Why Do I Need an Annual PPD?

Every health care provider needs an annual assessment for various federal and state requirements. Part of that assessment includes screening for the bacterial infection tuberculosis (TB). Basically, there are two forms of TB infection: (1) active infection and (2) latent tuberculosis infection (LTBI). With active TB infection, typically the symptoms are obvious: weight loss, coughing up blood, and night sweats. However, someone can have an active TB infection without a cough. LTBI on the other hand rarely shows any symptoms at all. Thus, the need for annual screening. The World Health Organization estimates that 2 billion people worldwide have LTBI. The problem with LTBI is that if untreated, may activate at some point and cause an acute infection.

Screening for TB is done via skin testing by the Tuberculin Skin Test (TST) containing purified protein derivative (PPD) and/or with the newer blood tests called Interferon Gamma Release Assays (IGRAs). It depends upon the institution regarding their policy on TB screening and which method they use as both have advantages and disadvantages.

For the TST, 0.1 cc of PPD is drawn up in a tuberculin syringe and is implanted intradermally into the forearm area. A proper implantation results in a wheal formation at the site. The area is then checked in 48-72 hours to measure any induration in millimeters. Many institutions require an initial two-step test which is essentially 2 TSTs about a week apart to catch any delayed response to the initial PPD.  Healthcare staff are considered positive if the induration measures 10 mm or more.

The main advantage to the TST is it’s low cost. The disadvantages include making the employee return twice for one test (one for the implantation and one for the reading), variability of implantation technique and interpretation (i.e. ensuring the results are read in mm), relative low specificity and sensitivity, and positively reacting to foreign born individuals that were administered BCG vaccine.

The IGRA blood tests are a relatively new way to test individuals for both active TB infection and LTBI. Advantages of IGRAs include: just 1 visit for the blood draw, higher sensitivity and specificity, and the ability to distinguish whether a person is truly positive if they were vaccinated with the BCG vaccine. The disadvantages include higher costs and the shorty window in which the sample must be processed.

A positive TST or IGRA requires additional screening and will include a chest x-ray (or sometimes sputum cultures) to rule out active infection.  Assuming a negative chest x-ray, the recommendation will be made to treat the LTBI with an antibiotic for 9 months to ensure eradication of the bacteria. One important thing to note is that if someone has LTBI, it can not be spread to anyone else. Active TB infection can only be spread via airborne droplets.

So the next time you are required to have a mandatory annual assessment, you know why and how you will be screened for TB!