Positive TB Skin Test

Criteria for a positive PPD intermediate skin test include

(Jasmer RM, Nahid P, Hopewell PC. Latent tuberculosis infection. N Engl J Med 2002; 347:18601866.)

  1. Induration of 5 mm or more:
    • HIV infected persons
    • Close contacts of persons with infectious TB
    • Person with an abnormal chest x-ray consistent with previous TB
      • An abnormal chest x-ray consistent with TB reveals fibrotic opacities occupying more than two square centimeters of the upper lobe
      • Chest x-rays showing pleural thickening or isolated calcific granulomas are not considered suggestive of previous TB
    • Immunosuppressed patients receiving the equivalent of 15 mg or more of prednisone per day for 1 month or more

  2. Induration of 10 mm or more:
    • Foreign-born persons recently arrived (less than 5 years earlier) from a country with a high prevalence of TB
    • Persons with a medical condition, which increases the risk of TB
      • Silicosis
      • End-stage renal disease
      • Malnutrition
      • Diabetes mellitus
      • Carcinoma of the head and neck
      • Immunosuppressive therapy
      • Leukemia
      • Loss of more than 10 % of ideal body weight
      • Gastrectomy or jejunoileal bypass
    • Illicit injection drug users
    • Members of underserved, low-income, populations (e.g. homelessess)
    • Residents and staff members of long-term care facilities (e.g. nursing homes, correctional institutions, homeless shelters)
    • Health care workers
    • Children less than 4 years
    • Persons with conversion on a tuberculin skin test (increase in induration of greater than 10 mm within a 2-year period)

  3. Induration of 15 mm or more:
    • All others

Routine tuberculin skin testing is not recommended for populations at low risk for a latent TB infection (LTBI).

If treatment of a LTBI after a positive TB skin test is to be undertaken, it should only be instituted after negative results have been obtained via a normal chest x-ray and if the chest x-ray is suspicious for active TB, a TB culture must be obtained to rule out active TB.

Booster Phenomenon

All health care workers must be aware of what is called a “booster” phenomenon to a tuberculin skin test. Over time, delayed hypersensitivity resulting from a mycobacterial tuberculin skin test despite the presence of true infection may wane in some persons, resulting in a non-reactive tuberculin skin test despite the presence of true infection.

However, in a patient who is infected with TB whose positive TB skin test (PPD) has waned and become negative with time, the negative PPD tuberculin test may “boost” (increase the size of the reaction) to a second test administered soon thereafter, resulting in a positive tuberculin test and misleadingly suggesting a tuberculin skin test conversion.

To avoid this problem, persons who will undergo annual tuberculin testing (e.g. health care workers) should undergo two-step testing on initial evaluation with a second tuberculin skin test administered one week after a negative initial PPD tuberculin skin test.

* Note well that a true negative PPD skin test cannot be boosted.

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