Diagnosis of TB


The steps to diagnose TB infection and disease include:
  • A medical evolution that includes history and risk assessment
  • The tuberculin skin test
  • A chest x-ray
  • A bacteriological examination


The Medical History

The medical history includes the medical, family, social history, and occupational aspects of the patient’s life.

The clinician should ask about :

  • Exposure to a person who has infectious TB

  • Any symptoms of TB (productive cough longer than 3 weeks, fever, unexplained weight loss, and night sweats)

  • The patient’s history of TB infection or TB disease

  • Risk factors for developing the disease (intravenous drug use, condition of the immune system , diabetes)

  • Country of birth

  • Length of immigration time


The Tuberculin Skin Test

The tuberculin skin test is used to determine if a person has TB infection. A substance, called tuberculin, is injected into the skin. Tuberculin is a purified protein. It is not a vaccine and does not contain any live-germ cells. The immune system of most people who have TB infection recognizes tuberculin and this will cause a reaction in the skin.

The Mantoux method of application is the preferred type of skin test because it is the most accurate method. When the Mantoux skin test is performed, a needle is injected into the upper skin layer of the patient's arm. The arm is examined 48 to 72 hours after the tuberculin injection in order to evaluate the reaction on the patient's skin. Any swelling that can be felt around the site of the injection is measured. The diagnosis of TB infection depends on the size of the measured induration and the patient's individual risk factors. If you were vaccinated with BCG, you may have a positive reaction to a TB skin test, but the skin reaction is not large.



A positive reaction probably means that :

  • your skin test reaction is strong

  • you were vaccinated many years ago (because the BCG reaction gets smaller over time)

  • you have spent time with a person with infectious TB

  • someone in your family has had TB

  • you are from a country where TB disease is very common (most countries in Latin America and the Caribbean, Africa, and Asia, except for Japan)


A Chest X-Ray

If TB infection or disease is suspected, a chest x-ray should be taken. The chest x-ray helps the clinician determine if achieve TB disease is present or old healed TB disease. In TB infection, the chest x-ray is usually normal.


A Bacteriological Examination

This is an examination of the secretions from the lungs (sputum), directly under a microscope or by culture. To achieve this examination, the specimen should contain an adequate quantity of TB bacteria.

The faster the bacteriological examination is done, the better the benefit to the patient. In Thailand at present, an effective technique for TB bacteria testing has been developed, called TB Fast Track. This project has been established by the cooperation of the National Center for Genetic Engineering and Biotechnology (BIOTEC), the National Science and Technology Development Agency (NSTDA) and the Faculty of Medicine Siriraj Hospital, Mahidol University. The processes include DNA detection, bacterial culture, and drug susceptibility. These are the most up to date, accurate, precise and sophisticated techniques.


Copyright © 2010 Microbiology Department, Faculty of Medicine, Siriraj Hospital, Mahidol University. All rights reserved.