What is tuberculin purified protein derivative (PPD)?
Tuberculin purified protein derivative (PPD) is a lab-made mixture of proteins derived from Mycobacterium tuberculosis used in skin testing for tuberculosis (TB) infection. It is the antigen used in the Mantoux tuberculin skin test, where a small amount is injected under the skin and the site is examined later for an immune reaction. [1]
How is the PPD skin test done, and when is it read?
In the Mantoux method, a clinician injects a measured dose of PPD into the skin (intradermal injection). The reaction is then checked after a set interval to measure the skin’s response (induration). [1]
What does a positive PPD test mean?
A positive PPD test indicates that the person’s immune system has been sensitized to TB antigens, which can happen after:
- TB infection (including latent TB infection), and/or
- prior BCG vaccination, depending on the situation, and/or
- exposure to other mycobacteria that share antigens. [1]
The PPD test does not, by itself, prove active TB disease; people with positive tests generally need further evaluation based on symptoms, risk factors, and sometimes imaging and microbiology testing. [1]
How do clinicians distinguish latent TB from active TB after a PPD result?
After a positive PPD (or other TB test), clinicians look at:
- symptoms consistent with active TB,
- risk factors for progression,
- chest imaging results,
- and possibly additional lab tests to confirm active disease. [1]
Who uses PPD, and what are the main limitations?
PPD (via Mantoux testing) is commonly used to screen for TB infection, including latent TB. Key limitations include false positives (for example, after BCG vaccination or non-TB mycobacterial exposure) and false negatives (for example, in some immunocompromised people or very early infection). [1]
PPD versus other TB tests: how does it compare?
PPD refers to the antigen used for the skin test. Other TB infection tests include blood-based interferon-gamma release assays (IGRAs), which use different TB antigens than PPD and can be used in similar screening scenarios. The best choice depends on patient factors and local guidance. [1]
Sources
[1] https://www.cdc.gov/tb/publications/factsheets/testing/skintesting.htm