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TB Infection

When a person has been infected with Mycobacterium tuberculosis they have a tuberuclosis infection, which is different than having tuberculosis. Tuberculosis infection is asymptomatic 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:

• 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 the disease)
• inadequate health care (i.e., Third World populations, prison populations, nursing home residents, mental illness facility residents)

Treatment of TB Infection

The reason for treating a patient with asymptomatic tuberculosis infection is that there is a 5% risk for developing TB during the first year after infection and an additional 5% risk for developing TB over a lifetime. Also, treatment of asymptomatic tuberculosis infection is much simpler than treating TB.

The size of induration (swelling) and the clinical setting affect the likelihood that a positive PPD skin test represents a real infection. For instance, 5 mm of induration is significant in someone who has had recent contact with active TB, but not in a person without risk factors. The main indications for preventive therapy for tuberculosis-infected patients are listed below:

• PPD > 5 mm in the following patient groups:
  • Adults and children with HIV infection
  • Close contacts with infectious cases of TB
  • Patients with fibrotic lesions on chest x-ray
  
  
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• PPD > 10mm in the following patient groups:
  • Adults and children at risk for developing TB
  • Intravenous drug users
  • Immigrants from countries where tuberculosis is prevalent
  • Members of racial and ethnic minorites with an increasing incidence of tuberculosis
  • Medically underserved persons
  • Residents of long term care facilities, including prisons, mental institutions, and nursing homes
  • Patients with silicosis
  • Postgastrectomy patients
  • Postjejunoileal bypass patients
  • Malnutrition
  • Diabetes
  • Chronic renal failure
  • Immunosuppressive therapy (e.g., transplant patients)
  • Workers in health care facilities, hospital microbiology laboratories, schools, and daycare facilities
  • Children under age 4
• Patients over age 35 with an increase > 15 mm in the area of PPD over 2 years
• Patients under age 35 with an increase > 10 mm in area of PPD over 2 years
• Patients with HIV infection exposed to an active case of TB, regardless of PPD reaction

Isoniazid
Treatment lasts 6 months in an adult without HIV, 12 months in an adult with HIV, and 9 months in children. This drug has side effects.

Rifampin
This is the second most commonly used drug. Generally, the course of therapy with rifampin is 6 to 12 months. If a patient has been exposed to multidrug-resistant Mycobacterium tuberulosis, individualization of drug therapy is required. Side effects have been noted with this drug.

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