PPD

A 5-year-old girl is referred to your clinic for evaluation of her tuberculin skin test. She had a Mantoux test containing 5 tuberculin units of purified protein derivative placed intradermally 48 hours ago. Evaluation of the test demonstrates 18 mm of induration. She moved to the United States 3 years ago and was immunized with bacillus Calmette-Guérin (BCG) vaccine at birth. Findings on chest radiography are negative.

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The recommended time for assessing the TST is 48 to 72 hours after administration. The area of induration (not erythema) should be measured using the ballpoint pen technique and recorded in millimeters. A negative TST result never excludes the diagnosis of tuberculosis.

Any child who has a positive tuberculin skin test (TST) of greater than 15 mm induration should be considered to be infected with Mycobacterium tuberculosis.

• +PPD and abnormal CXR. Should be evaluated by the use of early morning gastric aspirates in an attempt to recover the organism for antimicrobial susceptibility testing.

Results should be considered positive if there is more than 5 mm of induration in the following situations:

• children suspected of having tuberculosis (has clinical symptoms or + CXR)
• children in close contact with a known or suspected contagious case of tuberculous disease
• children receiving immunosuppressive medications or who have immunosuppressive conditions.

Positive results are associated with more than 10 mm of induration in the following situations:

• children at risk of disseminated disease (eg, <4 years of age, chronic medical conditions or malnutrition)
• children who have increased exposure to tuberculosis disease (eg, travel and exposure to high-prevalence regions of the world)
• those frequently exposed to adults who are HIV-infected, homeless, users of illicit drugs, residents of nursing homes, incarcerated or institutionalized, or migrant farm workers

Positive results are associated with more than 15 mm of induration in the following situations:

• Children 4 years of age or older without any risk factors

The BCG Vaccine

Live vaccine from M. bovis

• Used primarily in young infants to prevent disseminated, life-threatening TB disease
• Does not prevent infection (LTBI)
• Generally, interpretation of PPD results in BCG recipients is the same as for those who have not received BCG. (The size of a PPD reaction attributable to BCG depends on many factors –age when BCG given, quality and strain of BCG vaccine, number of doses, immune status of child, frequency of PPD.)

PPD and BCG. Reactivity of the TST after receipt of the bacillus Calmette-Guérin (BCG) vaccine depends the following factors (below).  In general, TST results in individuals who have received BCG should be interpreted similarly to those who have not received BCG. Therefore, a positive test result should not be considered a false-positive

• age at which BCG was received
• quality and strain of BCG used
• nutritional and immune status of the recipient
• frequency of the TST administration.
• In fact, some individuals never react to the tuberculin skin test, and repeating the skin test in 3 months for the girl in the vignette is of no value.

References:
Abernathy RS. Tuberculosis: an update. Pediatr Rev. 1997;18:50-58
Full text is available online for subscription or fee.

American Academy of Pediatrics Committee on Infectious Diseases. Active immunization. In: Pickering LK, ed. Red Book: 2003 Report of the Committee on Infectious Diseases. 26th ed. Elk Grove Village, Ill: American Academy of Pediatrics; 2003:7-53