIV, recent close contacts of someone with active TB, organ transplant recipients, people on immunosuppressive medications (such as long-term corticosteroids or TNF-alpha blockers), and anyone with certain scarring visible on a chest X-ray.
- 10 mm or more is positive for people born in countries where TB is common (including Mexico, the Philippines, Vietnam, India, China, Haiti, and Guatemala), people who live or work in nursing homes, homeless shelters, or correctional facilities, lab workers who handle TB specimens, people with diabetes, severe kidney disease, silicosis, certain cancers, or low body weight, children under five, and people who misuse drugs or alcohol.
- 15 mm or more is positive for people with no known risk factors for TB.
This tiered system exists because missing a true infection is more dangerous for someone whose immune system is already compromised. Lowering the threshold for high-risk groups catches more real cases, even at the cost of a few more false positives.
Why the Test Can Be Wrong
The TB skin test is not perfect in either direction. False positives happen most commonly in people who received the BCG vaccine, which is a TB vaccine given routinely in many countries outside the United States. Because BCG contains a related strain of mycobacteria, vaccinated people can develop a skin reaction to PPD even though they were never infected with actual TB. Infection with other, non-tuberculosis mycobacteria found in the environment can also trigger a false positive.
False negatives are possible too. People with weakened immune systems, including those with HIV, may not mount a strong enough response to produce measurable induration, even if they carry TB bacteria. Recent viral infections, live virus vaccines given in the prior four to six weeks, very recent TB infection (before the immune system has had its three-to-eight-week learning period), and overwhelming active TB disease can all blunt the reaction. Older adults sometimes lose their skin test reactivity over time, even if they were previously infected.
The Two-Step Testing Process
Some workplaces, especially hospitals and long-term care facilities, require what’s called two-step testing when you first start. This addresses a phenomenon called the booster effect. In people who were infected with TB years ago, the immune memory can fade just enough that a single skin test produces a weak or negative result. But that first injection “reminds” the immune system about TB proteins, so a second test done one to three weeks later produces the true, stronger reaction.
Without two-step testing at baseline, that boosted second reaction could be mistaken for a brand-new infection on a future annual test. Running both tests upfront establishes your true baseline.
Caring for the Injection Site
Between the injection and your reading appointment, leave the site alone as much as possible. Don’t scratch, rub, or pick at it. Avoid putting bandages, lotions, or creams directly over the spot, as these can irritate the skin and make the result harder to interpret. Washing your arm gently with soap and water is fine. If the area itches, placing a cool, damp cloth over it can help without affecting the test.
TB Skin Test vs. Blood Test
A blood test for TB (called an interferon-gamma release assay, or IGRA) works differently. Instead of injecting anything under your skin, a blood sample is drawn and mixed with TB proteins in a lab. The lab measures whether your immune cells react. Both tests detect immune memory rather than active disease, but the blood test has a practical advantage: it requires only one visit and is not affected by prior BCG vaccination. For people who received BCG as children, a blood test avoids the false-positive problem entirely.
Either test can be used for routine screening in most situations. The skin test remains widely used because it’s inexpensive, doesn’t require specialized lab equipment, and has decades of data behind it. A positive result on either test typically leads to the same next step: a chest X-ray to check for signs of active TB disease, followed by treatment decisions based on what that imaging shows.