Diagnosing TB

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

• Fever

• Fatigue

If TB disease is in the lungs (pulmonary), symptoms may include:

• Coughing for  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 Gold Test

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.