The Recall or Booster Effect

On sequential Mantoux testing, some persons may show a marked increase in the size of their tuberculin reactions; this may be due to tuberculosis infection in the interval between tests, or to recall (boosting) of a previous ability to react to the test dose of initial infection. Thus, older persons who previously had been infected with tuberculosis may now have negative reactions when tested. Their lymphocytes no longer react to a 5 TU test dose. That dose of tubercle protein, too small to initiate sensitivity, can "remind" the lymphocytes that they once had been sensitized and "boost" sensitivity to its former level.

  • 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.

  • It usually is evident within one week after the test dose.

  • Small reaction are most affected.

How can we distinguish between a conversion reaction caused by boosting and one caused by a new infection? For example, if a health care worker had a negative reaction a year ago and is now positive, he could have acquired a new infection during that period, or alternatively, he could have had a boosted reaction. A two-step initial testing procedure allows the clinician to distinguish between conversions caused by boosting and those caused by new infections. With this strategy, the initial test with 5 TU PPD is given, read, and recorded two to three days later. If the size of the reaction is classified as positive, no further tuberculin testing is needed, and the person should be given the appropriate diagnostic tests and treatment. If the reaction is negative, the test should be repeated in one to two weeks.

A reaction that converts from negative to positive within one week and in which the area of induration increases by at least 6 mm is considered the result of boosting. It is unlikely that someone would have become infected during a single week.

Two Stage Tuberculin Testing

  1. 1st test given, read and record

  2. If 1st test is positive -- no further testing

  3. If 1st test is negative -- repeat in 1 - 2 weeks

  4. Considered boosting -- 2nd test is positive - induration increases 6 mm.


This information is provided by Tuberculosis Prevention and Control Program within the Department of Public Health.