A PPD test is a widely used skin test to identify individuals who may have been exposed to the bacterium causing tuberculosis. It involves introducing a small amount of a specific protein derivative under the skin to observe the body’s immune reaction, screening for a prior immune response.
Understanding the PPD Test
The PPD test uses Purified Protein Derivative (PPD), a sterile solution containing proteins from Mycobacterium tuberculosis. These proteins do not cause disease but elicit an immune response in individuals previously exposed to the bacterium. The test indicates whether a person’s immune system has encountered the bacterium, not if they have active tuberculosis disease. It functions by detecting a delayed-type hypersensitivity reaction, a type of immune memory.
When the PPD solution is introduced, immune cells (T-lymphocytes) recognize these proteins if prior exposure occurred. This triggers a localized inflammatory response at the injection site. This reaction signifies the immune system has been sensitized to the tuberculosis bacterium, even if exposure did not lead to active illness. The PPD test is a valuable tool for identifying individuals who may require further evaluation for tuberculosis infection.
Preparing for PPD Administration
Selecting the correct injection site is important, with the inner surface of the forearm being preferred. This area has less hair and pigmentation, making it easier to observe and measure the reaction. The site should be free of lesions, scars, or veins.
Before injection, thoroughly clean the chosen skin area with an alcohol swab. Allow the alcohol to air dry completely, as residual alcohol can inactivate the PPD solution. Necessary equipment includes a tuberculin syringe, designed for intradermal injections, and the PPD solution. Inform the patient about the procedure, including the slight discomfort and the need to return for reading.
The PPD Administration Procedure
The standard dose for a PPD test is 0.1 mL of Purified Protein Derivative solution. This precise volume ensures a consistent amount of antigen for standardized interpretation. Use a tuberculin syringe with a short, fine-gauge needle (typically 26-27 gauge).
To perform the injection, hold the syringe almost parallel to the skin surface, at a 5 to 15-degree angle, with the bevel facing upwards. Insert the needle just beneath the skin’s surface, ensuring the bevel is entirely covered but still visible. Slowly inject the 0.1 mL of PPD solution, which should create a distinct, pale wheal or bleb, approximately 6 to 10 mm in diameter. After the injection, carefully withdraw the needle at the same angle it was inserted, without applying pressure or massaging the site, as this could disperse the solution and affect accuracy.
Reading and Interpreting PPD Results
The test site must be examined between 48 and 72 hours after the injection. Reading outside this timeframe can lead to inaccurate results, either false negatives if read too early or false positives if read too late. The key measurement is the diameter of the induration, the firm, raised area, not the surrounding redness.
To measure the induration, gently palpate across the forearm to locate the raised area. Use a flexible ruler to measure its widest diameter, perpendicular to the forearm’s long axis. Record the measurement in millimeters. Different induration sizes have varying significance based on an individual’s risk factors for tuberculosis. For instance, an induration of 5 millimeters or more may be considered positive in individuals with recent close contact to active TB or those with weakened immune systems. An induration of 10 millimeters or more can be positive for individuals with certain medical conditions or those from high-prevalence countries. For individuals with no known risk factors, an induration of 15 millimeters or more is generally considered a positive result.
What PPD Results Indicate
A “positive” PPD result generally indicates that an individual has been infected with the tuberculosis bacterium at some point. Their immune system has developed a response to the PPD proteins, suggesting prior exposure, but it does not differentiate between latent tuberculosis infection and active tuberculosis disease. Individuals with a positive PPD result require further medical evaluation to determine if they have active disease or a latent infection.
Conversely, a “negative” PPD result typically suggests that the individual has not been infected with the tuberculosis bacterium. This indicates their immune system did not react to the PPD solution, implying no prior exposure or no detectable immune response. A negative result does not completely rule out tuberculosis infection, particularly in severely immunocompromised individuals or those with very recent exposure. A positive PPD result often prompts additional diagnostic tests, such as a chest X-ray or sputum tests, to assess for active tuberculosis disease.