Pediatrics & Therapeutics
Open Access

Xpert MTB/RIF in pediatric tuberculosis

Ashraf Tag-Eldeen^{* *}Correspondence: Professor and Management Consultant. Ashraf Tag-Eldeen, Alexandria University, Egypt, Egypt, Email:

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Introduction

The global burden of tuberculosis worldwide is estimated to be 9 million of which India contributes to 2.7 million cases. The incidence of tuberculosis in Indian children under 14 years of age was 224000 in 2017 (1). This high burden of disease is compounded by difficulty in reliably establishing a microbiological diagnosis. Traditionally microbiological confirmation was done by identification of acid-fast bacilli on smears and culture on solid or liquid medium. Recently the World Health Organization (WHO) approved of a new diagnostic modality called Xpert MTB/RIF, which is a catridge-based nucleic acid amplification test (CB-NAAT) for rapid and reliable diagnosis of tuberculosis (2). In this study we compare the diagnostic usefulness of this new test with the more traditional methods of diagnosis.

Methods

This prospective study was conducted, after ethical clearance from the institutional review board, in Bombay Hospital Institute of Medical Sciences and Research between January 2014 and January 2016. All children from 1 month to 18 years of age with clinical suspicion of tuberculosis were included in the study. Informed consent was taken from the parents of all children under 16 years of age and from the patient if over 16 years of age. Children who had received anti tubercular treatment in the last two years were excluded. Tuberculosis was clinically suspected in children who had unexplained significant weight loss or poor weight gain, unexplained documented fever (temperature >38oC) (3).

All cases were classified as pulmonary and extra pulmonary tuberculosis based on additional symptoms like cough or findings in the respiratory system examination, seizures, altered sensorium, lymphadenopathy, abdominal pain etc.and all children underwent mantoux test. Cases with clinical features suggestive of TB were classified as clinical TB while children who were finally given an alternative diagnosis or responded without anti tubercular medications were classified as non-TB. Imaging was done based on symptoms (CT/MRI for TB meningitis, chest X-ray for pulmonary TB, ultrasound/CT abdomen for suspected abdominal TB). Sputum/gastric lavage/ tissue/body fluid samples were tested for acid-fast bacilli by smear, Xpert MTB/RIF, and mycobacterium growth inhibitor tube (MGIT) with drug sensitivity. Ethics committee and Institutional review board approval for the study was obtained (Ref no.BHIMS/6468)

Results

Demography of the study population

54 children were recruited for the study. The mean age of the study population was 10 years(IQR-4years-16years). 29 children were suspected and investigated for pulmonary TB while 25 children were investigated for extra-pulmonary tuberculosis. Of the total children investigated for either pulmonary or extra-pulmonary tuberculosis 13 were eventually found to have an alternative diagnosis and improved without anti tubercular medications. TB lymphadenitis (8/25,32%) was the most common form of extrapulmonary Tb followed by CNS tuberculosis (7/25, 28%)

Clinical profile

Out of the 41 children with microbiologically proven tuberculosis all presented with fever (100%). Significant weight loss was seen in 24% (10/41) of the children with pulmonary TB and in 9% (4/41) of children with extra-pulmonary TB. A history of significant contact with a case of tuberculosis was seen in 19 % (8/41) of pulmonary and 14% (6/41) of the extra-pulmonary cases. Mantoux was positive in 20% (5/25) children with proven TB who were tested for it. Microbiological results

Acid-fast bacilli were observed using fluorescence microscopy in 27% (8/29) children with suspected pulmonary tuberculosis while 20% (5/25) extra-pulmonary samples were positive for acid-fast bacilli. Xpert MTB/RIF was positive in 65% (19/29) pulmonary and 52% (13/25) extra-pulmonary samples. MGIT was positive in 48.2% (14/29) pulmonary samples and 56% (14/25) extra-pulmonary samples.

In cases with a clinical diagnosis of tuberculosis AFB smear had a sensitivity of 37.3% (CI-17.2% to 59.3%) and specificity of 100% (71.5%-100%)Xpert MTB/RIF had a sensitivity of 75% (CI55.1% to 89.3%) and a specificity of 100% (CI- 75.2% to 100%) while MGIT had a sensitivity of 68.2 % (CI- 51.9% to 81.8%) and specificity of 100% (CI- 75.2% to 100%).

In culture positive cases of pulmonary TB, AFB smear had a sensitivity of 42.2% (CI-17.6% to 71.1%)and specificity of 75.0% (CI-34.8% to 96.8%) while Xpert MTB/RIF had a sensitivity and specificity of 78.5% (CI-49.2 to 95.3%) and 47.6% (CI-21.2%- 73.4%) respectively. In extra-pulmonary TB AFB had a sensitivity of 35.5% (CI 12.7 to 64.8%) and 100%(CI 47.8 to 100%) while Xpert MTB/RIF had sensitivity and specificity of 71.4% (CI - 41.9 to 91.6%) and 54.5% (CI-23.3 % to 83.2%).

Xpert MTB/RIF had 100% sensitivity (CI-79.4% to 100%) and specificity (CI- 76.8% to 100%) for detection of rifampicin resistance. 53.3% (16/30) of the isolates were resistant to rifampicin and isoniazid. All samples that were rifampicin resistant on Xpert MTB/RIF were also resistant to isoniazid tested by conventional drug sensitivity test. Resistance to moxifloxacin was noted in 78% (11/14) while ofloxacin was resistant in 50% of the isolates(7/14).

Seeds sterilized with 0.1% mercuric chloride solution for 5 minutes gives 90 percent sterile culture. It is observed that after inoculation of 5-6 days the seeds coats starts breaking and germination took place within 10 days.

Discussion

Childhood tuberculosis often masquerades as other diseases, making clinical diagnosis difficult. To add to this, obtaining a microbiological confirmation, in cases where it is suspected, is difficult given the paucibacillary nature of the disease and the poor sensitivity of the traditional screening tests like fluorescent microscopy. In our study fluorescent microscopy had a sensitivity of 37.2% in pulmonary samples of children with clinically suspected pulmonary TB. This was far lower than the sensitivity of Xpert MTB/RIF that tested positive in 75% of the cases with clinically suspected pulmonary tuberculosis. In children with culture positive tuberculosis the sensitivity of fluorescent microscopy was 42.2% while that of Xpert MTB/RIF was 75.0%. The world health organization in its policy update systematically analysed the existing data and quoted the sensitivity of Xpert MTB/RIF to vary from 50%-77% for sputum samples and 55%-81% for gastric lavage samples. The same update reported the sensitivity of microscopy to vary from 30-60% in sputum and 0-50% in gastric lavage samples in children with culture positive tuberculosis (2). This review included analysis from both traditional ZN stain and LED fluorescent microscopy. A study from India compared LED fluorescent microscopy with Xpert MTB/RIF and found that the latter outperformed the former in all type of samples (4).

Apart from rapid detection of the bacilli, the ability of Xpert MTB/RIF to reliably detect rifampicin resistance is the other major advantage attributed to it. In our study Xpert MTB/RIF had 100% sensitivity in detecting rifampicin resistance. Additionally, all samples that showed rifampicin resistance on Xpert MTB/ RIF also had resistance to isoniazid acting as a surrogate marker for MDR TB. Boheme et al reported 99.1% sensisitivity of Xpert MTB/RIF for the detection of rifampicin resistance (5).

An appalling finding in our study is the high rates of multidrug resistance. In northern India the prevalence of drug resistant TB is estimated to be 13% in 2015 where some states like Uttar Pradesh it is estimated to be as high as 35% (6). A study of epidemiology of drug resistant TB in children in the city of Mumbai estimated the prevalence of drug resistant TB to be around 9.6% (7). Also disturbing is the high resistance to the second line fluoroquinolones found in our isolates. The easy over the counter access of these antibiotics (8) and irrational use to treat viral respiratory illness may be the cause for this high burden of drug resistance in tuberculosis.

Traditional screening tests like Mantoux test, which picked up only 20% of the microbiologically proven cases in our studies, have long proven to be unreliable resulting in its removal from various diagnostic algorithms (9).

The main limitation in our study is the sample size, which had to be restricted due to limited funds.

Conclusion

The latest endorsement of Xpert MTB/RIF by WHO (2) and its inclusion in the RNTCP guideline (9), while justified, the practical applicability is limited due to financial constraints in resource limited settings like India. Also, Xpert MTB/RIF though an excellent diagnostic test cannot replace liquid culture and drug-sensitivity test, considering the high resistance to other first line and several second line anti-tubercular therapy.

In this day and age of multi drug resistance it may be worthwhile chasing a microbiological diagnosis in children. With increasing sensitivities of the nucleic acid amplification tests paucibacillary disease can no longer be an excuse to start empiric anti tubercular medications. Obtaining adequate, appropriate samples and utilization of all the available diagnostics efficiently by health-care providers and financial aid to replace low yielding, labor intensive investigations like smear microscopy with more reliable nucleic acid amplification tests at district and rural centers by governments will help plan the treatment more efficiently and will eventually contribute towards the goal of disease eradication.

Acknowledgement

-Ms.Sonali rakesh the statistician for her help with the analysis of the data and Quest Diagnostics which carried out the microbiological tests including smear microscopy, Xpert MTB/Rif and MGIT.

References

1. ANNEX 2 Country profiles for 30 high tb burden countries, 20 high TB burden countries based on absolute number of incident cases 10 high TB burden countries based on severity of disease burden (incidence per capita).
2. Automated real-time nucleic acid amplification technology for rapid and simultaneous detection of tuberculosis and ri- fampicin resistance: Xpert MTB/RIF assay for the diagnosis of pulmonary and extrapulmonary TB in adults and children.
3. Alvarez Uria G, Azcona JM, Midde M, Naik PK, Reddy S, Reddy R. Rapid Diagnosis of Pulmonary and Extrapul- monary Tuberculosis in HIV-Infected Patients. Tuberc Res Treat.2012;134:1â??4.
4. Boehme CC, Nicol MP, Nabeta P, Michael JS, Gotuzzo E, Ta- hirli R, 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 imple mentation study. Lancet 2011;377:1495â??505.
5. Shah MA, Shah I. Increasing Prevalence of Pediatric Drug- Resistant Tuberculosis in Mumbai, India and its Outcome. Pediatr Infect Dis J. 2018; 37: 1261-1263.
6. Shet A, Sundaresan S, Forsberg BC. Pharmacy-based dispens- ing of antimicrobial agents without prescription in India: Ap- propriateness and cost burden in the private sector. Antimi- crob Resist Infect Control. 2015;4.
7. Chaudhuri A. Recent changes in technical and operational guidelines for tuberculosis control programme in India - 2016: A paradigm shift in tuberculosis control. J Assoc Chest Physicians. 2017;5:1.