STEVEN C BUSSELEN wrote:
Does anyone know of clear PPD guidelines that include a CLEAR/CONCISE explanation of how and why to do the 2-step PPD?
Among other questions, a couple of members of our staff are wondering if there is a clear deadline for the 2nd PPD.
I reviewed the UpToDate article, which is neither clear nor concise. It suggests waiting 1-3 weeks as a general guideline. I can't see any reason to subject a patient to two more PPDs, just because they waited a month or two after the first PPD. Isn't the goal to deal with the booster phenomenon by having the 2nd PPD reasonably soon after the 1st, so patients getting annual PPD's don't have to face tough questions if the 2nd PPD comes back positive (Did they get exposed in the last year?)
Steven Busselen, MD
The "booster" phenomenon was studied as means of reducing false negative PPD-TST frequencies in specific populations whose immune system was compromised or WEAK. It was thought to be particularly useful in new admission to nursing homes when the admittee was old and debilitated. These patients were not classical immuno-compromised patients, but the 2 step principles apply to the entire group of AIDS, metastatic Ca, dementia, etc patients. Generally, the second PPD was given two weeks later, and several gained professorships in the study of this phenomenon.
But, I know of no controlled studies indicating that lives were saved. It seemed logical, and hordes "true believers" sought to have the 2 step PPD as a regulatory mandate. Some succeeded. I would prefer that knowledgable physicians be allowed to use their judgement on a case by case basis.
One medical school in Tennessee, for a while, required that all entering students submit to a two step test. BayesTheorem would suggest that they had misapplied his basic principles sensitivity and specificity. Nevertheless the student health section did it anyway. Another false step for the American Medical Education system.
Similarly there have been no head to head comparisons selecting one "best" interval since the basic test remains untested by randomized outcome studies.
But physicians and nurses have to be very careful in this area, since America is home to the "TB police" with actual incarceration for those who do not follow these arcane regulations in an exact way.
With best wishes for your professional success,
Wm. MacMillan Rodney MD, FAAFP, FACEP
American Board of Family Medicine Obstetrics
Chair, Academic Affairs
Medicos para la Familia
Nashville, Memphis, and International
Two-step (tuberculin) skin test. A procedure used for baseline skin testing of persons who will periodically receive TSTs (e.g., health-care workers or residents of long-term--care facilities) to reduce the likelihood of mistaking a boosted reaction for a new infection. If an initial TST result is classified as negative, a second test is repeated 1--3 weeks later.
If the reaction to the second TST is positive, it probably represents a boosted reaction, indicating that the infection was most likely in the past and not recent. If the second TST is also negative, the person is classified as not being infected. Two-step skin testing has no place in contact investigations or in other circumstances in which ongoing transmission of M. tuberculosis is suspected.
Booster Phenomenon and Two-Step Tuberculin Skin Testing
Periodic use of the tuberculin skin test is valuable for the surveillance of tuberculin-negative persons at risk for exposure to M. tuberculosis.
Repeated testing of uninfected persons does not sensitize them to tuberculin. However, delayed-type hypersensitivity resulting from mycobacterial infection or BCG vaccination may gradually wane with years. Although subsequent initial skin testing may be negative, the stimulus of a first test may boost or increase the size of the reaction to a second test administered 1 week to 1 year later and thus may suggest an apparent -- but false -- tuberculin conversion.
Although the booster phenomenon may occur at any age, its frequency increases with age and is highest among persons >55 years of age and/or among those persons who have had prior BCG vaccination (18).
When tuberculin skin testing of adults is repeated periodically, as in employee-health or institutional screening programs, an initial two-step approach can reduce the likelihood that a boosted reaction will be misinterpreted as a recent infection. If the first tuberculin test result is negative, a second 5-TU test should be administered 1 week to 3 weeks later. A positive second result probably indicates boosting from a past infection or prior BCG vaccination.
Persons having a boosted reaction should be classified as reactors, not converters. If the second result is negative, the person is probably uninfected, and a positive reaction to subsequent tests indicates a true tuberculin skin-test conversion (see Definition of a Tuberculin Skin-Test Conversion).
Because of problems with continued cross-reactions with other mycobacteria, the specificity of the tuberculin test is less when serial skin testing is performed than when a single test is administered. Thus, serial skin-testing programs tend to overestimate the incidence of new TB infection in the tested population. Because of this potential for overestimation of incidence, serial skin-testing programs should be targeted to populations at high risk for continued exposure to infectious TB.
UK has a nice article on TB testing...
Boosting is best distinguished from conversion on clinical grounds. One can attribute an increase in reaction size to boosting when the increase in reaction is seen after an interval of 1 to 5 wk during which there has been no possibility of exposure, such as preemployment testing of a health care worker. Conversion can be confidently stated to have occurred when a previously tuberculin-negative individual becomes tuberculin test positive after receiving BCG vaccination, or following significant exposure such as during an outbreak or as a result of close contact with a highly contagious index case. An increased reaction is more likely because of new mycobacterial infection (i.e., true conversion) if several prior tuberculin tests were negative, particularly if baseline two-step tuberculin testing was performed (12)
This resource says boosting up to 1-year later, but no reference.
Some people with LTBI may have a negative reaction to the TST if many years have passed since they became infected. They may have a positive reaction to a subsequent TST because the initial test stimulates their ability to react to the test. This is commonly referred to as the “booster effect” and may incorrectly be interpreted as a skin test conversion (going from negative to positive). For this reason, the “two-step method” is recommended at the time of initial testing for individuals who will be tested periodically (e.g., health care workers). If the first test result in the two-step baseline testing is positive, consider that the person has LTBI and evaluate and treat the person accordingly. If the first test result is negative, the second step of the two-step baseline testing should be repeated in 1–3 weeks. If the second test result is positive, consider that the person has LTBI and evaluate and treat the person accordingly; if both steps are negative, consider the person uninfected and classify the TST as a negative baseline (see Figure 1).
Figure 1: Two-Step Tuberculin Skin Test (TST) Method
1st TST Negative → Repeat TST in 1–3 weeks
2nd TST Negative → Person probably does not have infection
Positive → Boosted reaction due to infection in the past
Note: A single step approach would be used for serial testing at baseline with QFT because boosting does not occur with QFT.
Kelly Locke, MD
Index--TB screening, 2 step PPD, booster pnehnomenon