Making a diagnosis of tuberculosis typically includes a review of the patient's medical history, a physical exam, and a TB skin test. However, a positive TB skin test doesn't necessarily mean the person has active tuberculosis. There are other tests, such as x-rays and sputum samples, that can be administered after a positive TB skin test result to confirm whether the person has active tuberculosis. Because tuberculosis bacteria grow very slowly, it can take up to 4 weeks to confirm the diagnosis.
An Overview of Tuberculosis Diagnosis
Tuberculosis may be suspected based on the patient's medical history (including current symptoms and medical conditions) and a physical exam.
Specific tests will help the doctor make the tuberculosis diagnosis and determine whether it is active or latent tuberculosis (see Latent Versus Active Tuberculosis). These tests can include a TB skin test (PPD test) and other tests, such as a chest x-ray and samples of sputum.
TB Skin Test for Diagnosing Tuberculosis
To identify those who may have been exposed to tuberculosis, healthcare providers typically inject a substance called tuberculin under the skin of the forearm. If a red welt forms around the injection site within 72 hours, the person may have been infected. This doesn't necessarily mean he or she has active tuberculosis. People who may test positive on the tuberculin test include:
- Most people with previous exposure to tuberculosis bacteria (Mycobacterium tuberculosis)
- Some people exposed to bacteria related to the TB bacteria
- Some people born outside the United States, who were vaccinated with the TB vaccine used in other countries.
Other Tests Used to Make a Diagnosis
If people have a positive TB test, other tests can help to show if they have active tuberculosis. In making a diagnosis of active tuberculosis, doctors rely on symptoms and other physical signs, the person's history of exposure to tuberculosis, and x-rays that may show evidence of a tuberculosis infection.
The healthcare provider will also take sputum and other samples to see if the TB bacteria will grow in the lab. If bacteria are growing, this positive culture confirms a tuberculosis diagnosis. Because M. tuberculosis grows very slowly, it can take up to 4 weeks to confirm the diagnosis. An additional 2 to 3 weeks are usually needed to determine which antibiotics to use to treat the disease.