Purified protein derivative (PPD) skin tests

When and why

A PPD test involves injecting small amounts of protein from the mycobacteria that cause tuberculosis into the skin. PPD stands for 'purified protein derivative', indicating that the substance injected is non-infectious. This is known as the Mantoux-method TST (tuberculin skin test) .

PPD is only recommended in people who have not had a prior positive test. It should be repeated annually in anyone who is at high-risk of acquiring TB. A person's vulnerability to TB rises as CD4 cell counts fall. If there is induration (swelling, hardening, redness) of 5mm or greater at the injection area 48 to 72 hours after the test, then the test is considered positive.

If there is immune reconstitution in someone on ARV therapy and the CD4 count increases to over 200 cells/mm3, the test should be repeated.

The PPD test is usually given in the forearm. The patient feels a slight prick, but the needle just barely slips under the skin, so it is virtually painless. The patient needs to return to their testing site within two to three days to have their reaction 'read' by a staff member.

How it will help

PPD is used to identify people who have recently been exposed to TB or who have active disease. If subsequent confirmatory testing indicates active infection,  treatment for TB will be started. A PPD test should be offered to all household and close contacts.

Unfortunately, a negative PPD test does not exclude the possibility of active TB. False negative results increase as CD4 cell counts decrease.1 A false negative result may just indicate that the ability to respond to a PPD test is gone, even though TB may be present and/or causing disease. 

A positive PPD does support a diagnosis of culture-negative pulmonary tuberculosis, as well as latent tuberculosis infection in people with stable abnormal chest radiographs.

If a a BCG (Bacille Calmette-Guérin) vaccine was ever given to someone, the PPD result will be positive.

References

  1. Markowitz N et al. Tuberculin and anergy testing in HIV-seropositive and HIV-seronegative persons. Ann Intern Med 119(3):185-93, 1993
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This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.