What is PPD?
PPD stands for purified protein derivative. It is an antigen derived from the culture media filtrates of a human strain of Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). In a TB skin test, this protein is injected into the skin to see if a person has a hypersensitivity reaction to it, which would indicate past exposure to the bacteria. The PPD test is also known as the Mantoux tuberculin skin test (TST). A positive reaction—measured as a firm, raised, hardened area (induration)—indicates that a person's immune system has previously encountered TB bacteria.
What is Tubersol?
Tubersol is a specific brand name for tuberculin purified protein derivative, or PPD. It is a standardized, stabilized solution of PPD licensed by the Food and Drug Administration (FDA) specifically for TB screening. The product is available in multi-dose vials for intradermal injection. Tubersol, along with other tuberculin reagents, is used by healthcare professionals for diagnostic purposes, most commonly to aid in the detection of latent TB infection. In essence, when a healthcare provider performs a 'PPD test' using the Tubersol product, they are administering the Tubersol brand of PPD.
The Tuberculin Skin Test (TST) Procedure
Administering a TST is a multi-step process that requires a healthcare professional and two separate visits.
- Injection: A small amount (typically 0.1 mL) of the tuberculin PPD solution, such as Tubersol, is injected intradermally (just under the top layer of the skin) on the inner surface of the forearm. This creates a small, pale wheal or bump that is absorbed within a few hours.
- Observation: The patient must return to the clinic for a follow-up visit, typically between 48 and 72 hours after the injection.
- Measurement: The healthcare provider measures the amount of induration (firm swelling), not just the redness, at the injection site. The measurement is taken across the forearm (perpendicular to the long axis) in millimeters.
- Interpretation: The result's significance is determined based on the size of the induration and the patient's individual risk factors for TB exposure.
PPD vs. Tubersol: A Comparison
To further clarify the distinction, here is a comparison table outlining the key differences between the generic term PPD and the specific brand name Tubersol.
| Feature | Purified Protein Derivative (PPD) | Tubersol (Brand Name) |
|---|---|---|
| Definition | A sterile, purified protein fraction from M. tuberculosis. | A standardized, stabilized, FDA-licensed solution containing tuberculin PPD. |
| Classification | The generic active ingredient. | A specific proprietary brand of PPD. |
| Purpose | The antigen used in the tuberculin skin test (TST). | Used to aid in the diagnosis of TB infection by administering the TST. |
| Availability | Can be manufactured under different brand names, including Tubersol and Aplisol. | A single product manufactured by Sanofi Pasteur Limited. |
| Usage | General term for the substance used in the Mantoux test. | The specific product a clinician might use for the test. |
Factors Affecting Test Results
It is important to understand that a positive PPD test, regardless of the brand used, does not automatically mean a person has active, contagious TB disease. The test simply shows that the person has been exposed to TB bacteria at some point. Additional tests, such as a chest X-ray, are required to check for active disease.
Several factors can influence the results of a PPD test:
- BCG Vaccination: Individuals who have received the Bacille Calmette-Guérin (BCG) vaccine, typically administered outside the United States, may have a false-positive reaction to the PPD test. Healthcare providers should consider a patient's BCG history when interpreting results.
- Weakened Immune System: People with compromised immune systems, such as those with HIV/AIDS or those taking immunosuppressive drugs, may have a false-negative result because their body may not mount a strong enough immune response.
- Recent TB Exposure: The test may be negative immediately following a new infection, as it takes time for the immune system to develop a reaction.
- Other Mycobacteria: Exposure to other species of mycobacteria, not just M. tuberculosis, can also cause a false-positive reaction.
Alternatives to TST
While the TST is a common and effective tool, other diagnostic methods are available, especially for those with a BCG vaccination or weakened immune systems. These include blood tests, often referred to as interferon-gamma release assays (IGRAs), such as the Quantiferon-TB Gold test. IGRAs measure the immune system's response to TB proteins in a blood sample, providing an alternative to the skin test.
For more detailed guidance and information on TB testing and interpretation, refer to resources from authoritative health organizations like the Centers for Disease Control and Prevention.
Conclusion
To recap, Tubersol is a brand name for the purified protein derivative (PPD) that is used to perform a tuberculin skin test. Therefore, the two are not different substances, but rather a specific product (Tubersol) and the general term for the active ingredient (PPD). The TST, whether using Tubersol or another PPD product, is a reliable tool for screening for prior TB exposure, but positive results require further investigation to rule out active disease. Understanding this distinction helps clarify the nature of this common and important diagnostic test.
