- Email: [email protected]
RIF: The newly endorsed TB diagnostic
p e d i a t r i c i n f e c t i o u s d i s e a s e 6 ( 2 0 1 4 ) 7 5 e7 8
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/pid
Notes from the Lab
Xpert MTB/RIF: The newly endorsed TB diagnostic Chaitali Nikam, Camilla Rodrigues* Department of Microbiology, P.D. Hinduja Hospital & Medical Research Centre, Veer Savarkar Marg, Mahim, Mumbai, Maharashtra 400016, India
article info
abstract
Article history:
Early diagnosis of drug-resistant tuberculosis is desirable to treat patient's regimens and to
Received 30 June 2014
prevent the transmission of resistant strains. Real-time PCR assays have been developed to
Accepted 7 July 2014
detect drug resistance rapidly, but none of them have been widely applied due to their
Available online 22 July 2014
complexity, high cost, or requirement for advanced instruments. GeneXpert is now approved by WHO for use in low income settings and can be used to screen pulmonary as
Keywords:
well as extra pulmonary specimens for resistance to Rifampicin with as low as 2 h of
Tuberculosis
Turnaround Time (TAT).
GeneXpert
Tuberculosis (TB) in children is rarely confirmed due to the lack of effective diagnostic
MDR-TB
tools; approximately 10e15% of pediatric TB is smear-positive due to paucibacillary sam-
EPTB
ples and the difficulty of obtaining high-quality specimens from children. TB is one of the
RT-PCR
top ten killers of children worldwide. Every day, up to 200 children lose their lives to TB and every year, up to 74,000 needlessly die. It is estimated that over half a million cases of TB occur in children every year, although researchers believe this figure is much higher as childhood TB is under reported. The true burden of TB in children is unknown due to inadequate surveillance and reporting of childhood TB cases. Copyright © 2014, Indian Academy of Pediatrics, Infectious Disease Chapter. All rights reserved.
1.
Global scenario
According to the World Health Organization (WHO), Mycobacterium tuberculosis (MTB) is considered to be vastly under diagnosed today. Each year an estimated 8.6 million people worldwide develop tuberculosis disease (TB) and less than 6 million cases are reported and treated.1 In addition, of the more than 300,000 estimated multi-drug-resistant TB (MDRTB) cases among all notified cases, less than a third were reported in 2012. Recent estimates indicate that infants and children comprise 15e20% of cases of tuberculosis disease in resource-limited countries2 and in some regions, this may be
as high as 50%.3 In developing regions, an increasing proportion of these children are co-infected with HIV, which results more severe TB disease and higher mortality.4 Children often acquire their disease from infected adults in their own family or in the community.5 In fact, the incidence of TB in children, who usually present with symptoms within a year of infection, provides a snapshot of transmission within a community.6 This is a direct result of current MTB testing methods requiring weeks to deliver a definitive result, which can lead to patients being left untreated or placed on ineffective therapies. These patients may continue to spread MTB to others in the community, increasing the disease burden. Great challenges face the global community, with the worldwide re-
* Corresponding author. Tel.: þ91 22 24447795; fax: þ91 22 24442318. E-mail address: [email protected] (C. Rodrigues). http://dx.doi.org/10.1016/j.pid.2014.07.003 2212-8328/Copyright © 2014, Indian Academy of Pediatrics, Infectious Disease Chapter. All rights reserved.
76
p e d i a t r i c i n f e c t i o u s d i s e a s e 6 ( 2 0 1 4 ) 7 5 e7 8
emergence of TB and increased number of multi-drug-resistant (MDR) and emergence of extensively drug-resistant (XDR) TB strains. The need for fast and accurate TB diagnosis with indication for the drug resistance is greater than before. Diagnosis of tuberculosis in children, however, is a major challenge. Infants and young children may cough, but they cannot be instructed to expectorate the sputum that they produce. If they cough, they usually swallow the sputum. That is the premise on which the collection of gastric aspirates is based: the swallowed acid fast bacilli (AFB) can be recovered from the stomach, especially in the morning before the stomach contents are emptied into the gastrointestinal tract. At least one study has shown that stool (ultimate destination of swallowed sputum) is another potentially useful specimen for detection of TB in children, although others have not had such positive results.7,8
2.
What is Xpert MTB/RIF ?
The GeneXpert MTB/RIF assay (“Xpert”, Cepheid Diagnostics, Sunnyvale, California) is a novel, Semi-automated Nucleic Acid Amplification Test (NAAT) with similar diagnostic accuracy to first-generation commercial NAATs.9,10 It is based on molecular beacon technology. Many clinical laboratories already use the Xpert platform for other diagnostic applications, and its minimal labor requirements make it simpler, faster, and potentially cheaper than previous NAATs. The European Union and World Health Organization have endorsed Xpert for TB evaluation in 2010,11 and on July 25, 2013, U.S. Food and Drug Administration (FDA) authorized its use for TB evaluation in the U.S.12 WHO published policy for use of GeneXpert in EPTB in 2013.13
sodium hydroxide and isopropanol-containing sample reagent (SR). The SR is added to the sample (currently recommended at a 3:1 ratio for sputum pellets and a 2:1 ratio for unprocessed sputum samples) and incubated at room temperature for 15 min. This step is designed to reduce the viability of M. tuberculosis in sputum at least 106 fold to reduce biohazard risk.15 The treated sample is then manually transferred to the cartridge which is loaded into the GeneXpert instrument, following processing is fully automated. Results are easy to read (Table 1).
4.
What happens inside each cartridge?
The cartridge incorporates a syringe drive, a rotary drive and a filter upon which M. tuberculosis bacilli are deposited after being liberated from the clinical material. The test platform employs a sonic horn that inserts into the cartridge base to cause ultrasonic lysis of the bacilli and release of the genetic material. The assay also contains lyophilized Bacillus globigii spores which serve as an internal sample processing and PCR control. The B. globigii PCR assay is multiplexed with the M. tuberculosis assay. M. tuberculosis is detected by the five overlapping molecular probes (probes AeE) that collectively are complementary to the entire 81 bp rpoB core region.12,13 M. tuberculosis is identified when at least two of the five probes give positive signals with a cycle threshold (CT) of 38 cycles and that differ by no more than a pre-specified number of cycles.
5.
Result interpretation
Table 1 e Interpretation of results.16 Interpretation and proposed minimum laboratory report laboratory for results from the Cepheid Xpert MTB/RIF assay GeneXpert instrument generated result using Xpert MTB/RIF assay
Interpretation of Xpert MTB/RIF assay result
MTB detected, RIF resistance detected
MTB target is detected within the sample. A mutation in the rpoB gene has been detected.
MTB detected, RIF resistance not detected
MTB target is detected within the sample. A mutation in the rpoB gene has not been detected.
MTB detected, RIF resistance indeterminate
MTB target is detected within the sample. A mutation in the rpoB gene because of insufficient signal detection. MTB target is not detected within the sample.
MTB not detected
3.
The Xpert MTB/RIF assay: how it work?
The assay utilizes single-use plastic cartridges with multiple chambers that are preloaded with liquid buffers and lyophilized reagent beads necessary for sample processing, DNA extraction and hemi nested RT-PCR.14 Clinical sputum samples (or decontaminated sputum pellets) are treated with a
Minimum laboratory report language MTBC detected. A mutation in rpoB gene has been detected, indicating possible RMP resistance. Confirmatory testing should follow. MTBC detected. No rpoB gene mutations detected; probably RMP susceptible. MTBC detected; presence of rpoB gene mutations cannot be accurately determined. MTBC not detected.
6. Limit of detection (LOD), analytical performance (sensitivity) and specificity in sputum and Extrapulmonary tuberculosis (EPTB) specimens The MTB/RIF assay has a calculated limit of detection of 131 CFU/ ml of sputum and was able to detect as few as 10 CFU/ml of
p e d i a t r i c i n f e c t i o u s d i s e a s e 6 ( 2 0 1 4 ) 7 5 e7 8
sputum in 35% of samples. The assay showed 100% sensitivity in smear-positive samples and 71.7% in smear-negative samples those were positive in culture. Specificity was 100% in each case. It detected as few as 1 genome/reaction approximately 40% of the time and detected 7.5 genomes/reaction 100% of the time. These results predicted an LOD for DNA analysis (where LOD is defined as the target number at which there is a 95% probability of a positive assay) of 4.5 genomes/reaction.14 The limit of detection of the Xpert MTB/RIF assay in sputum appeared to be close to that of culture and was at least 2 orders of magnitude more sensitive than the AFB smear. The Xpert MTB/RIF assay showed 100% sensitivity specificity for rifampicin resistance detection.15 In Cochrane review published on 2014, out of 27 unique studies (integrating nine new studies) involving 9557 participants. Sixteen studies (59%) were performed in low- or middle-income countries showed following results. As an initial test replacing smear microscopy, Xpert MTB/ RIF pooled sensitivity was 89% [95% Credible Interval (CrI) 85%e92%] and pooled specificity 99% (95% CrI 98%e99%), (22 studies, 8998 participants: 2953 confirmed TB, 6045 non-TB). As an add-on test following a negative smear microscopy result, Xpert MTB/RIF pooled sensitivity was 67% (95% CrI 60%e74%) and pooled specificity 99% (95% CrI 98%e99%; 21 studies, 6950 participants). For smear-positive, culture-positive TB, Xpert MTB/RIF pooled sensitivity was 98% (95% CrI 97%e99%; 21 studies, 1936 participants). For people with HIV infection, Xpert MTB/RIF pooled sensitivity was 79% (95% CrI 70%e86%; 7 studies, 1789 participants), and for people without HIV infection, it was 86% (95% CrI 76%e92%; 7 studies, 1470 participants). In comparison with smear microscopy, Xpert MTB/RIF increased TB detection among culture-confirmed cases by 23% (95% CrI 15%e32%; 21 studies, 8880 participants). In Rifampicin resistance detection, Xpert MTB/RIF pooled sensitivity was 95% (95% CrI 90%e97%; 17 studies, 555 rifampicin resistance positives) and pooled specificity was 98% (95% CrI 97%e 99%; 24 studies, 2411 rifampicin resistance negatives).15 In EPTB specimens like, tissue biopsies/fine-needle aspirates; pleural fluid; gastric aspirates; pus; CSF; urine; peritoneal/synovial/pericardial fluid sensitivity is 81.3% (CI: 76.2e85.8) and specificity is 99.8% (CI: 99.4e100).17 Tissue biopsies (105); pus (98); body fluids (24), it's having sensitivity and specificity of 80.6% (CI: 75.5e85) and 99.6% (97.8e100) respectively. Xpert MTB/RIF should be used in preference to conventional microscopy and culture as the initial diagnostic test for CSF specimens from patients suspected of having TB meningitis.18 The utility of Xpert MTB/RIF in diagnosing pediatric pulmonary TB was evaluated in 13 studies that included 2603 participants. Studies either collected the same specimen type from all children or different types of specimens from different subgroups of children (for example, samples of expectorated sputum were collected from older children; samples of induced sputum or gastric lavage or aspirate were collected from younger children). In three studies different types of specimens were collected from each child. As a result, a total of 3347 specimens were assessed: expectorated sputum (4 studies, 270 children), induced sputum (7 studies, 1279 children), nasopharyngeal aspirate (1 study, 474 children), gastric lavage or aspirate (6 studies, 1324 children). For samples of expectorated sputum, sensitivity varied from 55% to
77
90%; for induced sputum sensitivity varied from 40% to 100%; and for gastric lavage or aspirate it varied from 40% to 100%. Confidence intervals overlapped for each specimen type, suggesting that no specimen type was superior. Specificities for all studies and specimen types ranged from 93% to 100%.19 Induced sputum, nasopharyngeal aspirates and, stool, can be recommended sample types for GeneXpert in children.
7.
Conclusion
Xpert is an exciting innovation in TB diagnostics. It has the potential for TB case detection in situations where traditional diagnostics are inadequate. The possibility to diagnose TB in 2 h will lead to fewer deaths and less transmission of disease. Xpert is however not a panacea. Its implementation presents challenges related to cost and infrastructure. It is not a point-of-care test which remains an important need for TB diagnostics. Fast and accurate detection of TB and MDR-TB is required at the community level with a point-of-care test, a strong laboratory network with a good referral system to ensure that patients have access to all the diagnostic and follow-up testing they need.
Conflicts of interest All authors have none to declare.
references
1. World Health Organization. Global Tuberculosis Report 2013. Geneva, Switzerland: World Health Organization; 2012. 2. Hesseling AC, Schaaf HS, Gie RP, Starke JR, Beyers N. A critical review of diagnostic approaches used in the diagnosis of childhood tuberculosis. Int J Tuberc Lung Dis. 2002;6(12):1038e1045. 3. Marais BJ, Schaaf HS. Childhood tuberculosis: an emerging and previously neglected problem. Infect Dis Clin North Am. 2010;24(3):727e749. 4. Mwinga A. Challenges and hope for the diagnosis of tuberculosis in infants and young children. Lancet. 2005;365:97e98. 5. Wood R, Robertson SJ, Pieter Uys, et al. Tuberculosis transmission to young children in a South African community: modeling household and community infection risks. Clin Infect Dis. 2010;51:401e408. 6. Stefan DC, Dippenaar A, Detjen AK, et al. Interferon-gamma release assays for the detection of Mycobacterium tuberculosis infection in children with cancer. Int J Tuberc Lung Dis. 2010;14:689e694. 7. Donald PR, Schaaf HS, Gie RP, Beyers N, Sirgel FA, Venter A. Stool microscopy and culture to assist the diagnosis of pulmonary tuberculosis in childhood. J Trop Pediatr. 1996;42:311e312. 8. Oberhelman RA, Oberhelman Richard A, SotoCastellares Giselle, et al. Diagnostic approaches for paediatric tuberculosis by use of different specimen types, culture methods, and PCR: a prospective case-control study. Lancet Infect Dis. 2010;10:612e620.
78
p e d i a t r i c i n f e c t i o u s d i s e a s e 6 ( 2 0 1 4 ) 7 5 e7 8
9. Boehme CC, Nabeta P, Hillemann D, et al. Rapid molecular detection of tuberculosis and rifampin resistance. N Engl J Med. 2010;363:1005e1015. 10. Boehme CC, Nicol MP, Nabeta P, et al. Feasibility, diagnostic accuracy, and effectiveness of decentralised use of the Xpert MTB/RIF test for diagnosis of tuberculosis and multidrug resistance: a multicentre implementation study. Lancet. 2011;377:1495e1505. 11. World Health Organization. Automated Real-time Nucleic Acid Amplification Technology for Rapid and Simultaneous Detection of Tuberculosis and Rifampicin Resistance: Xpert MTB/RIF System: Policy Statement. Geneva: World Health Organization; 2011. 12. U.S. Food and Drug Administration. FDA Permits Marketing of First U.S. Test Labeled for Simultaneous Detection of Tuberculosis Bacteria and Resistance to the Antibiotic Rifampin; July 25, 2013. Available from: http://www.fda.gov/NewsEvents/Newsroom/ PressAnnouncements/ucm362602.htm. 13. Helb D, Jones M, Story E, et al. Rapid detection of mycobacterium tuberculosis and rifampin resistance by use of on-demand, near-patient technology. J Clin Microbiol. 2010;48:229e237.
14. Blakemore R, Story E, Helb D, et al. Evaluation of the analytical performance of the Xpert(R) MTB/RIF assay. J Clin Microbiol. 2010;48:249e251. 15. Banada PP, Sivasubramani SK, Blakemore R, et al. Containment of bioaerosol infection risk by the Xpert MTB/ RIF assay and its applicability to point-of-care settings. J Clin Microbiol. 2010;48:3551e3557. 16. http://www.ipaqt.org/wp-content/uploads/2013/04/IPAQTTechnical-FAQs-on-Xpert-MTB_V5.pdf. 17. Vadwai V, Boehme C, Nabeta P, Shetty A, Alland D, Rodrigues C. Xpert MTB/RIF, a new pillar in the diagnosis of extrapulmonary tuberculosis? J Clin Microbiol. 2011;49:2540e2545. 18. Denkinger CM, Schumacher SG, Boehme CC, Dendukuri N, Pai M, Steingart KR. Xpert MTB/RIF assay for the diagnosis of extrapulmonary tuberculosis: a systematic review and metaanalysis. Eur Respir J. 2014 Apr 17 [Epub ahead of print]. 19. XPert MTB/RIF Assay for the Diagnosis of Pulmonary and Extrapulmonary TB in Adults and Children. Policy Update. Available from: http://www.who.int/tb/laboratory/xpert_ launchupdate/en/.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/pid
Notes from the Lab
Xpert MTB/RIF: The newly endorsed TB diagnostic Chaitali Nikam, Camilla Rodrigues* Department of Microbiology, P.D. Hinduja Hospital & Medical Research Centre, Veer Savarkar Marg, Mahim, Mumbai, Maharashtra 400016, India
article info
abstract
Article history:
Early diagnosis of drug-resistant tuberculosis is desirable to treat patient's regimens and to
Received 30 June 2014
prevent the transmission of resistant strains. Real-time PCR assays have been developed to
Accepted 7 July 2014
detect drug resistance rapidly, but none of them have been widely applied due to their
Available online 22 July 2014
complexity, high cost, or requirement for advanced instruments. GeneXpert is now approved by WHO for use in low income settings and can be used to screen pulmonary as
Keywords:
well as extra pulmonary specimens for resistance to Rifampicin with as low as 2 h of
Tuberculosis
Turnaround Time (TAT).
GeneXpert
Tuberculosis (TB) in children is rarely confirmed due to the lack of effective diagnostic
MDR-TB
tools; approximately 10e15% of pediatric TB is smear-positive due to paucibacillary sam-
EPTB
ples and the difficulty of obtaining high-quality specimens from children. TB is one of the
RT-PCR
top ten killers of children worldwide. Every day, up to 200 children lose their lives to TB and every year, up to 74,000 needlessly die. It is estimated that over half a million cases of TB occur in children every year, although researchers believe this figure is much higher as childhood TB is under reported. The true burden of TB in children is unknown due to inadequate surveillance and reporting of childhood TB cases. Copyright © 2014, Indian Academy of Pediatrics, Infectious Disease Chapter. All rights reserved.
1.
Global scenario
According to the World Health Organization (WHO), Mycobacterium tuberculosis (MTB) is considered to be vastly under diagnosed today. Each year an estimated 8.6 million people worldwide develop tuberculosis disease (TB) and less than 6 million cases are reported and treated.1 In addition, of the more than 300,000 estimated multi-drug-resistant TB (MDRTB) cases among all notified cases, less than a third were reported in 2012. Recent estimates indicate that infants and children comprise 15e20% of cases of tuberculosis disease in resource-limited countries2 and in some regions, this may be
as high as 50%.3 In developing regions, an increasing proportion of these children are co-infected with HIV, which results more severe TB disease and higher mortality.4 Children often acquire their disease from infected adults in their own family or in the community.5 In fact, the incidence of TB in children, who usually present with symptoms within a year of infection, provides a snapshot of transmission within a community.6 This is a direct result of current MTB testing methods requiring weeks to deliver a definitive result, which can lead to patients being left untreated or placed on ineffective therapies. These patients may continue to spread MTB to others in the community, increasing the disease burden. Great challenges face the global community, with the worldwide re-
* Corresponding author. Tel.: þ91 22 24447795; fax: þ91 22 24442318. E-mail address: [email protected] (C. Rodrigues). http://dx.doi.org/10.1016/j.pid.2014.07.003 2212-8328/Copyright © 2014, Indian Academy of Pediatrics, Infectious Disease Chapter. All rights reserved.
76
p e d i a t r i c i n f e c t i o u s d i s e a s e 6 ( 2 0 1 4 ) 7 5 e7 8
emergence of TB and increased number of multi-drug-resistant (MDR) and emergence of extensively drug-resistant (XDR) TB strains. The need for fast and accurate TB diagnosis with indication for the drug resistance is greater than before. Diagnosis of tuberculosis in children, however, is a major challenge. Infants and young children may cough, but they cannot be instructed to expectorate the sputum that they produce. If they cough, they usually swallow the sputum. That is the premise on which the collection of gastric aspirates is based: the swallowed acid fast bacilli (AFB) can be recovered from the stomach, especially in the morning before the stomach contents are emptied into the gastrointestinal tract. At least one study has shown that stool (ultimate destination of swallowed sputum) is another potentially useful specimen for detection of TB in children, although others have not had such positive results.7,8
2.
What is Xpert MTB/RIF ?
The GeneXpert MTB/RIF assay (“Xpert”, Cepheid Diagnostics, Sunnyvale, California) is a novel, Semi-automated Nucleic Acid Amplification Test (NAAT) with similar diagnostic accuracy to first-generation commercial NAATs.9,10 It is based on molecular beacon technology. Many clinical laboratories already use the Xpert platform for other diagnostic applications, and its minimal labor requirements make it simpler, faster, and potentially cheaper than previous NAATs. The European Union and World Health Organization have endorsed Xpert for TB evaluation in 2010,11 and on July 25, 2013, U.S. Food and Drug Administration (FDA) authorized its use for TB evaluation in the U.S.12 WHO published policy for use of GeneXpert in EPTB in 2013.13
sodium hydroxide and isopropanol-containing sample reagent (SR). The SR is added to the sample (currently recommended at a 3:1 ratio for sputum pellets and a 2:1 ratio for unprocessed sputum samples) and incubated at room temperature for 15 min. This step is designed to reduce the viability of M. tuberculosis in sputum at least 106 fold to reduce biohazard risk.15 The treated sample is then manually transferred to the cartridge which is loaded into the GeneXpert instrument, following processing is fully automated. Results are easy to read (Table 1).
4.
What happens inside each cartridge?
The cartridge incorporates a syringe drive, a rotary drive and a filter upon which M. tuberculosis bacilli are deposited after being liberated from the clinical material. The test platform employs a sonic horn that inserts into the cartridge base to cause ultrasonic lysis of the bacilli and release of the genetic material. The assay also contains lyophilized Bacillus globigii spores which serve as an internal sample processing and PCR control. The B. globigii PCR assay is multiplexed with the M. tuberculosis assay. M. tuberculosis is detected by the five overlapping molecular probes (probes AeE) that collectively are complementary to the entire 81 bp rpoB core region.12,13 M. tuberculosis is identified when at least two of the five probes give positive signals with a cycle threshold (CT) of 38 cycles and that differ by no more than a pre-specified number of cycles.
5.
Result interpretation
Table 1 e Interpretation of results.16 Interpretation and proposed minimum laboratory report laboratory for results from the Cepheid Xpert MTB/RIF assay GeneXpert instrument generated result using Xpert MTB/RIF assay
Interpretation of Xpert MTB/RIF assay result
MTB detected, RIF resistance detected
MTB target is detected within the sample. A mutation in the rpoB gene has been detected.
MTB detected, RIF resistance not detected
MTB target is detected within the sample. A mutation in the rpoB gene has not been detected.
MTB detected, RIF resistance indeterminate
MTB target is detected within the sample. A mutation in the rpoB gene because of insufficient signal detection. MTB target is not detected within the sample.
MTB not detected
3.
The Xpert MTB/RIF assay: how it work?
The assay utilizes single-use plastic cartridges with multiple chambers that are preloaded with liquid buffers and lyophilized reagent beads necessary for sample processing, DNA extraction and hemi nested RT-PCR.14 Clinical sputum samples (or decontaminated sputum pellets) are treated with a
Minimum laboratory report language MTBC detected. A mutation in rpoB gene has been detected, indicating possible RMP resistance. Confirmatory testing should follow. MTBC detected. No rpoB gene mutations detected; probably RMP susceptible. MTBC detected; presence of rpoB gene mutations cannot be accurately determined. MTBC not detected.
6. Limit of detection (LOD), analytical performance (sensitivity) and specificity in sputum and Extrapulmonary tuberculosis (EPTB) specimens The MTB/RIF assay has a calculated limit of detection of 131 CFU/ ml of sputum and was able to detect as few as 10 CFU/ml of
p e d i a t r i c i n f e c t i o u s d i s e a s e 6 ( 2 0 1 4 ) 7 5 e7 8
sputum in 35% of samples. The assay showed 100% sensitivity in smear-positive samples and 71.7% in smear-negative samples those were positive in culture. Specificity was 100% in each case. It detected as few as 1 genome/reaction approximately 40% of the time and detected 7.5 genomes/reaction 100% of the time. These results predicted an LOD for DNA analysis (where LOD is defined as the target number at which there is a 95% probability of a positive assay) of 4.5 genomes/reaction.14 The limit of detection of the Xpert MTB/RIF assay in sputum appeared to be close to that of culture and was at least 2 orders of magnitude more sensitive than the AFB smear. The Xpert MTB/RIF assay showed 100% sensitivity specificity for rifampicin resistance detection.15 In Cochrane review published on 2014, out of 27 unique studies (integrating nine new studies) involving 9557 participants. Sixteen studies (59%) were performed in low- or middle-income countries showed following results. As an initial test replacing smear microscopy, Xpert MTB/ RIF pooled sensitivity was 89% [95% Credible Interval (CrI) 85%e92%] and pooled specificity 99% (95% CrI 98%e99%), (22 studies, 8998 participants: 2953 confirmed TB, 6045 non-TB). As an add-on test following a negative smear microscopy result, Xpert MTB/RIF pooled sensitivity was 67% (95% CrI 60%e74%) and pooled specificity 99% (95% CrI 98%e99%; 21 studies, 6950 participants). For smear-positive, culture-positive TB, Xpert MTB/RIF pooled sensitivity was 98% (95% CrI 97%e99%; 21 studies, 1936 participants). For people with HIV infection, Xpert MTB/RIF pooled sensitivity was 79% (95% CrI 70%e86%; 7 studies, 1789 participants), and for people without HIV infection, it was 86% (95% CrI 76%e92%; 7 studies, 1470 participants). In comparison with smear microscopy, Xpert MTB/RIF increased TB detection among culture-confirmed cases by 23% (95% CrI 15%e32%; 21 studies, 8880 participants). In Rifampicin resistance detection, Xpert MTB/RIF pooled sensitivity was 95% (95% CrI 90%e97%; 17 studies, 555 rifampicin resistance positives) and pooled specificity was 98% (95% CrI 97%e 99%; 24 studies, 2411 rifampicin resistance negatives).15 In EPTB specimens like, tissue biopsies/fine-needle aspirates; pleural fluid; gastric aspirates; pus; CSF; urine; peritoneal/synovial/pericardial fluid sensitivity is 81.3% (CI: 76.2e85.8) and specificity is 99.8% (CI: 99.4e100).17 Tissue biopsies (105); pus (98); body fluids (24), it's having sensitivity and specificity of 80.6% (CI: 75.5e85) and 99.6% (97.8e100) respectively. Xpert MTB/RIF should be used in preference to conventional microscopy and culture as the initial diagnostic test for CSF specimens from patients suspected of having TB meningitis.18 The utility of Xpert MTB/RIF in diagnosing pediatric pulmonary TB was evaluated in 13 studies that included 2603 participants. Studies either collected the same specimen type from all children or different types of specimens from different subgroups of children (for example, samples of expectorated sputum were collected from older children; samples of induced sputum or gastric lavage or aspirate were collected from younger children). In three studies different types of specimens were collected from each child. As a result, a total of 3347 specimens were assessed: expectorated sputum (4 studies, 270 children), induced sputum (7 studies, 1279 children), nasopharyngeal aspirate (1 study, 474 children), gastric lavage or aspirate (6 studies, 1324 children). For samples of expectorated sputum, sensitivity varied from 55% to
77
90%; for induced sputum sensitivity varied from 40% to 100%; and for gastric lavage or aspirate it varied from 40% to 100%. Confidence intervals overlapped for each specimen type, suggesting that no specimen type was superior. Specificities for all studies and specimen types ranged from 93% to 100%.19 Induced sputum, nasopharyngeal aspirates and, stool, can be recommended sample types for GeneXpert in children.
7.
Conclusion
Xpert is an exciting innovation in TB diagnostics. It has the potential for TB case detection in situations where traditional diagnostics are inadequate. The possibility to diagnose TB in 2 h will lead to fewer deaths and less transmission of disease. Xpert is however not a panacea. Its implementation presents challenges related to cost and infrastructure. It is not a point-of-care test which remains an important need for TB diagnostics. Fast and accurate detection of TB and MDR-TB is required at the community level with a point-of-care test, a strong laboratory network with a good referral system to ensure that patients have access to all the diagnostic and follow-up testing they need.
Conflicts of interest All authors have none to declare.
references
1. World Health Organization. Global Tuberculosis Report 2013. Geneva, Switzerland: World Health Organization; 2012. 2. Hesseling AC, Schaaf HS, Gie RP, Starke JR, Beyers N. A critical review of diagnostic approaches used in the diagnosis of childhood tuberculosis. Int J Tuberc Lung Dis. 2002;6(12):1038e1045. 3. Marais BJ, Schaaf HS. Childhood tuberculosis: an emerging and previously neglected problem. Infect Dis Clin North Am. 2010;24(3):727e749. 4. Mwinga A. Challenges and hope for the diagnosis of tuberculosis in infants and young children. Lancet. 2005;365:97e98. 5. Wood R, Robertson SJ, Pieter Uys, et al. Tuberculosis transmission to young children in a South African community: modeling household and community infection risks. Clin Infect Dis. 2010;51:401e408. 6. Stefan DC, Dippenaar A, Detjen AK, et al. Interferon-gamma release assays for the detection of Mycobacterium tuberculosis infection in children with cancer. Int J Tuberc Lung Dis. 2010;14:689e694. 7. Donald PR, Schaaf HS, Gie RP, Beyers N, Sirgel FA, Venter A. Stool microscopy and culture to assist the diagnosis of pulmonary tuberculosis in childhood. J Trop Pediatr. 1996;42:311e312. 8. Oberhelman RA, Oberhelman Richard A, SotoCastellares Giselle, et al. Diagnostic approaches for paediatric tuberculosis by use of different specimen types, culture methods, and PCR: a prospective case-control study. Lancet Infect Dis. 2010;10:612e620.
78
p e d i a t r i c i n f e c t i o u s d i s e a s e 6 ( 2 0 1 4 ) 7 5 e7 8
9. Boehme CC, Nabeta P, Hillemann D, et al. Rapid molecular detection of tuberculosis and rifampin resistance. N Engl J Med. 2010;363:1005e1015. 10. Boehme CC, Nicol MP, Nabeta P, et al. Feasibility, diagnostic accuracy, and effectiveness of decentralised use of the Xpert MTB/RIF test for diagnosis of tuberculosis and multidrug resistance: a multicentre implementation study. Lancet. 2011;377:1495e1505. 11. World Health Organization. Automated Real-time Nucleic Acid Amplification Technology for Rapid and Simultaneous Detection of Tuberculosis and Rifampicin Resistance: Xpert MTB/RIF System: Policy Statement. Geneva: World Health Organization; 2011. 12. U.S. Food and Drug Administration. FDA Permits Marketing of First U.S. Test Labeled for Simultaneous Detection of Tuberculosis Bacteria and Resistance to the Antibiotic Rifampin; July 25, 2013. Available from: http://www.fda.gov/NewsEvents/Newsroom/ PressAnnouncements/ucm362602.htm. 13. Helb D, Jones M, Story E, et al. Rapid detection of mycobacterium tuberculosis and rifampin resistance by use of on-demand, near-patient technology. J Clin Microbiol. 2010;48:229e237.
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