The Purified Protein Derivative (PPD) test, also known as the Tuberculin Skin Test (TST) or Mantoux test, screens individuals for infection with Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). The test determines if a person’s immune system has mounted a response to components of the bacteria, suggesting a past or present infection. A negative PPD result typically indicates that the person has not been infected with the TB bacterium.
Administering the PPD Test
The PPD test involves injecting a small, carefully measured amount of the PPD solution into the skin. This injection is performed intradermally, into the top layer of the skin, usually on the inner surface of the forearm. The procedure uses a short needle inserted at a shallow angle, typically between 5 and 15 degrees, with the bevel facing up. A correct injection results in the formation of a small, pale, raised blister or wheal, measuring about 6 to 10 millimeters in diameter, which usually disappears shortly after administration.
The reaction relies on the body’s delayed-type hypersensitivity immune response, so it is not immediate. For an accurate reading, the patient must return to a healthcare professional between 48 and 72 hours after the injection. If the reading is performed outside this window, the result is considered unreliable, and the test must be repeated.
Visual Appearance of a Negative Result
A negative PPD test site looks largely unremarkable when examined during the 48- to 72-hour reading window. The most telling characteristic is the complete absence of a firm, raised area (induration) at the injection site. The skin at the testing spot should appear flat and feel soft, blending in with the surrounding forearm tissue.
It is important to differentiate between a true reaction and minor irritation from the injection itself. Slight, superficial redness or a small bruise (erythema) at the site is common and does not indicate a positive result. Redness is a normal, localized response to the needle and fluid. A negative result shows only this minor irritation or the faint mark where the needle entered the skin, with no significant structural change.
Measuring Induration for a Negative Interpretation
Healthcare professionals rely on a specific measurement, not a visual assessment of color, to definitively interpret the test. The measurement focuses exclusively on the diameter of the induration—the hard, dense, and raised area caused by the influx of immune cells. The absence of this induration classifies the result as negative.
To measure the induration, a trained professional palpates the area to locate the margins of the hardened swelling. They then use a millimeter ruler to measure the diameter of this firmness across the forearm, perpendicular to the long axis of the arm. A measurement of 0 millimeters indicates no induration is present, which is a clear negative result.
The official cut-off point for a negative interpretation is generally any induration measuring less than 5 millimeters. This criterion is applied across all risk categories. For individuals with no known risk factors for TB, a positive reaction requires an induration of 15 millimeters or more. Therefore, a reading of less than 5 millimeters is considered a negative result regardless of risk.
Implications of a Negative Result
A negative PPD test result generally means the person has not been infected with M. tuberculosis bacteria. This suggests the immune system has not been sensitized to the tuberculin proteins and did not mount a measurable delayed hypersensitivity reaction. For most individuals, this signifies they do not have a latent TB infection and are not at risk of developing active TB disease.
A negative result does not guarantee freedom from infection in all circumstances, as the possibility of a “false negative” exists. This occurs particularly in individuals whose immune function is compromised, such as those with HIV or those receiving immunosuppressive medications. This reduced immune response, known as anergy, can prevent the body from reacting strongly enough to the PPD solution even if the bacteria are present.
False negatives can also occur if the person was tested too soon after a recent exposure, as it takes the immune system two to eight weeks to develop a reaction. In cases of recent exposure or if symptoms of active TB disease are present, a healthcare provider may recommend repeat testing or a different screening method. This alternative is often a blood test, such as the Interferon-Gamma Release Assay (IGRA), used for confirmation.