Dr Madhukar Pai, MD, PhD
Director, McGill GlobalHealth Programs, McGill University, Montreal, Canada
Associate Director,McGill International TB Centre
India alone accounts for a quarter of all TB cases in the world. Over 2.2 million new TB cases occur every year and TB kills nearly 900 people every day in India. The emergence of severe forms of drug-resistant TB has worsened the situation, especially in cities like Mumbai.
TB is a bacterial infection caused by Mycobacterium tuberculosis. While TB most affects the lungs, it can affect other parts of the body as well (e.g. lymph nodes, skin, bones, abdomen,urinary tract, nervous system).
When should we worry about TB?
All persons with otherwise unexplained productive cough lasting two weeks or more, or with unexplained chronic fever and/or weight loss should be tested for TB. Apart from chronic cough and fever, TB causes weight loss, loss of appetite, and tiredness. Night sweats can also occur.
What tests are acceptable for TB diagnosis and what samples should be collected?
Sputum is the ideal specimen to collect for pulmonary TB. All patients suspected of having pulmonary TB should have at least two sputum specimens submitted for microscopic examination and/or a World Health Organization (WHO) approved molecular test such as Xpert MTB/RIF (also called GeneXpert). Liquid culture is very useful in diagnosing smear-negative TB,and drug-resistant TB.
Where are these TB tests available?
In the public sector,sputum smears are widely available in designated microscopy centers and DOTS centers. GeneXpert is also available in the public sector, at the district level. Cultures are only available at reference hospitals and medical colleges.All TB tests are free in the public sector.
In the private sector,tests such as GeneXpert, liquid cultures and line probe assays are available atmore affordable prices via the Initiative for Promoting Affordable, Quality TBtests (www.ipaqt.org). More than 115 accredited labs are part of the IPAQTnetwork.
What about chest X-rays?
Chest radiography is useful but cannot provide a conclusive diagnosis on its own, and needs to be followed by sputum testing. Abnormal X-rays do suggest TB, but other lung conditions can also produce abnormalities on radiography. So, only relying on chest x-ray can result in over-diagnosis. Tuberculosis can only be confirmed by microbiological tests such as sputum smear microscopy, GeneXpert, and cultures.So, it is very important to order sputum tests that can directly detect Mycobacterium tuberculosis.
What about blood test sand skin tests?
There is no accurate blood test for active TB at this time. There is no clinical role for blood-based antibody tests (e.g. IgG/IgM ELISA or rapid tests) and interferon-gamma release assays (e.g. TB Gold). They are not accurate and should not be used for pulmonary TB diagnosis.
The Mantoux (tuberculin)skin test cannot distinguish latent TB infection from active TB disease, and has no utility for diagnosing pulmonary TB in adults. It has clinical utility in children, along with other tests such as chest x-ray, smears, and clinical history. Tests like Mantoux and TB Gold were designed to detect latent infection, and cannot separate latency from active disease.
How is drug-resistant TB diagnosed?
Drug-resistance testing can be done using two methods: genotypic and phenotypic. Genotypic methods are based on molecular tests that detect mutations in TB bacteria that confer drug-resistance. For example, mutations in the rpoB gene of Mycobacterium tuberculosis is strongly associated with rifampicin resistance. Examples of genotypic tests include Xpert MTB/RIF (GeneXpert), and Hain Genotype MTBDRplus (a commercial line probe assay). Phenotypic methods are based on detection of culture growth with and without TB drugs added to the culture media. Phenotypic methods include solid and liquid cultures. While solid cultures can take up to 2 months, liquid cultures (e.g. MGIT culture) can produce useful results within 2 weeks. All of these tests are available via the IPAQT network of private laboratories. They can also be accessed for free in the public/government TB program.