For more than 100 years, the world has been relying on the same basic technology to detect tuberculosis: a smear test from sputum that is peered at through a microscope.
But a few months ago, the World Health Organization (WHO) endorsed an automated molecular test (Xpert MTB/RIF) that many hope will revolutionise the detection of this killer disease, especially for strains of multi-drug resistant TB (MDR-TB) and TB in those living with HIV.
The most important advantage of this test over the usual smear is its greater reliability in identifying TB cases, otherwise known as the sensitivity rate.
In studies, the WHO says the sensitivity of the “Xpert” test was 91 per cent, compared with 59.5 for the standard smear test.
The test’s sensitivity to TB that is resistant to an important antibiotic – Rifampicin – was 95.1 per cent. Ordinary smears cannot detect antibiotic resistance. It can also help to diagnose cases of TB that coexist with HIV, which may be missed in smear tests.
The gold standard for detecting TB remains the culture of the mycobacterium that causes the disease, but this has drawbacks: it takes many weeks to grow the culture and it requires expensive infrastructure, with laboratories and technicians, which are scarce in poorer countries.
Xpert, on the other hand, can be used in more basic labs because it is simpler and safer to use and results are available in a couple of hours.
This is particularly important for MDR-TB, a disease that is of extreme concern to public health officials because it is so hard to treat.
Gilles Van Cutsem, medical co-ordinator for the charity Médecins sans Frontières for South Africa and Lesotho, says: “If you don’t test for MDR-TB, you don’t find it. It’s the hidden epidemic. But where you test, you find it in very high numbers …Survival improves when you test, because you can start early treatment and you can treat in local clinics. Also, just as important, early diagnosis cuts transmission. It’s the key to decreasing the epidemic.”
Typically, this dangerous form of TB is diagnosed from culture only after treatment with first line antibiotics fails.
As Karin Weyer, co-ordinator at the WHO’s Stop TB Department, explains: “To diagnose MDR-TB, you have to rely on expensive tests that are only available at reference labs nationally. Today, less than 10 per cent of MDR-TB patients are tested …The sooner you can start treating patients with MDR-TB, the more lives you will save. [It] is spreading fast in vulnerable populations, such as those with HIV co-infection, who die prematurely, often before the diagnosis of MDR-TB is made.” The WHO believes the new test could lead to a threefold increase in the diagnosis of patients with MDR-TB and a doubling for HIV-associated TB in areas where there are high rates of HIV.
More than 20 countries have received the new test, according to the Foundation for Innovative New Diagnostics (Find), one of Xpert’s developers. Its advocacy officer, Lakshmi Sundaram says uptake will partly depend on how “flexible and proactive” large donors are.
For all the excitement, Xpert is not the perfect diagnostic test – one that can be cheaply and reliably performed anywhere (like the urine dipstick test for diabetes or pregnancy).
The test machines require a regular power supply. They need to be calibrated each year and require some training. Also they do not come cheap.
Even after big discounts – more than 60 per cent – for low and middle-income countries, the device will cost roughly $17,000. This is much more than the $1,500 cost of a microscope, though much less than that of equipping a lab for culture according to the WHO.
The costs per test will range from $10.72 to $16.86 depending on volumes, roughly comparable – at the low end – to the cost of repeating a smear test several times, as is often done.
The WHO has recommended that Xpert be used as the initial test where patients are suspected to have MDR-TB or HIV-associated TB.
In other cases, microscopy will remain the mainstay.
According to Dr Van Cutsem, cost is an important constraint. “The price …needs to be reduced for scale-up to be possible. It is the main barrier at the moment. The situation where there’s only one manufacturer for a diagnostic test that has the potential for an enormous public health impact …is not optimal and the emergence of generic competition is highly desirable,” he says.
Developments such as cheaper power supplies and remote calibration would improve the situation. There is also research into whether the devices could be used to run other tests, such the viral load in HIV cases.
At the Global Fund to fight Aids, Tuberculosis and Malaria, Mohamed Abdel Aziz, senior TB adviser, sums up the importance of this new technology: “This is a big step forward, but it is not the end of a long route to discover a diagnostic tool that is available everywhere …This test will not replace microscopy.”
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