Mycobacterium tuberculosis: Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis bacteria. TB primarily targets the lungs but may affect any area of the body such as the urinary tract, central nervous system, bones and/or joints, other organs, abdominal area, or lymph nodes. A tuberculosis infection that has spread through the blood to various organs of the body is termed miliary tuberculosis. With respiratory infections, TB is spread through the air from person to person through droplets of respiratory secretions such as sputum or aerosols released by coughing, sneezing, laughing, or breathing. Most of those who become infected with M. tuberculosis manage to confine the mycobacteria to a few cells in their lungs, where they stay alive but in an inactive form. This latent TB infection does not make the person sick or infectious and, in most cases, it does not progress to active tuberculosis.

However, some people – especially those with compromised immune systems – may progress directly from initial TB infection to active tuberculosis. People who have HIV are much more likely to become sick if they contract TB. In another 5-10% of those with latent TB infection that has not been treated and who are not co-infected with HIV, the mycobacteria will later be reactivated and begin to multiply, leading to active progressive tuberculosis disease. Often, new cases of TB are resistant to the antibiotics typically prescribed to treat the disease (multidrug- resistant, MDR), making the infections more difficult to cure. Extensively drug-resistant tuberculosis (XDR-TB) is an emerging concern. It is even more resistant and difficult to treat than MDR-TB. XDR-TB has been defined by WHO and the Centers for Disease Control and Prevention as M. tuberculosis that is resistant to the drugs isoniazid and rifampin, to the drug class fluoroquinolone, and to at least one of three injectable “second-line” drugs (amikacin, kanamycin, or capreomycin). Cases of XDR-TB are being closely monitored by the world medical community and measures are being taken in hopes of limiting its spread. TB molecular tests (nucleic acid amplification test, NAAT) detect the genetic material of M. tuberculosis. They are useful because they can generally provide results in about 24 hours as opposed to the several weeks required for culture. The 2009 guidelines from the Center for Disease Control and Prevention (CDC) recommend that a TB NAAT be performed on at least one sample from someone with signs and symptoms of TB. The results are evaluated in conjunction with results of an AFB smear. Like AFB smears, both positive and negative results must be confirmed with AFB cultures. However, if both AFB smear and molecular test are positive, the doctor will begin treatment of the affected person before results of the culture are available.

In 15–20% of active cases, the infection spreads outside the respiratory organs, causing other kinds of TB. These are collectively denoted as “extrapulmonary tuberculosis”. Extrapulmonary TB occurs more commonly in immunosuppressed persons and young children. In those with HIV, this occurs in more than 50% of cases. Notable extrapulmonary infection sites include the pleura (in tuberculous pleurisy), the central nervous system (in tuberculous meningitis), the lymphatic system (in scrofula of the neck), the genitourinary system (in urogenital tuberculosis), and the bones and joints (in Pott’s disease of the spine), among others. When it spreads to the bones, it is also known as “osseous tuberculosis”. A form of osteomyelitis. A potentially more serious, widespread form of TB is called “disseminated” TB, commonly known as miliary tuberculosis. Miliary TB makes up about 10% of extrapulmonary cases

The symptoms of active TB depend on what part(s) of the body are involved. The classic symptoms tend to be pulmonary (TB in the lungs) and include:

  • Chronic cough, sometimes with bloody sputum
  • Fever, chills
  • Night sweats
  • Unexplained weight loss
  • Weakness
  • Chest pain

When a TB infection occurs outside of the lungs (extrapulmonary disease), symptoms can vary depending on the site that is infected. It may cause few noticeable symptoms or a wide range, including:

  • Back pain and paralysis (spinal TB)
  • Weakness due to anemia (TB in the bone marrow)
  • Joint pain
  • Pain associated with the reproductive system or urinary tract and possibly resulting in infertility
  • Abdominal pain
  • Fever and shortness of breath (TB in the pericardium or miliary TB, multiple small sites of TB often in many organs)
  • Altered mental state, headache, and coma (TB in the brain and/or central nervous system, meningitis)


Methodology: Taqman Real time PCR assay

Clinical Use:

  • Due to the risk of the spread of the disease, the potential for the emergence of drug-resistant strains and the severity of the disease in patients infected with HIV-1, the rapid diagnosis of the M. tuberculosis complex is extremely important. Assist in MTB diagnosis and patient management
  • Identify MTB viral load quantitation/ Detection for epidemiologic and prognostic purposes
  • Assess bacterial  response to treatment as measured by changes in the MTB DNA levels

Screening: Centers for Disease Control recommendations

  • NAAT (nucleic acid amplification test) be performed on at least one sample from someone with signs and symptoms of TB.
  • Patient has AIDS & HIV-positive persons
  • Immunocompromised persons (meaning people with weakened immune systems),

Such as organ and bone marrow transplant recipients, cancer patients, patients receiving immunosuppressive drugs

Performed:  Every day
Reported: 2-3 days

Specimen required: C.S.F. Plueral fluid, Synovial fluid, Sputum, Pus, tissue, bronchial lavage, urine, endometrium, Culture Cells, biopsy, Fine Needle Aspirations, ascites. Cerebrospinal fluid or body tissue (biopsy) .Stability collection to initiation of testing On Cells: Ambient: 6 hours: Refrigerated: 4 0C.

NOTE-Blood should only be sent in cases of miliary or disseminated tuberculosis.


Storage/Transport Temperature: Frozen-20 0C. Refrigerate specimens at 2°C-4°C.

Unacceptable Conditions
:  Heparinized specimens, Hemolysis sample, Quantity not sufficient for analysis, specimen grossly contaminated, specimen too old, frozen whole blood specimen, specimen leaky or tube broken.


Interpretation:  Specific amplification curve for MTC complex group indicate presence of MTC in given sample. Whereas no amplification indicates absence of MTC in this test. Amplification of internal control indicates no PCR inhibition. Negative result does not rule out MOTT infections.  This test can quantitate /detect MTB complex DNA over the range 70-109 copies/mL. However this does not mean that lower copies or higher copies cannot be detected. The lower copies can be detected in some cases. This is a limitation of the currently available extraction systems. A negative result does not preclude the presence of MTB complex infection because results depend on adequate/proper patient sample storage and transportation, absence of inhibitors and sufficient DNA to be detected.

NOTE-The result of this test must always be correlated with clinical status and history of the patient and other relevant data and should not be used alone for the interpretation.                                                                                                                                                                                                                                                     


One Thought to “Mycobacterium tuberculosis”

  1. karan

    Nice wirte up about mtb diagnostic .very helpful info.

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