|Year : 2015 | Volume
| Issue : 3 | Page : 233-240
Genexpert technology: A new ray of hope for the diagnosis of tuberculour pericardial effusion
Neema Negi, Bimal Kumar Das
Department of Microbiology, All India Institute of Medical Sciences, New Delhi, India
|Date of Web Publication||23-Feb-2016|
Bimal Kumar Das
Department of Microbiology, All India Institute of Medical Sciences, Ansari Nagar, New Delhi - 110 029
Source of Support: None, Conflict of Interest: None
Tuberculous pericardial effusion is a well-known complication of tuberculosis (TB). However, the greatest challenge in front of clinicians is its diagnosis as the conventional methods lack the required sensitivity and specificity of detection. The emergence of drug resistance and co-infections such as human immunodeficiency virus further complicates the situation making it difficult to diagnose such cases. GeneXpert technology is a major breakthrough in the field of TB diagnosis and has opened newer avenues for many new molecular tests to be launched in the future. The World Health Organization endorsed this technology in 2010 for rapid and simultaneous detection of Mycobacterium tuberculosis (MTB) and rifampicin (RIF) resistance. GeneXpert-MTB/RIF assay was highly recommended as an initial diagnostic platform in high burden countries for early and quick detection of TB cases. Until date, very few studies have evaluated the performance of this brilliant assay in pericardial effusion cases, thus, more studies are required to address the unanswered questions left so far. Our review attempts to recapitulate the achievements, the potential impacts and the prospective use of this novel technology in early diagnosis of TB, especially focussing pericardial effusion cases.
Keywords: Extra pulmonary tuberculosis, pericardiocentesis, tuberculous pericardial effusion
|How to cite this article:|
Negi N, Das BK. Genexpert technology: A new ray of hope for the diagnosis of tuberculour pericardial effusion. J Pract Cardiovasc Sci 2015;1:233-40
|How to cite this URL:|
Negi N, Das BK. Genexpert technology: A new ray of hope for the diagnosis of tuberculour pericardial effusion. J Pract Cardiovasc Sci [serial online] 2015 [cited 2018 Dec 16];1:233-40. Available from: http://www.j-pcs.org/text.asp?2015/1/3/233/177230
| Introduction|| |
In the era of vaccination and treatment, rapid diagnosis of some infectious diseases still remains a distant dream despite technological advances. Tuberculosis (TB) caused by Mycobacterium Tuberculosis (MTB) is one such disease that manifests itself in two forms, either pulmonary tuberculosis (PTB) or extrapulmonary tuberculosis (EPTB). Pulmonary TB is the deadliest form of the disease while EPTB is localized in areas other than lungs, especially lymph nodes, genitourinary tract, pleura, bones, joints, meninges, central nervous system, peritoneum, and other abdominal organs.  Pericardial effusion is one such manifestation of EPTB. Pericardial effusion cases correlated to TB have been reported to be increasing over the years in developing countries (>60, and >80% with human immunodeficiency virus (HIV) infection).  In this review, we provide an overview of the current knowledge in the field of pericardial effusion diagnosis caused by MTB. It is also an attempt to highlight the role of GeneXpert technology and its usage as future diagnostic tool for pericardial effusion.
| Epidemiology|| |
As per the recently published reports by World Health Organization (WHO) (global TB report 2015),  the effective treatment and diagnostic strategies employed against TB has drastically reduced mortality by 47% since 1990, yet TB remains a major cause of death, especially in low- and middle-income countries. India has been reported to hold the highest number of TB cases (23%) among the list of high burden countries. Of the 9.6 million reported TB cases globally, an estimated 0.8 million (15%) reported cases in 2012 were of EPTB and the European Center for Disease prevention and Control eurosurveillance report found that 22% of notified TB cases in Europe as of 2011 were extrapulmonary. EPTB is also one of the most common opportunistic infection reported in people living with HIV/acquired immune deficiency syndrome (AIDS). 
Although pulmonary TB is the leading cause of mortality, EPTB has not received high priority status from a public health perspective, the way pulmonary TB is being addressed internationally. EPTB has also significantly contributed in affecting disease transmission, TB-related morbidity and is responsible for life-threatening consequences.  Hence, there is an urgent need for early diagnosis and treatment of EPTB cases as they do have a significant impact on treatment strategies.
| Pericardial Tuberculosis|| |
Another well-known complication of EPTB is pericardial TB, and it has been reported to be present in approximately 1% of all autopsied cases of TB and has 1-2% incidence in case of pulmonary TB. It has been estimated by many studies that TB pericarditis occurs in one to 8% patients with PTB. ,, Afro-Asian countries have high rate of chronic pericardial effusion cases, and there has been a steady increase in the incidence since the occurrence of HIV infection.  There are three different clinical forms of tuberculous pericarditis namely, pericardial effusion, constrictive pericarditis, and a combination of effusion and constriction.  Late complication of effusive pericarditis is constrictive tuberculous pericarditis which occurs in 30-60% of cases.  Effusive constructive pericarditis is another rare manifestation of pericardial disease which is basically a hemodynamic syndrome wherein constriction of the heart by the visceral pericardium occurs in the presence of a compressive pericardial effusion. 
| Tuberculous Pericardial Effusion|| |
The pericardium is a protective barrier that surrounds the heart and blood vessels and is composed of a thin visceral membrane, a fibrous parietal membrane, and the pericardial space which is filled with an ultrafiltrate of plasma known as pericardial fluid.  Small changes in pericardial fluid volume do not affect pericardial pressure; however, a sudden increase in cardiac volume causes a steep change in cardiac volume resulting in pericardial effusion. The membrane is able to withstand the pressure of the growing fluid until the limit of its stretch. When pericardial fluid volume increases beyond the limit of membrane stretch, cardiac tamponade results.  There is not a single cause of pericardial effusion; many factors can lead to this condition that could be idiopathic, infectious (most commonly viral), cardiac, autoimmune, medication-induced, radiation-induced, traumatic, metabolic, malignant, or the result of other causes.  Patients with TB pericarditis usually present with chronic pericardial effusion.  Fever, night sweats, fatigue, and weight loss are usually the non specific and initial symptoms of tuberculous pericardial effusion (TPE). However, patients may report cardiac tamponade and could manifest severe distress, retrosternal compression, tachycardia, and raised jugular venous pressure with blunt γ descent. ,, It has also been reported that the most common presentation in African patients with TPEs is chronic cardiac compression that mimics heart failure. 
| Obstacles in the Diagnosis of Pericardial Effusion|| |
The diagnosis and management of pericardial diseases remains challenging due to wide variety of manifestations and discrepancy between clinical symptoms and amount of pericardial effusion. , The first challenge in front of clinicians is to establish an etiologic diagnosis. Diagnosis becomes smooth in patients complaining chest pain of pericarditic characteristics or in patients with other co-infections that could be possible reasons for pericardial involvement (renal failure, chest irradiation). However, diagnosis becomes difficult in others with clear chest X-rays. Currently, a number of investigation options are available which includes cytology, biochemistry, X-ray, for the diagnosis of pericardial effusion cases. Echocardiography remains the gold standard for evaluation of pericardial effusion, imaging by computed tomography scanning or magnetic resonance imaging can also be used but is often remains unavailable in rural areas of low- and middle-income countries. Pericardial fluid appears on an echocardiogram as an echolucent space between the pericardium and the epicardium. Pericardiocentesis is recommended in all patients in whom TB is suspected. However, it is still an invasive procedure that is cumbersome to perform and is uncomfortable for the patients.  Hazardous complications associated with pericardiocentesis may include hemopericardium, pneumopericardium, cardiac perforation, introduction of infections, and arrhythmias.  Pericardial paracentesis for diagnostic and therapeutic purposes are carried out under echocardiographic scanning. On aspiration, the fluid is sent for cytology, biochemistry, Ziehl-Neelsen (ZN) stain, and culture for tubercle bacilli. Laboratory tests are reliable for confirmation of tuberculous pericarditis, however; extraction of tubercle bacilli from pericardial fluid is difficult which is further delayed by poor sensitivity and performance of these laboratory diagnostic procedures. Moreover, Burgess et al. have also demonstrated that conventional diagnostic procedures employed for detection of tuberculous pericarditis are usually insensitive and require long culture periods.  The paucibacillary nature of disease and nonuniform distribution of microorganisms leads to false negative results in most of the laboratory assays. Precise and accurate diagnosis of pericardial effusion requires a good infrastructure of testing laboratory with highly trained and efficient staff, but they seems to be rarely available in resource-constrained settings.  In developing nations, diagnosis is entirely dependent on conventional methods including ZN microscopy, histology, solid culture, and tuberculin test which are likely to give a false negative test result in pericardial effusion cases. 
| Challenges in Conventional Diagnostic Methodology|| |
Early diagnosis and prompt treatment can save millions of lives. Rapid and accurate diagnosis would definitely lead to early initiation of effective treatment, but this is often difficult as the traditional methods lack the required sensitivity and specificity. ZN staining and culture methods remain the gold standard for the detection of pulmonary TB samples but have not proved to be very effective testing strategies for the diagnosis of TPE cases. Previously published reports suggest that ZN stainin, direct microscopy, and culture are all poorly sensitive for detecting MTB.  These traditional methods are expensive, complex, delayed by 2-6 weeks and are less sensitive for the detection of EPTB, especially from pericardial effusion cases. Until date, there are no specific guidelines for diagnosis of TPE cases. Recently, Bayazit et al. introduced an enzyme-linked immunosorbent spot assay for counting the numbers of antimycobacterial effector T-cells that produce interferon (IFN)-gamma against MTB - specific antigens encoded by genes located in region of difference.  However, it is very early to be fully dependent on this method as the feasibility and reliability of this assay for diagnosing TPE has not been evaluated by other studies. However, the last few years have witnessed the advancement of molecular tests that have very low negative predictive value and high positive predictive value (98-99%) for extrapulmonary cases. , These tests have completely revolutionized the field of TB diagnosis by targeting specific genes or gene segments, and have shortened the turnaround time of detection from weeks to days.  Currently, many molecular methods are available for rapid diagnosis and identification of TB bacilli. In addition, these assays have contributed immensely for detection of drug resistance pattern in clinical samples reducing the turnaround time of detection.  Nucleic acid amplification tests (NAATs) have proven their worth in EPTB cases by having very high positive predictive value, and with a sensitivity of 60-100% even with inadequate amount of sample. Polymerase chain reactions (PCRs) have also benefited clinicians in predicting disease outcome in MDR-TB cases by direct amplification of rpoB gene responsible for rifampicin (RIF) resistance in MTB.  The diagnostic accuracy and effectiveness of PCR were also examined by Cegielski et al. in 13 specimens of pericardial fluid and 15 specimens of pericardial tissue from twenty patients. They reported that accurate diagnosis of TB was correctly made in 13 patients (81%); there was one false-positive result for a patient with Staphylococcus aureus pericarditis.  Although these tests have become popular due to their sensitivity, specificity, speed and accuracy, still these techniques are not being utilized as widely as they were predicted to be used in resource-limiting settings.
| GeneXpert Technology: A Bird's Eye View|| |
One such novel, automated real-time nucleic acid amplification technology is Xpert-MTB/RIF system. The GeneXpert-MTB/RIF method can simultaneously detect and identify MTB and resistance to RIF within two working hours. This technology was recommended and implemented by WHO in December 2010 in developing nations as a substitution over conventional methods for rapid detection of MTB in extrapulmonary samples.  This Cepheid GeneXpert system is based on molecular beacon technology and performs ultrasensitive hemi nested PCR on clinical samples with an accurate (99-100%) diagnosis and detects RIF resistance in single assay.  Being a closed system, this assay does not require much expertise and there is minimal risk of contamination and biohazard. Initially, the WHO endorsed the assay specifically for diagnosing pulmonary TB patients; recently attempts have been made to explore other affected areas including EPTB clinical samples.
| Sensitivity and Specificity of the Assay|| |
Emerging data suggests that this assay has the potential for diagnosing EPTB as the sensitivity and specificity were found to be almost similar for both PTB and EPTB cases. These facts are further supported by a large number of studies conducted until date. Ligthelm et al. showed excellent diagnostic accuracy of the Xpert assay in terms of sensitivity (96.7%) and specificity (86.6%) in patients with tuberculous lymphadenitis.  Similarly, Lawn and Zumla demonstrated high sensitivity (81.3% for EPTB) of Xpert MB/RIF on large number of nonrespiratory samples.  Furthermore, Iram et al. also compared and evaluated the performance of Xpert-MTB/RIF assay over conventional methods and reported that this assay is highly efficient in detecting TB, especially in smear-negative and in EPTB compared to the conventional ZN staining.  Recently, published meta-analysis data by WHO have shown that GeneXpert achieved an overall pooled sensitivity of 88% (95% credible interval (CrI) 84-92%) and pooled specificity of 99% (95% CrI 98-99%) as investigated by 22 studies involving 9008 participants.  Similarly, 21 studies (8880 participants) showed that the pooled sensitivity of GeneXpert was found to be 99% (95% CrI 98-99%) while the pooled sensitivity of smear microscopy was reported to be 65% (95% CrI 57-72%). Therefore, Xpert technology increased TB detection among culture-confirmed cases by 23% (95% CrI 15-32%).  The performance of Xpert assay has been demonstrated for some types of EPTB such as TB lymphadenitis, pleural TB and TB meningitis. ,, However, there are no comprehensive data about TBP to guide clinical practice. Pandie et al. evaluated for the 1 st time the diagnostic accuracy of Xpert-MTB/RIF test in a large comprehensive study and reported that Xpert-MTB/RIF, when combined with either adenosine deaminase (ADA) or unstimulated IFN-gamma (uIFNγ), offers >97% sensitivity and specificity for TBP diagnosis.  In addition, Sharma et al. further evaluated the diagnostic performance of this assay in a cohort of 1274 patients enrolled in a tertiary care center in India and reported that the sensitivity and specificity of GeneXpert assay are high (71% and 95% respectively) compared to cultural methods.  Summary of all the studies is performed are summarized in [Table 1] and [Table 2].
|Table 1: Sensitivity and specificity of GeneXpert polymerase chain reaction for the diagnosis of extrapulmonary tuberculosis|
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|Table 2: Detection of Mycobacterium tuberculosis in pericardial fluid-cases of pericardial effusion|
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Pandie et al. evaluated for the 1 st time the diagnostic accuracy of Xpert-MTB/RIF test in a large comprehensive study and reported that Xpert-MTB/RIF when combined with either ADA or uIFNγ, offers > 97% sensitivity and specificity for TBP diagnosis.  In his study, 151 consecutive patients with suspected TBP were enrolled at a single center in Cape Town, South Africa. MTB culture and/or pericardial histology served as the gold standard. Forty-nine percent (74/151) were classified as definite TBP, 33% (50/151) as probable TBP and 18% (27/151) as non TBP. Xpert-MTB/RIF had a sensitivity and specificity of 63.8% and 100%, respectively. Concentration of pericardial fluid by centrifugation and using standard sample processing did not improve Xpert-MTB/RIF accuracy. ADA (≥35 IU/L) and uIFNγ (≥44 pg/ml) both had a sensitivity of 95.7%98.5%) and a negative likelihood ratio of 0.05 (0.02 to 0.10). However, the specificity and positive likelihood ratio of uIFNγ was higher than ADA. The sensitivity and negative predictive value of both uIFNγ and ADA were higher than Xpert-MT/RIF.
In Tortoli's study, 4% of the patients had pericardial, peritoneal or synovial effusions. They investigated consecutive extrapulmonary clinical specimens (1,068 patients) including both pediatric (494) and adult samples. They found an overall sensitivity and specificity of 81.3% and 99.8% for Xpert, in contrast, lower than 50% for cavitary fluids like pericardial effusion [Table 1] and [Table 2].
| The Diagnostic Platform of GeneXpert Assay|| |
• Disposable plastic cartridges having multiple chambers, preloaded with buffers and reagents are employed for sample processing, purification, DNA extraction and amplification. Clinical samples containing sputum are first treated with sodium hydroxide and isopropanol-containing sample reagent (SR). SR helps in reducing the viability of MTB bacilli to 10 6 fold for minimizing biohazard risk. Treated samples are manually loaded into the cartridge which is then inserted into the GeneXpert machine ,
• After washing and purification, TB bacilli are concentrated, and DNA is released from the sample by ultrasonic lysis. The captured DNA is then amplified within the 192 bp segment of the rpoB gene using a hemi-nested rt-PCR reaction ,
• Bacillus globigii spores serve as an internal sample processing, and PCR control and this assay is multiplexed with MTB assay 
• Detection of Rifampicin resistance: Molecular beacon technology is employed for detection of RIF resistance wherein the 81bp core region of the rpoB gene encoding bacterial RNA polymerase is amplified using five different nucleic acid hybridization probes. , These molecular beacons are oligonucleotide probes which are covalently linked to fluorophore at one end and a quencher at the other end. These probes bind to the complementary and amplified rpoB gene sequence. Five differently labeled probes specific to different sequences within the wild type rpoB gene segment are used. Binding of the probe results in emission of florescence light indicating sensitivity of the strain to RIF. While, RIF resistance is predicted when there is mutation in rpoB gene segment that results in either delayed or less florescence of probes.  Diagrammatic representation of GeneXpert assay is depicted in [Figure 1].
|Figure 1: Schematic representation of GeneXpert technology depicting the step wise procedure of the assay.|
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Hence, GeneXpert technology is a boon for resource limited settings for instant diagnosis and detection of resistance pattern against MTB. In addition, as sample processing, PCR amplification and detection are integrated in one single unit making the system fully automated, GeneXpert is definitely an attractive tool for point of care testing.
| Diagnosis of Mycobacterium Tuberculosis Among People Living with Human Immunodeficiency Virus/Acquired Immune Deficiency Syndrome|| |
The mortality rate of TB/HIV co-infection is rising annually in high burden countries including India; therefore, there is an urgent need for accurate diagnosis of TB among people living with HIV (PLHIV)/AIDS. It has also been reported by WHO that perinatal HIV have a 29-fold increased risk of TB compared to HIV-uninfected people.  The clinical pattern, as well as the immune response to TB changes as the immune deficiency, progresses in HIV infected individuals. , In EPTB, smears tend to be negative because of paucity of TB bacilli at the onset of disease and HIV infection further complicates the scenario by expanding the horizon for other opportunistic infections. In HIV prevalent settings, GeneXpert is the best available option for quick diagnosis of TB cases for timely initiation of treatment.  Studies conducted in South Africa and Ethiopia suggests high diagnostic accuracy of GenXpert for pulmonary TB among HIV infected individuals and reported significant increase in detection of TB cases (53%) with a smear-negative microscopy result. , Similarly, findings from South Korea and Greece suggest high positive predictive value (97.7%) of Xpert-MTB/RIF for simultaneous detection of RIF resistance. ,, Meta analysis data of PLHIV showed a statistically significant result wherein Xpert-MTB/RIF pooled sensitivity was 61% (95% CrI 42-79%) for smear-negative, culture-positive TB compared with 97% (95% CrI 91-99%) for smear-positive, culture-positive TB.  Raizada et al. also reported high treatment success rate and recommends not only the revision of existing national policies for accurate diagnosis of TB among PLHIV but also that Xpert could serve as a primary testing strategy for TB confirmation and should be made available in HIV clinics.  These compelling evidences clearly indicate that Xpert technology is a reliable option for preliminary diagnosis of TB among PLHIV where the risk of rapid progression to AIDS or MDR-TB is great.
| Implementation and Impact of GeneXpert Technology in Developing Nations|| |
Foundation for Innovative New Diagnostics developed Xpert TB/RIF to be implemented on a large scale and, later on, WHO released recommendation and guidance for countries to incorporate this new test into their programs. By the end of 2014, 110 high burden and low-income countries procured 3,553 GeneXpert machines and 8.8 million cartridges.  It is expected that scale up of this brilliant technology would be substantially accelerated in near future that would not only decrease the cost of this assay, but it is anticipated that the assay will be part of revised guidelines in high burden countries. Though numerous studies have demonstrated outstanding performance of this test yet it's very early to judge the contribution of this new test in accurate diagnosis, treatment and cure of TB worldwide. There are many unanswered questions that need to be addressed like how to scale up the use of this assay, especially in regional areas which lack proper infrastructure, whether the assay could be used as point of care testing in rural areas, how the funding for this expensive assay can be arranged in resource limited settings. In South Africa, Xpert-MTB/RIF assay has replaced smear microscopy for early diagnosis of TB and has been implemented on a very large scale at each and every sector of the country. By the end of 2012, the South African ministry announced more procurement of modules (37%) and cartridges (53%) globally.  As of October 2013, WHO again issued a policy update wherein they recommended the use of GeneXpert-MTB/RIF for early diagnosis of pediatric and extrapulmonary TB cases.  The TB Xpert project managed by WHO global TB program along with other STOP TB partners procured 222 GeneXperts by the end of 2013. This project is aimed at providing 1.4 million Xpert-MTB/RIF test cartridges and approximately 220 machines in 21 countries in 2013-2015.  Similarly, government of India is also planning to introduce more machines for initial diagnosis of TB cases. The Revised National TB Control Programme is expanding Cartridge Based NAAT (CBNAAT) testing in a big way. By the scale-up planned, by 2018-19, 1021 CBNAAT machines are expected to be available for use in the country. 
| Achilles Heel of Xpert - Mycobacterium Tuberculosis/Rifampicin ampicin Assay|| |
The only limitation of this incredible technology is its cost which prevents its widespread usage in resource constrained settings therefore its usage is restricted to those clinics or laboratories who can afford the assay. This assay is highly expensive compared to smear microscopy with one single machine costing around US$17,000-$62,000.  Even after negotiated discounts for high burden countries, each single use cartridge cost US$17-$120, and there are additional charges for delivery, installation and annual maintenance of the machine.  Its cost and other logistic constraints restrict its usage and implementation as point of care setting. In addition, the applicability of this assay has also not been evaluated on a large scale in pediatric population. Though cost of this machine is major challenge for low and middle-income countries but looking at the increasing number of PTB and EPTB cases, implementation of this assay is the need of the hour to control this deadly disease from killing more people.
| Concluding Remarks|| |
There is paucity of clinical data on the diagnosis and management of pericardial effusion cases. We recommend the usage of Xpert TB/RIF assay for early diagnosis of pericardial effusion cases as it is not just a test device platform; it is an exciting innovation in TB diagnostics that will also pave the way for many more affordable tests to be launched in future. Implementation of this assay in developing nations will dramatically improve rapid diagnosis of EPTB. To realize this dream of implementing this assay at the doorstep of every HIV/TB clinic, many stakeholders including policy makers, regulators needs to come together to provide appropriate funds for improving public health sectors.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2]