Despite Complexities, Test for TB Could Increase Rapid Diagnoses in Sub-Saharan Africa


Earth Institute Fellow Yanis Ben Amor trains laboratory technicians in Rwanda how to conduct a Line Probe Assay (LiPA), a test that detects the presence of the bacteria that causes tuberculosis.

Image credit: The National Reference Laboratory, Kigali

Sub-Saharan Africa shoulders 27 percent of the 8 million new annual tuberculosis cases. The current diagnostic for TB (Acid-fast Smear) used in much of Africa only works in half of all cases, leaving millions of people without treatment and many more susceptible to infection. Yanis Ben Amor, an Earth Institute Fellow, is hoping to reverse the situation by bringing a rapid and more sensitive diagnostic to the hardest hit areas of Africa.

In November 2005, Ben Amor traveled to Rwanda to test his belief that even in resource-poor settings, a technically complex rapid diagnostic test (Line Probe Assay) for tuberculosis that is currently not being used in Africa can be conducted efficiently, with some training of technicians and with the appropriate technologies.

Trained as a molecular biologist, Ben Amor brought decontaminated DNA samples of TB (wild type and drug resistant), donated by the Prince Leopold Institute for Tropical Medicine in Antwerp, to Kigali and, with a consultant, set up two experiments — one at the National Reference Laboratory in Kigali and the other at Butare University.

For both experiments, a laboratory technician not trained in molecular biology was taught how to conduct the test using local inputs such as water, which is required for testing. After two days, the technicians carried out the experiments on their own on 18 samples of drug resistant and "wild type," or drug responsive, TB. In order to rule out potential contamination, water was used as a negative control.

No one on the team, not even Ben Amor, knew which samples contained drug-resistant TB or wild type TB. The results of the tests, it turned out, were 95 percent consistent each time the samples were tested by the technicians and by the consultant. The Prince Leopold Institute confirmed the results.

With his experiment in Kigali and Butare complete, Ben Amor showed on a small scale that, with proper training and the right equipment, this complex but rapid test — known as PCR (polymerase chain reaction) testing — can be done effectively in resource-poor settings.

The most formidable challenge is equipping areas in sub-Saharan Africa with this critical diagnostic tool. At a cost of $30 to $40 per test and an additional $10 for treatment of susceptible (nonresistant) TB, the total burden of $50 per person would be far too expensive for individuals and families to bear. On the other hand, resistant cases can cost up to $3,000 over the course of 2 years to treat.

Currently, multi-drug resistent (MDR) TB cases are low, but the numbers are on the rise. Patients can develop drug-resistant TB if they are not treated appropriately according to WHO guidelines; if they stop taking the full course of treatment; or if they are infected with MDR-TB from another person.

"A better strategy," says Ben Amor, "is to invest in a plan that can detect MDR early."

Ben Amor is working to attract funding sources to help increase the availability of rapid diagnostic tests in areas where TB is endemic and afflicting entire communities. Such an initiative would be critical to any comprehensive national strategy to effectively address the need for improved diagnostics and availability of treatment.

In Rwanda, as in many countries in Africa, 37% of people infected with TB are also co-infected with HIV. In these cases, the two diseases accelerate the progression or severity of the other, complicating treatment. The ability to detect TB early and definitively is imperative in such circumstances.

The Global Fund to Fight AIDS, Tuberculosis and Malaria estimates that between 2000 and 2020, nearly one billion people will be newly infected with TB. Another 35 million will die from the disease. Tackling TB requires targeted screening of high-risk groups to identify individuals with latent TB infection or MDR infection.

"Once TB is detected," says Ben Amor, "it is largely treatable, even for those with drug-resistant disease, but rapid and effective diagnosis is the first and vital step."

Tuberculosis Drug Treatments -- Acid-fast Smear

One of the most prevalent diagnostic technologies detects presence of the TB bacteria in sputum using an acid-fast smear (AFS) through microscopic examination, but is problematic because the technique is cumbersome, both for patients who need to return to the health care center at least twice to provide sputum samples, and for the technician straining to count Mycobacterium tuberculosis (MTB) bacilli using a sun-lit microscope.

The test has a sensitivity of only 40-60 percent under field conditions, falling as low as 20 percent in the presence of HIV co-infection. AFS testing is ineffective in detecting TB bacteria in individuals with low load levels, leading to 'smear negative' results in people who have TB but are in the early stages of their infection.

Early-stage TB infection is less likely to cause death or be contagious to others; however, a delay in diagnosis of infection in these individuals, particularly those who may be drug resistant, can lead to increased illness, infection in others, or death.

Tuberculosis Drug Treatments -- Line Probe Assay

Currently, one of the best ways to detect TB as well as multi-drug resistant TB (MDR-TB) is through a platform called Line Probe Assay (LiPA). Rather than relying on visualizing bacteria under a microscope (as is done in acid-fast smear), LiPA indirectly detects presence of Mycobacterium tuberculosis by amplifying DNA present in the sputum by polymerase chain reaction (PCR). Amplified material can subsequently be visualized on a strip by the presence or absence of bands, much like a pregnancy test.

LIPA also detects resistance to rifampicin (a first-line drug used to treat TB), which often indicates multi-drug resistant TB.

“The benefits of LiPA include rapid diagnosis in patients who are under high suspicion for TB but have tested smear negative,” says Ben Amor. 

PCR testing involves fairly significant preparations. The test is also prone to cross contamination, leading to false positives, and is prohibitively expensive for millions infected with TB, especially in impoverished areas of Africa. Despites the drawbacks, some TB experts believe that LiPA, which is currently not available in the continent, can be an effective way to detect the disease, especially MDR cases.