From the Guidelines
The culture test is the definitive test for confirming tuberculosis, as it grows the Mycobacterium tuberculosis bacteria from a patient's sample and allows for drug susceptibility testing. This is based on the most recent and highest quality evidence available, including a study published in the Morbidity and Mortality Weekly Report in 2009 1. The culture test remains the gold standard for laboratory confirmation of TB, despite taking 2-8 weeks for results, because it confirms viable bacteria and allows for drug susceptibility testing.
Diagnosis of Tuberculosis
Diagnosis of tuberculosis typically involves multiple tests, including:
- A tuberculin skin test (TST) or interferon-gamma release assay (IGRA) blood test to screen for TB infection
- Sputum smear microscopy to provide preliminary results for active TB disease, although it is less sensitive than culture
- Nucleic acid amplification tests (NAATs) like GeneXpert MTB/RIF, which offer rapid results within hours and can detect drug resistance
- Chest X-rays, which are helpful but not definitive
- Samples from affected sites, such as lymph nodes or cerebrospinal fluid, for extrapulmonary TB
Importance of Culture Test
The culture test is essential for confirming the diagnosis of tuberculosis, as it provides a definitive identification of the bacteria and allows for drug susceptibility testing. This information is critical for guiding treatment and preventing the development of drug-resistant strains of TB. As noted in a study published in the American Journal of Respiratory and Critical Care Medicine in 2003, the decision to initiate treatment should be based on epidemiologic information, clinical and radiographic findings, and the results of microscopic examination of acid-fast bacilli (AFB)–stained sputum and cultures for mycobacteria 1.
Clinical Practice
In practice, doctors often start treatment based on strong clinical suspicion and preliminary test results while awaiting culture confirmation. This approach is supported by a study published in MMWR Recommendations and Reports in 2006, which notes that a culture of sputum or other clinical specimen that contains M. tuberculosis provides a definitive diagnosis of TB disease, and that identification of M. tuberculosis and drug-susceptibility results are available within 28 days using recommended rapid methods 1.
From the Research
Diagnosis of Tuberculosis
The diagnosis of tuberculosis (TB) can be confirmed through various tests, including:
- Bacilloscopy: a microscopic examination of sputum or other bodily fluids to detect the presence of Mycobacterium tuberculosis 2
- Liquid medium cultures: a laboratory test that involves growing the bacteria in a liquid medium to detect the presence of M. tuberculosis 2
- Genetic amplification techniques: such as polymerase chain reaction (PCR) or other molecular assays that can detect the genetic material of M. tuberculosis 2, 3, 4, 5, 6
- Tuberculin Skin Test (TST): a skin test that measures the immune response to M. tuberculosis, with a positive result indicating infection 2
- Interferon-Gamma Release Assays (IGRA): a blood test that measures the immune response to M. tuberculosis, with a positive result indicating infection 2
Molecular Assays
Molecular assays, such as PCR, have been shown to be sensitive and specific for the detection of M. tuberculosis in clinical samples, including sputum, pleural fluid, and cerebrospinal fluid 3, 4, 5, 6. These assays can detect the presence of M. tuberculosis DNA in clinical samples, even in cases where culture is negative.
Culture
Culture is considered the gold standard for the diagnosis of TB, but it can take several weeks to obtain results 2. Liquid medium cultures are more sensitive than solid medium cultures and can detect the presence of M. tuberculosis in clinical samples more quickly 2.
Combination of Tests
A combination of tests, including bacilloscopy, culture, and molecular assays, may be used to confirm the diagnosis of TB 2. The choice of test depends on the clinical presentation, the availability of resources, and the suspected location of the infection.