A Purified Protein Derivative (PPD) test, also known as a Mantoux tuberculin skin test, screens for infection with Mycobacterium tuberculosis (TB). This test involves injecting a small amount of purified protein derivative under the skin, typically on the forearm. If previously exposed to TB bacteria, the immune system develops a delayed-type hypersensitivity reaction to these proteins, which helps assess infection.
Visual Appearance of a Negative Result
A negative PPD test result shows no significant raised, hardened area (induration) at the injection site when read 48 to 72 hours after administration. The site typically shows only the original needle puncture mark. Faint redness (erythema) might be present, but it is not measured and does not indicate a positive reaction.
Healthcare providers measure the diameter of any firm, raised swelling, not the redness. For a negative result, there should be no measurable induration, or it should be very small, typically less than 5 millimeters, depending on individual risk factors. The skin at the test site should look almost the same as before the test, indicating no significant immune response.
What a Negative Result Means
A negative PPD test generally indicates that an individual has not been infected with Mycobacterium tuberculosis or has not developed an immune response to the bacteria. This result means that neither latent TB infection (where the bacteria are present but inactive) nor active TB disease (where the bacteria are causing illness) is likely present. The test relies on the body’s cell-mediated immunity to detect prior exposure. For most individuals, a negative result is reassuring, suggesting they do not harbor the TB bacteria.
Factors Affecting PPD Test Results
A negative PPD test result does not always definitively rule out TB infection, as several factors can lead to a false negative. One reason is recent infection, where the immune system has not yet developed a detectable response; this “window period” typically ranges from 2 to 12 weeks after exposure. Individuals with weakened immune systems may also show a false-negative result because their body cannot mount a sufficient immune reaction.
Certain medical conditions (such as HIV/AIDS, cancer, or severe malnutrition) and medications (like corticosteroids or chemotherapy) can suppress the immune system, leading to an inadequate response. Improper test administration or incorrect reading can also result in an inaccurate negative finding. In some cases, a very old TB infection might yield a false-negative result if the immune response has significantly waned.
If TB exposure is suspected despite a negative test, or if symptoms are present, further evaluation, such as blood tests (IGRA) or chest X-rays, may be recommended. Retesting might also be necessary for ongoing exposure or occupational health screenings.