Tuberculosis

Testing for TB Disease and Infection

TB should be considered in any patient who has:

  • A cough, persistent for 3 weeks.

  • Other signs or symptoms compatible with active TB, eg. bloody sputum, weight loss, fever and night sweats.
  • CXR changes consistent with pulmonary TB.

    Groups who should be tested

  • Close contacts of a person kmown or suspected to have TB
  • Overseas Immigrants from high-risk areas, eg. Asians, Hispanics, African Americans, Russians, Pacific Islanders, Homeless people and Aboriginal and Torres Strait Islanders
  • Healthcare Workers working with high-risk clients
  • Illicit drug and alcohol users
  • Residents and employees of Instutions, eg. prisons and nursing homes
  • Persons with HIV infection

    Tuberculin Skin Testing

    Skin testing for tuberculosis is useful in countries where the incidence of tuberculosis is low, and the health care system works well to detect and treat new cases. In countries where BCG vaccination has been widely used, the TB skin test is not useful, because persons vaccinated with BCG will have a positive skin test.

    The TB skin test is based upon the type 4 hypersensitivity reaction. If a previous TB infection has occurred, there are sensitised lymphocytes that can react to another encounter with antigens from TB organisms. For the TB skin test, a measured amount of tuberculin purified protein derivative (PPD) is injected intracutaneously to form a small wheal, typically measured on the forearm. In 48 to 72 hours, a positive reaction is marked by an area of red induration that can be measured by gentle palpation (redness from itching and scratching doesn't count). Reactions over 10 mm in size are considered positive in non-immunocompromised persons.

    Injecting PPD intracutaneously,gross.

    A TB skin test is usually performed on the forearm. After sterilising the skin, a measured amount of PPD is injected intracutaneously.

    A properly placed skin test, gross.

    After the PPD is injected intracutaneously, a small wheal appears on the forearm. The patient is instructed not to scratch the wheal, as it tends to itch.

    A positive skin test, gross.

    Within 48 - 72 hours, a positive skin test is marked by an area of reddish induration greated than 10mm. It is the induration(firm bump) that is gently palpated that determines the size, not the area of redness. This reaction is slightly larger than the average positive test. This one is 17mm in size. The positive reaction shown here was obtained with a TB skin test performed 20 years after the initial infection.

    Repeated testing may increase the size of the reaction (induration), but repeated TB skin testing will not lead to a positive test in a person not infected by TB. Anergy, or absence of PPD reactivity in persons infected with TB, can occur in immunocompromised persons, or it may even occur in persons newly infected with TB, or in persons with miliary TB.