Purified protein derivative, commonly called PPD, is a mixture of proteins extracted from the bacterium that causes tuberculosis. It is the active ingredient injected just under the skin during a TB skin test, also known as the Mantoux test. PPD doesn’t contain live bacteria and can’t cause TB. Its only job is to provoke an immune reaction in people whose bodies have previously encountered the TB bacterium, making it a diagnostic tool rather than a treatment or vaccine.
What PPD Is Made Of
PPD is produced by growing a human strain of Mycobacterium tuberculosis in liquid culture media, then filtering out the bacteria and purifying the proteins they left behind. The result is a sterile solution containing those protein fragments, which serve as antigens: substances the immune system can recognize and respond to. A standard test dose is 0.1 mL of solution containing 5 tuberculin units (TU) of purified protein. That tiny amount is enough to trigger a visible skin reaction in someone whose immune system has been primed by prior TB exposure.
How the Test Works
A healthcare provider injects the PPD solution into the top layer of skin on the inner forearm using a very small needle. The injection creates a small, pale bump (sometimes called a wheal) that disappears within minutes. At that point, the test is just waiting for your immune system to respond.
If your body has encountered TB bacteria before, either through actual infection or in some cases through the BCG vaccine given in many countries, a specific type of immune cell called a T cell will recognize the injected proteins. These T cells migrate to the injection site and release signaling molecules that cause local inflammation. This is a type IV hypersensitivity reaction, sometimes called delayed-type hypersensitivity, because it takes time to develop. Unlike an allergic reaction that happens in minutes, this immune response typically peaks 48 to 72 hours after the injection.
The visible result is a firm, raised area of thickened skin at the injection site called induration. Redness may also appear, but redness alone doesn’t count. Only the firm, swollen area matters when the test is read.
Reading the Results
You need to return to a trained healthcare provider between 48 and 72 hours after the injection to have the result read. If you miss that window, the test generally needs to be repeated. The provider measures the diameter of the induration in millimeters, running a finger across the injection site to find the edges of the firm area. Again, they measure the hard bump, not any surrounding redness.
The size of the induration that counts as “positive” depends on your personal risk factors. People with HIV, organ transplant recipients, and others with weakened immune systems have a lower threshold, meaning a smaller area of swelling is considered significant. People with moderate risk factors, such as healthcare workers or recent immigrants from countries where TB is common, have a slightly higher threshold. And for people with no known risk factors, the threshold is higher still. Your provider interprets the measurement in the context of your health history.
What a Positive Result Means
A positive PPD test means your immune system recognizes TB proteins. It does not necessarily mean you have active TB disease. Most people with a positive result have latent TB infection, meaning the bacteria are present in the body but dormant and not causing symptoms or spreading to others. Further testing, typically a chest X-ray and sometimes sputum samples, is needed to determine whether the infection is latent or active.
A negative result generally means you haven’t been infected with TB. However, certain conditions can suppress the immune response enough to produce a false negative. People with severely weakened immune systems, those who were recently infected (within the past 8 to 10 weeks), very young children, and people with overwhelming active TB may not mount a detectable reaction even though TB bacteria are present. Live virus vaccines given recently can also temporarily suppress the response.
Why BCG Vaccination Complicates Results
The BCG (Bacille Calmette-Guérin) vaccine, widely used outside the United States to protect against severe childhood TB, can trigger a positive PPD result even in someone who has never been infected. There is no reliable way to tell whether a positive skin test comes from the vaccine or from actual TB infection. People who received multiple BCG doses, as practiced in some countries, tend to have stronger and longer-lasting skin test reactions. BCG-related reactivity does tend to fade over time, but repeated skin testing can actually boost it back up.
This is one of the main reasons the United States does not routinely use the BCG vaccine, and it’s also why an alternative blood test has become increasingly popular for people who were vaccinated.
PPD Test vs. Blood Tests for TB
Interferon-gamma release assays (IGRAs) are blood tests that serve the same screening purpose as the PPD skin test but work differently. Instead of injecting antigens under your skin, a blood sample is mixed with TB-specific proteins in a lab, and the test measures the immune response in the tube.
The key advantage of blood tests is that they are unaffected by prior BCG vaccination and by most nontuberculous mycobacteria (environmental bacteria that are similar to TB but don’t cause it). This makes blood tests more specific, meaning fewer false positives. They also require only a single visit, since there’s no need to return for a reading. The PPD skin test remains widely used because it is less expensive, doesn’t require specialized lab equipment, and has decades of data behind it. In many settings, either test is acceptable.
Side Effects and Safety
For most people, a PPD skin test causes nothing more than mild soreness or a small bruise at the injection site. Local reactions like itching, swelling, or slight pain are the most common side effects. In rare cases, a strongly positive reaction can produce blistering or a small ulcer at the injection site.
Severe allergic reactions, including hives, difficulty breathing, or significant swelling of the face and throat, can occur but are uncommon. These reactions may appear within 15 minutes of the injection or between 6 and 12 hours afterward. Anyone who has previously had a severe reaction to a PPD test, such as skin blistering, tissue death at the injection site, or anaphylaxis, should not be tested with PPD again. A blood-based TB test is the appropriate alternative in those cases.
Fever and lightheadedness have also been reported occasionally. The test contains no live organisms and poses no risk of causing TB infection.