The Recall or Booster Effect
In most individuals infected with TB, tuberculin skin test (TST) sensitivity persists for many years after infection. However, over time in the absence of re-exposure to tuberculosis or to tuberculin, the size of the TST reaction may decrease or disappear because of waning immune memory.
If the TST is administered to TB-infected individuals with faded immune memory, the reaction (induration) may be small or absent; i.e., falsely negative. However, this TST may restore (or boost) immune memory and there may be a recall response on repeat testing resulting in a positive reaction. This reaction, now positive on the second test, reflects the prior TB infection.
- Because waning takes time, boosting is more common among older persons, especially those more than 55 years of age. However, it is unusual in those older than age 75; perhaps, after a sufficiently long period, sensitivity wanes beyond recall.
- Boosting usually is evident within one week after the initial TST.
While administration of a TST can similarly “boost” an IGRA response, boosting is not considered in interpreting IGRA results.
If an initial TST result is negative, a two-step TST procedure may be required to “boost” a potential reaction that has waned over time in order to establish a reliable baseline. This is important for persons who will undergo periodic (serial) skin testing (e.g., health care workers who may be required to undergo annual testing) and for residents in long-term care facilities, i.e., nursing homes and rest homes. In the two-step method, persons with a negative initial TST, who have not had a TST within the past year, undergo a second TST 1-4 weeks after the first.
Repeated skin testing with PPD will not induce a positive skin test reaction in individuals who have no hypersensitivity to the antigens in PPD. That is, a repeat TST does not produce a false positive reaction.
Two-Step Tuberculin Skin Testing
- Administer 5 tuberculin units of PPD intradermally, according to standard practice.
- Read in 48-72 hours; record induration in mm in patient’s medical record.
- If the initial test is positive (10 mm or greater), no further testing is required.
- If the initial test is negative, repeat the TST in 1 - 4 weeks.
- The result from the second test should be considered the baseline result and recorded in mm induration in the patient’s record.
- If the second test is positive, the individual has a “boosted” reaction and is considered to have a latent TB infection once active TB is excluded.
- If the second test is negative, the individual should be considered uninfected. In the uninfected person, a 10 mm or greater increase on any future TST should be interpreted as a skin test conversion.