Tubersol is used to test whether someone has been infected with tuberculosis (TB). It’s a purified protein derivative (PPD) solution injected just under the skin of the forearm, producing a visible reaction in people whose immune systems have previously encountered TB bacteria. The test doesn’t tell you whether you have active TB disease, only whether your body has been exposed to the bacteria at some point.
How the Test Works
Tubersol contains proteins extracted from the TB bacterium. When a small amount is injected into the skin, it acts as a trigger for immune cells that already “remember” a past encounter with TB. If you’ve been exposed before, specialized immune cells called T-cells rush to the injection site and release inflammatory signals. This causes redness, swelling, and a firm raised area called induration over the next two to three days.
This is a two-stage immune process. During the original TB exposure (which could have happened months or years earlier), your immune system learned to recognize TB proteins and created memory cells designed to respond if they ever saw those proteins again. The Tubersol injection is the second encounter, and the skin reaction is your immune system proving it remembers. The raised bump at the injection site is caused by an accumulation of immune cells, fluid, and localized swelling, not by the TB bacteria itself.
If you’ve never been exposed to TB, your immune system has no memory of the proteins, and the injection site shows little or no reaction.
What Happens During the Test
A healthcare worker injects a small amount of the Tubersol solution into the top layer of skin on the inner forearm, using a technique called the Mantoux method. The injection creates a small, pale bump (called a wheal) that disappears within minutes. The standard dose is 5 tuberculin units in 0.1 mL of solution.
You then return 48 to 72 hours later to have the site read by a trained healthcare worker. They measure the diameter of any firm, raised area (induration) at the injection site using a ruler. Redness alone doesn’t count. Only the firm, swollen portion is measured, and the result is recorded in millimeters.
How Results Are Interpreted
The size of induration that counts as “positive” depends on your personal risk level for TB. Not everyone is held to the same standard.
- 5 mm or more is considered positive for people at the highest risk: those with HIV, recent close contacts of someone with active TB, people with chest X-rays suggesting past TB, and organ transplant recipients or others on immune-suppressing medications.
- 10 mm or more is positive for people with moderate risk factors, including recent immigrants from countries where TB is common, healthcare workers, people who live or work in high-risk congregate settings (shelters, prisons), and children under age 4.
- 15 mm or more is positive for people with no known risk factors for TB.
A positive result means your body has encountered TB bacteria at some point. It does not mean you’re currently sick or contagious. Most people with a positive skin test have what’s called latent TB infection, where the bacteria are present but dormant. Additional testing, typically a chest X-ray, is needed to determine whether the infection is active.
The BCG Vaccine Complication
One well-known limitation of Tubersol involves the BCG vaccine, a TB vaccine given at birth or in childhood in many countries outside the United States. Because BCG is made from a related bacterium, it can train the immune system to react to Tubersol even without a true TB infection, causing a false-positive result.
The conventional guidance has been that BCG’s effect on skin test results fades within about 10 years. A long-term follow-up study tracking participants for up to 55 years tells a different story. BCG vaccination after infancy more than doubled the likelihood of a positive skin test in the first 15 years (with a hazard ratio of 2.33). Even 16 to 55 years later, vaccinated individuals still had a modestly elevated chance of testing positive. This means if you received a BCG vaccine at any point in your life, your provider should factor that into reading your results, regardless of how long ago you were vaccinated.
For people with a history of BCG vaccination, a blood test (called an interferon-gamma release assay) is often preferred over the skin test because it’s not affected by prior BCG vaccination.
Two-Step Testing
Some workplaces, particularly hospitals and long-term care facilities, use a two-step testing protocol when you’re being tested for the first time. The reason is a phenomenon called the booster effect. In some people who were infected with TB years ago, the immune memory has faded just enough that the first skin test produces a weak or negative result. But that first injection “reminds” the immune system, and a second test given one to three weeks later produces the true, stronger reaction.
Without two-step testing, that boosted second result could later be mistaken for a brand-new infection during routine annual testing. Two-step testing establishes an accurate baseline so future results can be compared reliably.
Who Gets Tested
Tubersol is primarily used for people at increased risk of developing active TB disease. This includes healthcare workers who may be exposed on the job, people who have spent time in countries where TB is widespread, residents and employees of congregate living facilities, close contacts of someone diagnosed with active TB, and individuals with weakened immune systems.
Routine testing of people with no risk factors is generally not recommended because in low-risk populations, false positives become more common relative to true positives, which can lead to unnecessary follow-up testing and treatment.
Side Effects
The most common reaction is mild discomfort, redness, or slight swelling at the injection site. In people with strong immune responses to TB, the induration itself can be uncomfortable or itchy. Severe reactions like blistering at the injection site are uncommon and typically occur in people with very strong sensitivity to TB proteins. Systemic allergic reactions are rare.
The test does not contain live bacteria and cannot cause a TB infection. If you’ve had a previous severe reaction to a tuberculin skin test, a blood-based TB test is a safer alternative.