The Gold Test For TB

The Quantiferon TB Gold test is primarily intended to detect latent infection with Mtb, and not for diagnosis and management of active tuberculosis, says Dr Avinash Phadke

In 1993, the WHO declared a global emergency and supported a strategy, which stated that control of infectious agents is even more important because of emergence of multiple drug resistant strains of Mycobacterium Tuberculosis (Mtb). TB is not the disease of the past, two million people died of TB last year and this year this number continues to rise. It is estimated that 1/5th of the world's population is infected with TB, causing at least eight million new cases and three million deaths annually. Annually, 15 million new medical cases are diagnosed in India, of which 90 per cent cases are of TB.

Mtb is the organism that is the causative agent for tuberculosis. Since TB is transmitted between persons, increase in TB cases in any segment of the population represents threat to all the segments of the population. TB remains a major public health problem in most of the developing countries. The point of concern is that TB is directly associated with Human Immunodeficiency Virus (HIV). With increase in HIV infection, the incidence of TB is on rise in developing countries and re-emerging as a major public health problem. HIV reduces or destroys the immune system of human body allowing normal Mtb to infect and cause disease in the host easily. In addition, drug resistant Mtb strains, normally slightly less virulent than normal strains, as well as other species that are not normally pathogenic to humans, find an HIV-compromised host an easy prey.

The rising incidence of TB is compounded by a rising incidence of drug resistance. The process of drug resistance is one of the most important predictors of mortality and hence it is essential that microbiological confirmation and drug sensitivity testing should be always available. This is where the gold test in TB emerges in the scene.

Quantiferon TB Gold (QFT-G) test is an invitro lab diagnostic test using the whole blood specimen. It is an indirect test for Mycobacterium TB complex (Mycobacterium TB, M avium, M africanium, M microti, M canetti) infection. It is used for both, TB and latent TB infection (LTBI). QFT-G can be used for patients who are being evaluated for possible MTB-complex infection, whether tuberculosis disease or LTBI. However, this test cannot distinguish between TB and LTBI, and hence the use of X-ray or any other diagnostic procedure is required. Quantiferon-TB Gold test was approved by the US FDA in 2005. It quantifies the amount of interferon when whole blood is exposed to antigens like T-SPOT. The test counts the number of activated T lymphocytes. This test should be considered as an alternative to the Mantoux test, but has the advantages that it can be used to detect LTBI in patients who have been vaccinated with BCG and have a positive Mantoux test. It can also be used when a Mantoux test is contraindicated.

QFT-G can be used in all circumstances in which the tuberculin skin tests (TST) is currently used. This includes contact investigations, evaluation of recent immigrants who have had BCG vaccination, and TB screening of healthcare workers and others undergoing serial evaluation for MTB. However, caution should be used when testing certain populations because of limited data in the use of QFT-G. This test is primarily intended to detect latent infection with Mtb, but not for the diagnosis and management of active tuberculosis. Should active TB be suspected, culture (and/or PCR testing) of appropriate clinical specimens is still necessary for a definitive diagnosis. This assay will not detect infection with M bovis. BCG strains used for vaccination or immunotherapy Mtb produces the antigens early secretory antigen target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10). These antigens are neither present in non-tuberculous mycobacteria, nor in BCG vaccine. The blood tests Quantiferon-TB Gold and (T-SPOT.TB) use these antigens to detect people with tuberculosis.

Principle Of The Test

Invitro diagnostic test uses peptide cocktails simulating ESAT-6 and CFP-10 proteins to stimulate cells in heparinised whole blood sample. These proteins are absent from all BCG strains and from most non-TB mycobacterium. Individuals infected with MTB complex organisms usually have lymphocytes in their whole blood that recognises these mycobacterial antigens. This recognition process involves generation and secretion of interferon-gamma. The detection and subsequent quantification of interferon gamma forms the basis of Quantiferon TB Gold test. Lymphocytes from the patient's blood are cultured with the antigens. If the patient has been exposed to tuberculosis before, T lymphocytes produce interferon in response. The test then uses ELISA to detect the interferon.

Advantages And Disadvantages

It requires a single patient visit to draw a blood sample. The results can be available within 24 hours. The test does not boost responses measured by subsequent tests, which can happen with TST. The Gold test is not subject to reader bias that can occur with TST. In addition, it is not affected by prior BCG (bacille Calmette-Guérin) vaccination. It differentiates TB infection from BCG and is suitable for repeat and serial testing — no booster phenomenon. The test is highly reproducible and objective. Lastly, it delivers more than 98.2 per cent specificity and about 90 per cent sensitivity. The disadvantages are blood samples must be processed within 12 hours after collection while white blood cells are still viable. Also, there is limited data on the use of QFT-G in children younger than 17 years of age, persons recently exposed to MTB, and in immunocompromised persons; errors in collecting or transporting blood specimens or in running and interpreting the assay can decrease the accuracy of QFT-G; limited data on the use of QFT-G to determine who is at risk for developing TB.

How To Interpret Test Results?

Interpretation of QFT-G results is based on IFN-gamma concentrations in test samples. Each QFT-G result and its interpretation should be considered in conjunction with other epidemiological, historical, physical, and diagnostic findings. A positive result suggests that MTB infection is likely; a negative result suggests that infection is unlikely; and indeterminate result suggests QFT-G results cannot be interpreted as a result of low mitogen response or high background response. Diagnosis of LTBI requires that TB disease be excluded by medical evaluation, which should include checking for signs and symptoms suggestive of TB disease, a chest radiograph, and, when indicated, examination of sputum or other clinical samples for the presence of MTB. QFT-G represents a breakthrough in TB diagnosis and control and offers several important advantages including the fact that it is more accurate than TST and only requires one visit.

Save Time And Money

The Quantiferon process eliminates the booster effects possible with the skin test and is highly reproducible in vitro procedure. The use of ESAT-6 and CFP-10 make the QFT Gold test highly specific for low-risk populations and more sensitive in determining active TB cases, according to Radford. The test is unaffected by 'nearly all tuberculous mycobacteria' and therefore is more accurate in populations that have been vaccinated with BCG or impacted by environmental factors. In addition, the test procedure creates savings as well. Because the patient need not return for a second visit, less office time is required from the physician. The patient, too, saves time and money, and the population at large benefits because many patients will not show up the second time, particularly if they do not believe they are ill.

QFT-G Vs The TST

• QFT results can be available within 24 hours without a second visit, whereas the TST requires a second visit for reading at 48-72 hours.
• The test responds to antigens more specific for MTB and is therefore independent of BCG status unlike the TST.
• The QFT-G is not subject to errors in placement and reading since it is a blood test.
• The QFT-G is also less dependent on previous nontuberculous mycobacterial infection.
• Injection PPD testing can boost subsequent TST responses, while the QFT-G is not affected by boosting from a previous TST.
• The QFT-G can detect both LTBI and active TB.

When To Use Both QFT And TST?

When the risk of TB is highest, and maximum sensitivity for detection of MTB infection is needed, a dual testing strategy is preferred if possible. Persons with clinical suspicion of active tuberculosis, as a diagnostic aide to radiographic and clinical evaluation need both tests. Also, immunocompromised individuals (HIV+ persons or those receiving immunosuppressive medications, including TNF-alpha antagonists) are in need of both. The Traditional Skin Test (TST): The TST originated in the 1890's — the decade that saw the invention of the gas-powered automobile — and until now was the only widely used method for detecting LTBI. However, the TST has many limitations and its effectiveness for controlling TB in the US was questioned in the US Institute of Medicines' 2000 major report, Ending Neglect. The report stated that "the greatest needs in the United States are new diagnostic tools for the more accurate identification of individuals who are truly infected". In contrast, QFT-G measures immune responses to peptides that simulate MTB proteins which are not present in the BCG vaccine or most non-tuberculosis mycobacteria. Thus, QTB-G is highly specific and a positive test result is strongly predictive of true infection with MTB.

The writer is Managing Director , NPIL & Dr Phadke's Pathology Lab & Infertility Centre Pvt Ltd, Mumbai.
E-mail: npilphadkelab@vsnl.net