Diagnosis of Tuberculosis Disease
Last Updated: April 2006
When Should You Suspect Tuberculosis (TB)?
TB is a disease caused by Mycobacterium tuberculosis. TB disease
should be suspected in persons who have the following symptoms:
- Unexplained weight loss
- Loss of appetite
- Night sweats
If TB disease is in the lungs (pulmonary), symptoms may include:
- Coughing for ³ 3 weeks
- Hemoptysis (coughing up blood)
- Chest pain
If TB disease is in other parts of the body (extrapulmonary),
symptoms will depend on the area affected.
How Do You Evaluate Persons Suspected of Having TB Disease?
A complete medical evaluation for TB includes the following:
1. Medical History
Clinicians should ask about the patient’s history of TB
exposure, infection, or disease. It is also important to consider
demographic factors (e.g., country of origin, age, ethnic or racial
group, occupation) that may increase the patient’s risk for
exposure to TB or to drug-resistant TB. Also, clinicians should
determine whether the patient has medical conditions, especially
HIV infection, that increase the risk of latent TB infection progressing
to TB disease.
2. Physical Examination
A physical exam can provide valuable information about the patient’s
overall condition and other factors that may affect how TB is treated,
such as HIV infection or other illnesses.
3. Mantoux Tuberculin Skin Test and/or QuantiFERON®-TB
The Mantoux tuberculin skin test (TST) and the QuantiFERON®-TB
Gold test (QFT-G) are used to test for M. tuberculosis infection.
Additional tests are required to confirm TB disease. The Mantoux
tuberculin skin test is performed by injecting a small amount of
fluid called tuberculin into the skin in the lower part of the arm.
The test is read within 48 to 72 hours by a trained health care
worker, who looks for a reaction (induration) on the arm.
The QFT-G is a blood test. It measures the patient’s immune
system reaction to M. tuberculosis. Once the blood samples
are taken, they must be processed within 12 hours. Interpretation
of QFT-G results is influenced by the patient’s estimated
risk for TB infection.
4. Chest Radiograph
A posterior-anterior chest radiograph is used to detect chest
abnormalities. Lesions may appear anywhere in the lungs and may
differ in size, shape, density, and cavitation. These abnormalities
may suggest TB, but cannot be used to definitively diagnose TB.
However, a chest radiograph may be used to rule out the possibility
of pulmonary TB in a person who has had a positive reaction to a
TST or QFT-G and no symptoms of disease.
5. Diagnostic Microbiology
The presence of acid-fast-bacilli (AFB) on a sputum smear
or other specimen often indicates TB disease. Acid-fast microscopy
is easy and quick, but it does not confirm a diagnosis of TB because
some acid-fast-bacilli are not M. tuberculosis. Therefore, a culture
is done on all initial samples to confirm the diagnosis. (However,
a positive culture is not always necessary to begin or continue
treatment for TB.) A positive culture for M. tuberculosis confirms
the diagnosis of TB disease. Culture examinations should be completed
on all specimens, regardless of AFB smear results. Laboratories
should report positive results on smears and cultures within 24
hours by telephone or fax to the primary health care provider and
to the state or local TB control program, as required by law.
6. Drug Resistance
For all patients, the initial M. tuberculosis isolate should be
tested for drug resistance. It is crucial to identify drug resistance
as early as possible to ensure effective treatment. Drug susceptibility
patterns should be repeated for patients who do not respond adequately
to treatment or who have positive culture results despite 3 months
of therapy. Susceptibility results from laboratories should be promptly
reported to the primary health care provider and the state or local
TB control program.
Thoracic Society (ATS) and CDC. Diagnostic standards and classification
of tuberculosis in adults and children.(PDF) Am J Respir
Crit Care Med 2000; 161.
and Infectious Diseases Society of America. Treatment of tuberculosis.(PDF)
MMWR 2003; 52 (No. RR-11).
Disease Control and Prevention. Guidelines for the investigation
of contacts of persons with infectious tuberculosis and Guidelines
for using the QuantiFERON®-TB Gold test for detecting
tuberculosis infection, United States. MMWR 2005; 54
Last Reviewed: 05/18/2008
Content Source: Division of Tuberculosis Elimination
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention