Mycobacterium tuberculosis - Tuberculosis

Indications for Testing

Risk of Latent Infection

Individuals who are at increased risk for M. tuberculosis exposure should undergo testing for latent TB infection.  Many institutions also require TB testing as a condition of employment or enrollment. Otherwise, testing is not recommended for individuals with a low risk for infection. 

Suspected Active Tuberculosis

Determining whether individuals should undergo diagnostic testing for active TB is based on whether they present with symptoms and/or show radiographic signs of disease. 

Pulmonary TB symptoms include a cough lasting longer than 3 weeks, hemoptysis, chest pain, and shortness of breath. 

Extrapulmonary TB may or may not present with classic symptoms of pulmonary TB and is characterized by symptoms affecting organs other than the lungs. Depending on the site(s) of infection, symptoms may include blood in the urine, headache or confusion, back pain, hoarseness, swollen glands, and swollen joints. 

Laboratory Testing

Tests for Latent Infection

Latent TB infection testing indirectly detects the presence of M. tuberculosis by evaluating an individual’s immune response to specific antigens, either through an interferon-gamma release assay (IGRA) or a tuberculin skin test (TST). ^, 

Interferon-Gamma Release Assay Blood Test

An IGRA is a blood test that detects M. tuberculosis infection by measuring the immune response in a blood sample to polypeptides resembling TB antigens.  These antigens are selected based on their presence in the M. tuberculosis complex but absence from the Bacille Calmette-Guérin (BCG) vaccine and most nontuberculous mycobacteria. 

When available and accessible, the IGRA is the preferred test for individuals 5 years and older,  particularly those who have received the BCG vaccine and/or are unlikely to return for the TST interpretation. 

Mantoux Tuberculin Skin Test

A TST is administered by injecting tuberculin, a protein derived from M. tuberculosis, into the skin of the lower forearm. The injection site is examined 48 to 72 hours later for induration, which is then measured for interpretation.  Clinical interpretation is based on the size of the induration and the individual’s risk for TB exposure and disease progression. 

A positive TST result suggests M. tuberculosis infection and warrants prompt clinical and radiographic evaluation.  A negative result makes infection less likely. However, a negative TST result does not rule out latent infection or active TB, as factors such as young age, malnutrition, immunosuppression, viral infections, and extrapulmonary TB can reduce TST reactivity.  Notably, some children with culture-confirmed TB may initially have a negative TST. Additionally, individuals who have received a BCG vaccine may have a false-positive TST due to cross-reactivity. 

Historically, TST was generally preferred in individuals 5 years or younger.  However, IGRA is considered an acceptable alternative in children 2 years and older and in those who may not return for TST readings. 

Tests for Active Tuberculosis

A definitive diagnosis of active TB requires bacteriologic confirmation. 

For pulmonary TB, the preferred sample type is freshly expectorated sputum.  For adult patients who are not spontaneously producing sputum, induced sputum is acceptable.  Bronchoalveolar lavage fluids are also alternative samples. For children and adolescents who are not spontaneously producing sputum, early-morning gastric aspiration is preferred over sputum induction, as the latter requires specialized expertise.  Nasopharyngeal aspirates and stool samples may be acceptable alternatives when preferred specimen types are difficult to obtain, although they may have lower sensitivity. 

For extrapulmonary TB, samples such as body fluids, tissue biopsies, or aspirates should be collected from the sites of suspected infection. 

For a complete and accurate medical evaluation, samples should be tested with a combination of the tests described in the following sections. These tests are generally ordered concurrently.

Acid-Fast Bacilli Smear

For all patients with suspected pulmonary TB, performing an acid-fast-bacilli (AFB) smear on three different sputum specimens collected at different time points is recommended.  A positive AFB smear can indicate TB disease but does not confirm TB infection. This is because a positive AFB result cannot differentiate M. tuberculosis from other mycobacteria.  AFB smears are rapid first-line tests, but species identification requires positive culture or NAAT results. ^, 

Nucleic Acid Amplification Tests

Performing a NAAT on the initial respiratory specimen in all patients with suspected pulmonary TB is recommended.  NAATs are rapid and highly specific, although they may be less sensitive than culture. A NAAT, unlike an AFB smear, can distinguish M. tuberculosis from other mycobacteria. A positive NAAT result with or without a positive AFB smear is considered sufficient evidence for TB diagnosis. 

NAATs and other molecular tests (such as sequencing) are also used to detect drug resistance. ^,  In individuals with positive AFB and/or NAAT results and risk factors for drug-resistant TB, molecular testing for drug resistance and susceptibility should be ordered immediately to inform timely treatment. 

Although NAATs continue to advance and are increasingly used in the diagnostic evaluation of TB, it is important to recognize that a negative result alone does not rule out TB or drug resistance.  This is because the detection capabilities of NAATs depend on known genetic markers and the technical limitations of each assay. 

Culture

Mycobacterial culture is recommended for all individuals with suspected TB disease, as culture enables a definitive diagnosis and yields isolates that may be needed for further testing and public health evaluation. ^,  Additional testing includes phenotypic drug susceptibility testing, genotyping, rapid molecular detection of drug-resistance genes, and precise species identification within the M. tuberculosis complex.  However, because of the relatively long turnaround time (up to 10 weeks with solid media and 1 to 6 weeks with liquid media),  more rapid tests such as NAATs are often ordered concurrently to guide timely clinical decisions.