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Diagnosis {{#invoke:main|main}}

M. tuberculosis (stained red) in sputum

Active tuberculosis

Diagnosing active tuberculosis based merely on signs and symptoms is difficult,<ref name=DiagP2011>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> as is diagnosing the disease in those who are immunosuppressed.<ref name=Clinic2009>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> A diagnosis of TB should, however, be considered in those with signs of lung disease or constitutional symptoms lasting longer than two weeks.<ref name=Clinic2009/> A chest X-ray and multiple sputum cultures for acid-fast bacilli are typically part of the initial evaluation.<ref name=Clinic2009/> Interferon-γ release assays and tuberculin skin tests are of little use in the developing world.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref name="Sester 100–11">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> IGRA have similar limitations in those with HIV.<ref name="Sester 100–11"/><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

A definitive diagnosis of TB is made by identifying M. tuberculosis in a clinical sample (e.g., sputum, pus, or a tissue biopsy). However, the difficult culture process for this slow-growing organism can take two to six weeks for blood or sputum culture.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> Thus, treatment is often begun before cultures are confirmed.<ref name=NICE2011/>

Nucleic acid amplification tests and adenosine deaminase testing may allow rapid diagnosis of TB.<ref name=DiagP2011/> These tests, however, are not routinely recommended, as they rarely alter how a person is treated.<ref name=NICE2011/> Blood tests to detect antibodies are not specific or sensitive, so they are not recommended.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Latent tuberculosis

See: Latent tuberculosis

The Mantoux tuberculin skin test is often used to screen people at high risk for TB.<ref name=Clinic2009/> Those who have been previously immunized may have a false-positive test result.<ref name=Rothel_2005>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The test may be falsely negative in those with sarcoidosis, Hodgkin's lymphoma, malnutrition, or most notably, in those who truly do have active tuberculosis.<ref name=Robbins/> Interferon gamma release assays (IGRAs), on a blood sample, are recommended in those who are positive to the Mantoux test.<ref name=NICE2011>National Institute for Health and Clinical Excellence. Clinical guideline 117: Tuberculosis. London, 2011.</ref> These are not affected by immunization or most environmental mycobacteria, so they generate fewer false-positive results.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> However, they are affected by M. szulgai, M. marinum, and M. kansasii.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> IGRAs may increase sensitivity when used in addition to the skin test, but may be less sensitive than the skin test when used alone.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

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