Tuberculosis Diagnosis

What is tuberculosis?

Tuberculosis (TB) is an infectious disease caused by bacteria named Mycobacterium tuberculosis. It is spread from person to person in tiny droplets during activities such as coughing, sneezing, or speaking. The disease usually infects the lungs, but the bacteria can attack any area of your body. Symptoms include a bad cough for about a month or more, chest discomfort or pain, and coughing up blood; other symptoms may include sweating at night, weight loss, fever, chills, and no appetite. Children under 15 years of age are more likely to develop life-threatening TB. People with latent TB have no symptoms and are not considered infectious.

Tuberculosis (TB) diagnostic testing

The diagnosis of TB is slightly complicated because of TB may occur in either a latent or active form. Most people think a skin test and chest X-ray will diagnose TB. This assumption is not completely correct. After a doctor has obtained your medical history and performed a physical exam, a skin test (Mantoux tuberculin skin test or TST) is done and read within 48-72 hours after your skin was injected and then examined by personnel trained to examine the skin for a reaction (induration). There is also a blood test termed IGRA (interferon-gamma release assays) that measures your immune response to Mycobacterium tuberculosis. However, both tests if positive only indicate that you have a heightened immune response to the bacteria but does not positively indicate active infection. You may only have a latent infection due to a prior exposure.

The same situation is seen with X-rays; positive findings like miliary lesions (many small lesions usually caused by Mycobacterium tuberculosis spread into the blood) may strongly suggest active TB but do not prove that you have active TB. Another test which is more specific is the acid-fast sputum stain used to show TB organisms that retain this colorful stain. Unfortunately, there are other bacteria that retain the stain that do not cause TB; consequently, this test like the skin, blood, and X-rays provide only presumptive evidence for an active TB diagnosis. It needs to be confirmed, most commonly with a nucleic acid amplification (NAA) test which looks for genetic evidence of the tuberculosis bacteria.

Definitive diagnosis is done when Mycobacterium tuberculosis organisms are cultured from sputum or biopsy tissue. However, most physicians will begin therapy before a definitive diagnosis occurs because it may take weeks to grow these organisms.Your culture of Mycobacterium tuberculosis can provide more than a definitive diagnosis because the bacteria can subsequently be tested for drug resistance. This information will allow doctors to design the best antibiotic regimen to treat you.

People with latent TB infections (LTBI) are diagnosed and treated according to the following 2013 CDC criteria:

  • No symptoms or physical findings suggestive of TB disease/li>
  • TST or IGRA result usually positive
  • Chest radiograph is typically normal
  • If done, respiratory specimens are smear and culture negative
  • Cannot spread TB bacteria to others
  • Should consider treatment for LTBI to prevent TB disease (strongly advised by the CDC)

Tuberculosis (TB) treatments

PPeople with LTBI are encouraged to undergo medical treatment because in some individuals the bacteria may reactivate and cause disease. People with active TB need to start treatments as soon as possible. Many patients begin treatments with only a presumptive diagnosis because their doctors know it is important to treat patients early in the infection. Treatment is with antimicrobial agents -- typically a combination of three or more drugs -- the bacteria are susceptible to in order to slow and/or halt infection-generated symptoms and bacterial spread to other body regions. The CDC lists more than 20 different drugs that are used in various combinations. Your doctor can design the best way to treat you for either latent (LTBI) or active TB infections. Guidelines are available from the CDC.

Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease


United States. Centers for Disease Control and Prevention. "Tuberculosis (TB): Testing & Diagnosis." Apr. 12, 2013. <>.