The dawn of a New Era for TB diagnostics?

HIV Update No. 8

December 2010

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Background

Tuberculosis (TB) continues to cause high disease burden in developing countries. In the year 2009

alone, 1.7 million people died from TB, equal to 4700 deaths a day. Of these, 380,000 deaths

occurred in women. In the same year, there were 9.4 million new TB cases, of which 3.3 million were

women and 1.1 million were among people living with HIV.

In addition, the burden of multi-drug resistant TB has continued to rise. Multi-drug resistant TB is a

form of TB that is difficult and expensive to treat and fails to respond to the most commonly used first-

line drugs. It is estimated that in 2009, 3.3% of all new TB cases had multi-drug resistant TB.

Extensively drug-resistant TB occurs when resistance to second-line drugs develops on top of multi-

drug resistant TB. Cases of extensively drug-resistant TB have been confirmed in more than 58

countries to date. Both multi-drug and extensive drug resistant TB pose an enormous challenge

because they are difficult to treat, use expensive drugs which are not easily available, and whose use

has to be sanctioned by the WHO.

One of the key challenges of tackling the burden of TB disease has been the poor development of

new strategies for detection and treatment of TB. Current diagnosis of TB relies on smear

microscopy, which can produce results within 24 hours, and culture, which may require up to 6 weeks

to produce results.

Summary

A new diagnostic tool has been developed, which represents an important breakthrough in the search for an efficient and accurate TB diagnostic tool.

The new method (marketed as GeneXpert® by Cepheid), is able to detect active TB and rifampicin resistance within 90 minutes.

In-order to ensure that communities in high burdened and developing counties can benefit from it, the technology should be made widely available to countries at reduced cost.

This method is still not suitable for point-of-care use; more research is required to produce a point-of-care diagnostic method for TB.

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Limitations of current diagnostic methods:

These methods (smear microscopy and

culture) have been in use for more than a

century and little innovation has been

witnessed since culture was first

described. Pharmaceutical and research

organizations have invested far less

resources into tuberculosis, (and other

neglected tropical diseases) which affects

many people in the low and middle-

income countries, as compared to their investments into diseases associated with affluence such as

cardiovascular and diabetes, thereby abrogating their moral duties to make new and effective

technology and medicines available to the most poor.

In addition these tools (smear microscopy and culture) often perform sub-optimally and may require

laboratory infrastructure (and electricity) that may be unavailable in some settings especially in rural

areas. In addition, although direct microscopy for AFB can lead to a diagnosis within 24 hours, this

method is limited by its poor ability to correctly identify infected persons while correctly excluding

persons who are not infected, especially in settings where TB is common.

How are smear microscopy performed?

A diagnosis of TB can be made by examining a sample form a patient under a microscope. Such a sample is first smeared on a glass slide and is then treated with a special stain which enables detection of the mycobacteria that cause TB when examined under a microscope. If bacteria are identified in the sample, the test is said to be positive and this indicates TB infection. Where available, results should be confirmed by culture.

How are cultures performed?

Cultures are performed on respiratory samples such as sputum. Normal respiratory bacteria are first removed from such samples in order to reduce the chances of misdiagnosis. The sample as are then concentrated to increase the likelihood of detecting TB mycobacteria. The samples are then incubated with nutrient so that the TB mycobacteria can multiply. Culture results are more reliable. However, since mycobacteria multiply very slowly, the process of culturing can take two to six weeks.

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In reality, and in most settings in the developing countries,

smear microscopy is the only method of diagnosis of TB, since

culture is not widely available. However, health care workers

may consider x-rays in combination with TB specific

symptoms as realistic indicators that somebody is infected

with TB.

But even then, and in some settings, TB treatment cannot be

initiated until confirmatory results (either a smear test or a

culture) are available, resulting in long delays in diagnosis

which can impede access to treatment and may also lead to

increased transmission of TB because people do not know

about their TB infection and therefore do not take the

necessary steps to avoid infecting others.

Consequently, and partly due to these diagnostic limitations,

TB control has not been achieved at the global level, as

evidenced by the growing TB burden as well as increased

cases of multidrug-resistant and extensively drug-resistant

tuberculosis. Rapid and reliable tests are required in-order to

ensure earlier diagnosis of TB which leads to earlier initiation

of treatment, reduces the likelihood of onward transmission

and increases the effectiveness of TB control programs.

New Development:

However a new method of diagnosis of TB, known as

GeneXpert® (Cepheid MTB/RIF Assay) appears promising.

It’s a closed, self-contained, and automated method which

diagnoses tuberculosis and detects resistance to one of the

most commonly used TB drugs (rifampicin) at the same time.

This new method of diagnosis has a number of key

advantages:

Firstly there is less risk of contamination since it is a closed

system. Often times, samples may get contaminated during

the process of culturing leading to erroneous results.

TB Diagnostic tools: Summary

Current tools

Smear microscopy

Culture

X-ray

Newly WHO approved tools (2005 - 2009)

Liquid culture

Molecular Line Probe Assay (LPA)

Strip speciation

Newly approved tool (2010) with potential for scale up

Automated detection and MDR screening (GeneXpert®)

Tools in late development

Colorimetric redox indicator

Front loaded smear microscopy

Interferon gamma release assay (QuantiFERON®–TB)

LED fluorescence microscopy

Tools in early development

Breathalyser screening test (has the best potential for community point of care)

LAM detection in urine

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Secondly, the risk of infection to the health worker is reduced since it is a closed system and requires

a relatively short sample-handling period, compared to the several weeks it takes to grow a pure

culture of TB mycobacterium. Health workers may get infected from handing sputum laden with TB

mycobacterium. This risk is reduced in this diagnostic method because the potential exposure of the

health care worker is reduced significantly.

Thirdly, GeneXpert® produces the results within 90 minutes. This is in contrast to the several weeks

it takes to make a diagnosis of TB using culture and the 1-2 days it may take to make a diagnosis on

smear microscopy.

Fourthly, the diagnostic platform not only makes a diagnosis of TB in 90 minutes possible, but it also

determines the resistance to rifampicin, a key drug of Tuberculosis treatment. This is an important

step towards diagnosing multidrug resistant TB. Rifampicin is one of the key drugs in the treatment of

TB. There is a growing trend of rifampicin-resistant tuberculosis which is difficult to treat, often

requiring expensive alternative drugs for a longer period of time.

Technology

The system relies on molecular analysis and combines on-board sample preparation with real-time

PCR (polymerase chain reaction) amplification and detection functions for an integrated and

automated nucleic acid analysis. The system is designed to purify, concentrate, detect and identify

targeted nucleic acid sequences thereby delivering answers directly from unprocessed samples.

Samples do not need to be pre-processed before using this system.

Process

The patient’s sputum sample is gathered in a bottle, a buffer fluid is added, and the sample is

transferred to a cartridge which is then placed in the machine. The operator pushes a button and a

result is obtained 90 minutes later.

Broad Utility

The diagnostic platform has a number of configurations and hence can be used for diagnosis of viral

infections (such as Influenza) as well as other bacterial infections (such as Staphylococcus and

Clostridium). It can also be used to monitor cancer patients.

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Towards the Implementation of GeneXpert®: What are the Issues?

Cost

Firstly, the price of the system is too high. Current indications show that the machine will be available

commercially at price estimated from 20,000 to 30,000 USD. This initial cost is prohibitively

expensive for a rapid roll out to countries with the highest burden of disease and which are in most in

need of such technology. The recurring cost of running the test is USD 30 for each test, mainly the

cost of a cartridge, which is used every time a test is run. This cost per test is extremely high,

considering the present global incidence of TB. Developing countries bear the burden of TB. Indeed,

all the 22 high burden countries globally are low or middle income countries.

Price reduction for both the machine and the recurrent cartridge cost is necessary in order to make

the machine (and technology) affordable in the regions where the epidemic is most serious, including

sub-Saharan Africa which has some of the highest rates of multi drug resistant TB. In addition, the

cost will be a major barrier to the availability to communities that require this technology. In-order to

have comprehensive access to the new technology, the tests cost and acceptability to the new

diagnostic method need to be carefully considered as key relevant factors.

Fitting the GeneXpert® diagnostic platform into the current health systems

The new diagnostic method will have to fit into one of the level of the health systems, whether

primary, secondary or tertiary. It is important for countries to determine where the diagnostic ability of

the GeneXpert® will most useful and coherently fit, in line with the infrastructure and health personnel

to sustain it. The level of fit will be different depending on the local context. However, in the long run,

the technology should be available at the lowest level of health systems in countries where TB is a

public health issue.

Availability at Community Level

The technology requires electricity and therefore this may be an immediate impediment for use of the

machine in remote and rural areas where there is no electricity. Secondly, this system is still

complicated for a lay person to operate and therefore may not match the skills set of a community

health worker. It is still way off from the desired point of care diagnostic method – such as the rapid

screening methods used for HIV testing. Community health workers, who perform intensified case

finding, contract tracing and community based screening of TB suspects need to be equipped with a

point-of-care tool which they can use at the community level with minimum technical skills.

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Health Systems Strengthening

The World Health Organization estimates that the current number of Tuberculosis cases reported by

countries is far lower than the estimated true incidence of TB. Thus significant cases of Tuberculosis

remain undiagnosed which continue to be an important source of new infections.

The new GeneXpert® Diagnostic method has the potential to achieve the much desired increase in

case detection of Tuberculosis disease as well as increased detection of Multidrug-resistant

Tuberculosis. However, this means that health systems in developing and high-burden countries will

have increased cases of Tuberculosis to treat immediately following widespread implementation of

the new technology.

Further, it is estimated that this new method of diagnosis, if widely scaled up, will also lead to early

detection of tuberculosis since it can detect TB at lower concentrations as opposed to culture which

depends on the presence of a significant amount of mycobacterium (at least 10,000 CFU/ML) for the

disease to be detected.

Consequently, widespread use of this method could lead to increased numbers of patients in need of

TB drugs. As a result, more TB drugs, health personnel and infrastructure will eventually be required

to keep up with the increased diagnostic capacity of the GeneXpert® and similar new technologies.

This calls for the urgent need to ensure that the health system will be in a position to handle this

increased demand for TB treatment and care.

Impact Assessment

New diagnostic methods such as the GeneXpert® are often scientifically-rated according to their

specificity and sensitivity – i.e. ability of the test to correctly identify cases and correctly exclude the

uninfected persons. However, in real terms, the ability of the test to increase the access to health

care is of greater importance. Concrete evidence of the ability of this new technology to increase

access to TB diagnosis, care and treatment needs to be examined and documented outside clinical

trial settings. This operational research is urgently required in-order to inform the best approaches to

scale up the implementation of the GeneXpert® (or similar) systems in different settings. It is

important for countries to be engaged in the creation of protocols for use of new technologies, and for

clinicians to be involved in demand creation. Community involvement is critical to ensure that the

diagnostic gaps are highlighted and acted upon. In addition there is an opportunity to ensure that

clear information on cost effectiveness is generated from settings outside clinical trials by early roll

out of new technologies in high burden countries.

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Children

It is noted that the new diagnostic method still relies on use of sputum sample for diagnosis of TB.

This method does not surmount the challenge faced in making a TB diagnosis among people who

are unable to produce sputum, such as gravely sick persons or children. There is therefore a need to

identify new biomarkers which could be used on the new diagnostic platform for enhanced diagnosis

using other samples (e.g. blood, urine or stool).

Conclusion

This new diagnostic platform has moved closer to the objectives of simplifying and improving the

detection of TB cases, including smear-negative sputum and drug- resistant cases. However, there is

still a need to innovate and widely distribute a simple and cheap test that can be performed at the

point-of-care level by both health care and community health workers. There is still no reliable

method of identification of all latently TB infected cases. The search for reliable biomarkers for the

various stages of TB infection needs to progress at a faster rate than presently is the case. Impact

assessments of the new diagnostic method be performed urgently in-order to inform implementation

in high burden and other countries.

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References and further reading

1. WHO 2010 Global tuberculosis control. Geneva WHO

2. WHO 2010 Multidrug and extensively drug-resistant TB (M/XDR-TB): 2010 global report on

surveillance and response. Geneva. WHO

3. Moore DF, Guzman JA, Mikhail LT. Reduction in turnaround time for laboratory diagnosis of

pulmonary tuberculosis by routine use of a nucleic acid amplification test. Diagn Microbiol Infect

Dis 2005;52:247--54.

4. Taegtmeyer M, Beeching NJ, Scott J, et al. Clinical impact of nucleic acid amplification tests on the

diagnosis and management of tuberculosis in a British hospital. Thorax 2008;63:317-21.

5. CDC. National plan for reliable tuberculosis laboratory services using a systems approach:

recommendations from CDC and the Association of Public Health Laboratories Task Force on

Tuberculosis Laboratory Services. MMWR 2005;54 (RR-6):1-12.

6. American Thoracic Society; CDC; Council of the Infectious Disease Society of America. Diagnostic

standards and classification of tuberculosis in adults and children. Am J Respir Crit Care Med

2000;161:1376-95.

7. Guerra RL, Hooper NM, Baker JF, et al. Use of the Amplified Mycobacterium tuberculosis Direct

Test in a public health laboratory: test performance and impact on clinical care. Chest

2007;132:946-51.

Glossary (Alphabetical list)

Assay: Laboratory test

CFU: Colony Forming Units: a quantitative indicator of the presence of infectious agent in a sample.

Extensively drug-resistant TB (XDR-TB) occurs when resistance to second-line drugs develops on top of MDR-TB

Mycobacterium tuberculosis: Bacteria responsible for causing Tuberculosis infection and disease

Multidrug-resistant TB (MDR-TB): Tuberculosis disease which does not respond to either Rifampicin or Isoniazid

Rifampicin: one of the key dugs for the treatment of Tuberculosis

Sputum: Viscous material that is derived from the lower air passages such as the lungs and bronchi

that may contain substances such as mucus, blood, pus and/or bacteria; it is not the saliva that is

produced by the glands in the mouth.

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For further information please contact:

Gitau Mburu, Senior Advisor: HIV and Health Systems

E-mail address:gmburu@aidsalliance.org Telephone: +44(0)1273 718929

International HIV/AIDS Alliance (International secretariat)

Telephone: +44(0)1273 718900 Fax: +44(0)1273 718901

mail@aidsalliance.org www.aidsalliance.org

Registered British charity number: 1038860