TB Infection

When a person has been infected with Mycobacterium tuberculosis they have a tuberculosis infection, which is different than having tuberculosis. Tuberculosis infection is asymptomatic (does not cause symptoms) and is confirmed by a positive purified protein derivative (PPD) skin test.

The PPD is derived from a protein found in the tuberculosis bacteria. The protein stimulates a response (localized swelling) in a person whose immune system has been exposed to the bacteria. A certain size swelling indicates a reaction in the PPD.

This test may produce false positives and false negatives. Because a false positive result may lead to unneeded therapy that carries its own risks, skin testing generally is performed only in those who are at risk for TB infections. Those at risk include, but are not limited to, people who have the following:

  • Signs, symptoms, and/or laboratory findings suggesting active TB
  • Recent contact with someone who has or may have active TB
  • HIV infection or AIDS
  • Abnormal chest x-ray compatible with past TB
  • Silicosis (respiratory disease caused by the inhalation of dust containing silica)
  • IV (intravenous) drug abuse habit
  • Diabetes mellitus
  • Long-term corticosteroid therapy
  • Immunosuppressive therapy (e.g., organ transplant recipients, cancer patients)
  • Endstage renal disease (kidney failure)
  • Clinical conditions associated with rapid weight loss
  • Hematologic (blood) and/or reticuloendothelial (affecting macrophages and liver, spleen, and bone marrow cells) diseases
  • Emigrated from Asia, Africa, Latin America, or Oceania (areas with prevalence of TB)
  • Inadequate health care (i.e., Third World populations, prison populations, nursing home residents, mental illness facility residents)

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 31 May 2000

Last Modified: 08 Oct 2015