Antiquated diagnostic methods accurate only half of the time
The most widely-used test for diagnosing active tuberculosis is sputum smear microscopy. This method involves a person suspected of having TB coughing up phlegm, or sputum, from their lungs, which is then examined under a microscope for evidence of TB mycobacterium. This method was developed nearly 140 years ago.
While microscopy is cheap and easy to use in all but the most remote areas, it detects less than half of all active TB cases. In addition, people with extra-pulmonary TB (infection outside the lungs), people who are infected with HIV, and children, often go undetected by this method. Without accurate diagnosis, people may receive incorrect treatment which could lead to drug-resistance, or no treatment at all, which risks the infection spreading to others.
Even more complex: diagnosing DR-TB, and TB in HIV-positive people and children
Testing for TB with a microscope can at best only reveal whether the TB bacteria are present in a person’s sputum or not. It is not able to tell if there is drug-resistant TB (DR-TB) present, which does not respond to one or more of the primary drugs used to treat TB. In order to identify if the patient has DR-TB, the ideal test is to culture or grow the TB and then determine which drugs the strain is resistant to, so that the best treatment options for the individual can be identified. But this test is complex to perform and until now has taken up to three months to deliver a result.
Diagnosing TB in people who are HIV-positive or in children is also extremely difficult, as they have fewer TB bacteria in their sputum, making detection harder. People co-infected with HIV are also more likely to have extrapulmonary TB, i.e. TB affecting other parts of the body apart from the lungs, making the sputum test unsuitable for them to begin with. In addition to children having less TB present in their sputum, they are also very difficult to get a sample from as they struggle to cough up sputum at all. This leaves doctors in most cases to make a diagnosis based on clinical symptoms alone.
A New TB Test Brings Hope
The introduction in 2010 of a new molecular-based TB test, which in addition to detecting TB can also detect resistance to one of the two primary TB drugs, could transform the current situation and lead to many more people being correctly diagnosed and treated.
Although still mainly dependent on a sputum sample, the new test can tell if the patient has TB and detects resistance to rifampicin, one of the main first-line drugs used in TB treatment – patients who are found to have DR-TB need a stronger and more complex treatment regimen. The test is able to give results in under two hours, allowing the correct treatment to be started much sooner. But the test is expensive and requires access to a steady electrical supply, so an even simpler, more affordable and easier-to-use test is still urgently needed that can provide results on the spot, in even the most resource-limited settings.
The Ideal Test for TB
The ideal TB test would:
• be able to be used in a small village clinic without a steady supply of electricity
• detect TB in a sample besides sputum, such as urine or blood
• diagnose extrapulmonary forms of the disease
• diagnose TB in children and HIV-positive people
• produce results in under three hours
Last Updated: July 2011