A PPD test (purified protein derivative), also called a Mantoux test or tuberculin skin test, is read by measuring the raised, firm bump on your forearm 48 to 72 hours after injection. The key detail most people get wrong: you measure the hard, swollen area (induration), not the redness around it. Whether that measurement counts as “positive” depends on the size in millimeters and your personal risk factors for tuberculosis.
When to Have Your Test Read
The reading window opens at 48 hours and closes at 72 hours after the injection. This timing matters. If you show up too early, the reaction hasn’t fully developed and can be misleading. If you miss the 72-hour window entirely, the result can’t be reliably interpreted, and you’ll need to schedule a new test. The replacement test can be placed right away, so missing your appointment doesn’t mean starting a long wait.
What Gets Measured (and What Doesn’t)
After a PPD injection, two things can appear on your forearm: redness and a firm, raised area of swelling. Only the firm swelling counts. The CDC is explicit on this point: measure the induration in millimeters, not the erythema (redness). Redness alone, no matter how vivid, is not a positive result.
To find the induration, run your fingertip lightly across the injection site. You’re feeling for a distinct, firm bump, almost like a small disc under the skin. It has clear edges you can detect by touch even if they’re hard to see. Some people have no bump at all, just a small needle mark. That’s a valid result too.
The measurement is taken across the forearm, perpendicular to the long axis of your arm. In practical terms, if your arm is stretched out in front of you, you measure side to side, not up and down toward your hand and elbow. A small, flexible ruler marked in millimeters is used. The result is recorded as a number: 0 mm, 5 mm, 12 mm, and so on.
What the Millimeter Number Means
There is no single cutoff that makes a PPD result “positive” for everyone. The threshold depends on how likely you are to have been exposed to tuberculosis or how vulnerable you’d be if you were infected. Three cutoff levels are used: 5 mm, 10 mm, and 15 mm.
5 mm or More
This is the lowest threshold, reserved for people at highest risk of serious TB infection. It applies if you have HIV, have received an organ transplant, are taking immunosuppressive medications (such as long-term corticosteroids), have had recent close contact with someone who has active TB, or have chest X-ray findings consistent with old, healed TB. For anyone in these groups, even a small bump is treated as a positive result because their immune systems may not mount a full response.
10 mm or More
This cutoff covers a broader set of risk factors. It applies to recent arrivals (within five years) from countries where TB is common, people who inject drugs, and residents or employees of high-risk group settings like prisons, nursing homes, hospitals, and homeless shelters. Lab workers who handle TB specimens also fall in this category. People with certain medical conditions that raise TB risk, including diabetes, chronic kidney disease, leukemia, significant weight loss, and chronic malabsorption syndromes, use this cutoff as well. Children under four years old and children or adolescents regularly exposed to adults in any of these groups are also read at the 10 mm threshold.
15 mm or More
For people with no known risk factors for TB, 15 mm or more of induration is required to call the test positive. This higher bar exists because in a low-risk person, smaller reactions are more likely to be caused by something other than actual TB infection.
Why Redness Can Be Misleading
It’s common to see a large red area around the injection site and assume the test is positive. But redness without a firm bump underneath is not meaningful. Some people develop a bruise or slight irritation from the needle itself. Others have mild allergic-type skin reactions to the solution. None of this counts unless there’s measurable induration beneath it. Conversely, a reaction can look subtle to the eye but feel clearly firm to the touch, and that firmness is what determines your result.
The BCG Vaccine Complication
If you received the BCG vaccine (a tuberculosis vaccine given routinely in many countries outside the United States), it can cause a positive PPD result even without actual TB infection. The vaccine primes your immune system to react to the tuberculin protein, producing induration that looks identical to a true positive. This is one of the most common sources of confusion with skin test results.
U.S. guidelines treat BCG-vaccinated individuals the same way as everyone else when interpreting the PPD. If your induration is 10 mm or more and you were born in or lived in a high-prevalence country, had contact with someone with infectious TB, or are regularly exposed to populations with high TB rates, your result is considered positive regardless of your vaccination history. For people in this situation, a blood test called an IGRA (interferon-gamma release assay) is often preferred because it is not affected by prior BCG vaccination and gives a clearer answer.
False Negatives and Weakened Immunity
A PPD test relies on your immune system mounting a visible response. If your immune system is suppressed, it may fail to react even if you carry TB. This phenomenon, called anergy, means the test reads as negative despite a real infection. It can occur in people with advanced HIV, those on chemotherapy, people taking immunosuppressive drugs after organ transplants, and individuals with severe malnutrition. This is exactly why the positive threshold is set so low (5 mm) for immunocompromised individuals: even a modest response in someone whose immune system is dampened deserves attention.
Very recent TB infection can also produce a false negative. It takes two to eight weeks after exposure for the immune system to develop a detectable response. A test given during that window might miss an infection that a later test would catch.
What Happens After a Positive Result
A positive PPD test does not mean you have active tuberculosis. It means your body has been exposed to TB bacteria at some point. The next step is a chest X-ray and a symptom evaluation. Your provider will ask about cough, fever, night sweats, and unexplained weight loss.
If the chest X-ray is clear and you have no symptoms, the diagnosis is typically latent TB infection. Latent TB means the bacteria are present but dormant. You’re not contagious and don’t feel sick, but treatment is recommended to prevent the infection from becoming active later. If the X-ray or symptoms suggest active disease, further testing follows, usually including sputum samples, to confirm the diagnosis and guide treatment.
Reading Your Own Result
While healthcare providers are trained to read PPD tests, understanding the process helps you make sense of what you’re told. When you go for your reading, the provider will press lightly on the injection site, mark the edges of any induration with a pen, then measure the distance between the marks. They’ll record the result in millimeters and interpret it based on your risk profile. If you’re told your result is “negative at 3 mm” or “positive at 12 mm,” you now know exactly what those numbers refer to and why they matter differently for different people.