Diagnosis of extrapulmonary tuberculosis (TB) can be challenging. Most established TB tests have been optimized for sputum samples and have low sensitivity in different sample types. Additionally, nonsputum samples are often paucibacillary, which further reduces test sensitivity. Because of this, negative test results cannot confirm the absence of extrapulmonary Mycobacterium tuberculosis (MTB) infection. A combination of acid-fast-bacilli (AFB) staining, culture, tissue biopsy, and histologic, immunologic, and nucleic acid-based tests may be required to diagnose extrapulmonary TB.
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People living with HIV are much more likely to become sick with tuberculosis (TB). In fact, TB is one of the leading causes of death among people living with HIV worldwide. HIV infection is the strongest risk factor for progression of latent TB infection (LTBI) to TB disease. Because of their elevated risk, frequent TB screening is recommended for persons living with HIV. Additionally, persons diagnosed with LTBI or TB disease who do not know their HIV status should receive testing to better understand their risk and help inform proper medical management.
Nontuberculous mycobacteria (NTM) include over 160 different bacterial species. The clinical symptoms and acid-fast-bacilli (AFB) smear results for diseases caused by NTM and Mycobacterium tuberculosis (MTB) are often indistinguishable. As a result, patients infected with NTM often receive unnecessary TB treatment. Improper diagnosis and treatment complicate patient management and contribute to the development of drug resistance. As such, the accurate diagnosis of TB and NTM infection is extremely important for the implementation of appropriate treatment and disease reporting. Laboratory approaches such as culture and nucleic acid-based testing are essential for accurate diagnosis.
Latent tuberculosis infection (LTBI) is diagnosed using laboratory tests that identify a positive immune response to Mycobacterium tuberculosis (MTB). These tests are recommended for individuals at high risk for TB exposure. Active tuberculosis (TB) is evaluated using a combination of medical history, physical examination, radiographic imaging, and laboratory testing. Tests such as an acid-fast-bacilli (AFB) smear, culture, and nucleic acid amplification tests (NAATs) indicate the presence of MTB. Additional laboratory testing is required to identify drug resistance or susceptibility. A definitive diagnosis of TB requires positive culture results, although a negative culture does not rule it out.
Individuals who are at increased risk for TB infection should undergo laboratory testing. Those with elevated risk include people who have spent time with someone who has TB disease, people from a country where TB is common, healthcare workers who care for high-risk patients, and people who work in high-risk settings such as correctional facilities or long-term care facilities. Many institutions require LTBI testing before employment or enrollment.
Individuals with clinical suspicion for TB disease should receive diagnostic testing. Pulmonary TB should be suspected in persons who experience coughing for longer than 3 weeks, hemoptysis, or chest pain. Other symptoms of TB disease include unexplained weight loss, loss of appetite, night sweats, fever, or fatigue.
LTBI tests investigate the presence of MTB bacterium in an individual by measuring immune response. Two main types of tests are used to determine the presence of LTBI: skin and blood tests. Screening tests cannot differentiate between LTBI and TB disease and should not be used in patients with active disease symptoms. Generally, screening tests should not be used for testing persons with a low risk of LTBI or TB disease. A tuberculin skin test (TST) or an interferon-gamma release assay (IGRA) can be used interchangeably for persons with moderate to high risk of infection, although clinical evaluation may help determine that one test is better suited for a specific situation.
A TST is performed by injecting tuberculin into the lower arm. The reaction should be evaluated at a second appointment 2-3 days after the initial injection. A positive result indicates the presence of MTB. A negative result indicates that TB infection is unlikely. The TST is the preferred test for children 5 years or younger who are at high risk for infection and disease progression. A TST is not recommended for persons who have been vaccinated for TB with bacille Calmette-Guérin (BCG) because of an increased risk of false-positive results. Persons who have had a severe reaction to a TST previously should instead undergo IGRA testing in future evaluations.
IGRAs measure the immune response to MTB. IGRA testing is preferred in individuals who may have difficulty returning for a follow-up appointment and in those who have received BCG as a vaccine or for cancer therapy. IGRA testing is relatively fast and does not require a follow-up appointment to determine results. However, IGRAs should not be used for children 5 years or younger.
A definitive diagnosis of active TB requires bacteriologic confirmation. For pulmonary TB, the preferred sample type is freshly expectorated sputum; a volume of 5-10 mL is preferred, but testing can be performed on volumes as low as 3 mL. If the patient is not spontaneously producing sputum, induced sputum is also acceptable. The recommended sample type varies for extrapulmonary TB. For a complete and accurate medical evaluation, samples should be tested with a combination of the tests described below. These tests are generally ordered concurrently.
For all patients with suspected pulmonary TB, the CDC recommends performing an acid-fast-bacilli (AFB) smear on three different sputum specimens. A positive AFB smear can indicate TB disease, but does not confirm MTB infection. This is because some nontuberculous mycobacteria (NTM) are also acid fast. AFB smears are rapid first-line tests, and their results correlate with the likelihood of TB transmission, but accurate species identification requires positive culture or NAAT results.
The CDC recommends performing a NAAT on the initial respiratory specimen from patients with suspected pulmonary TB. NAATs are rapid and highly specific, although they are often less sensitive than culture. A NAAT, unlike an AFB smear, can distinguish MTB from NTM. 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) can detect various drug-resistance genes. Molecular tests are recommended for rapid identification of drug resistance and susceptibility. Diagnostic test results and patient history should be taken into account when considering which NAAT is most appropriate.
Mycobacterial culture is a recommended test for the diagnosis of TB, and a positive culture is considered confirmatory. The CDC recommends that all specimens be cultured, regardless of AFB smear results. However, because of the relatively long turnaround time for culture (up to 6-8 weeks), a positive culture is not always necessary to begin or continue treatment. Once growth is detected, species identification can be performed if necessary.
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Once MTB growth is confirmed, drug susceptibility testing should be performed. Culture-based drug susceptibility testing is considered the gold standard to identify drug-resistant TB. At a minimum, susceptibility for isoniazid, rifampin, pyrazinamide, and ethambutol should be evaluated. As in diagnosis, culture is the most sensitive and specific test to elucidate drug resistance and susceptibility, but it is limited by its relatively long turnaround time. Patients who do not respond adequately to treatment or who continue to have positive culture results after 3 months of therapy should have drug resistance and susceptibility testing repeated.
An AFB smear, culture, and NAAT should be used to evaluate extrapulmonary TB, but the selection of sample type and quantity requires clinical evaluation.
This section provides brief descriptions of WHO-recommended technologies for the detection of TB and DR-TB, summarizes WHO recommendations for such technologies and refers to the WHO consolidated guidelines on tuberculosis Module 3: Diagnosis &#; rapid diagnostics for tuberculosis detection (7) for a thorough discussion of the technologies and recommendations.
The guidelines updated in (7) added three new classes of NAAT technologies, as shown in Table 2.1. The most recent update added a fourth new class of technology &#; targeted NGS.
Table 2.1. Classes of technologies and associated products included in current guidelines
The change from product-specific recommendations (e.g. Xpert MTB/RIF or Truenat MTB) to class-based recommendations (e.g. low complexity automated NAATs) was introduced in . We are currently in a transitionary period, during which the previous product-based recommendations will be integrated into the new classes.
The WHO-recommended tests have also been reorganized in this document to clearly delineate their intended use, as per the recommendations. The new organizational structure is as follows:
The initial tests for the diagnosis of TB are broadly grouped as WRDs; these are defined as diagnostic tests that employ molecular- or biomarker-based techniques for the diagnosis of TB (15). The newer, rapid and sensitive molecular tests recommended for the initial detection of MTBC and drug resistance are designated as mWRDs; they include Xpert MTB/RIF Ultra and Xpert MTB/RIF (Cepheid, Sunnyvale, United States of America [USA]), Truenat MTB, MTB Plus and MTB-RIF Dx tests (Molbio Diagnostics, Goa, India) and loop-mediated isothermal amplification (TB-LAMP; Eiken Chemical, Tokyo, Japan).
Also included as mWRDs are the new class of NAATs; that is, the automated moderate complexity NAATs, which detect not only MTBC and RIF resistance but also INH resistance. The four products evaluated and included in this class are Abbott RealTime MTB and MTB RIF/INH assays (Abbott Laboratories, Abbott Park, USA), the BD MAX MDR-TB assay (Becton, Dickinson and Company, Franklin Lakes, USA), the Hain FluoroType MTBDR assay (Bruker/Hain Lifescience, Nehren, Germany), and the Roche cobas MTB and MTB-RIF/INH assays (Hoffmann-La Roche, Basel, Switzerland).
In addition, the biomarker-based lateral flow lipoarabinomannan assay (LF-LAM) test (Alere Determine TB LAM Ag, USA) is recommended to assist in diagnosing TB in selected groups of people living with HIV/AIDS (PLHIV) presumed to also have TB. A positive LF-LAM result is considered to be bacteriological confirmation of TB in these people (15), and this test is also included as a WRD. A negative result does not rule out TB; therefore, LF-LAM should be implemented in parallel with mWRD testing among PLHIV.
WHO has reviewed and recommended each of these tests, and has developed recommendations for their use. In all settings, WHO recommends that rapid techniques be used as the initial diagnostic test to detect MTBC and RIF resistance, to minimize delays in starting appropriate treatment. Follow-on testing for resistance to INH and FQ is important.
Follow-on tests to detect drug resistance include:
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