Diagnostic accuracy of Xpert ultra for childhood tuberculosis: A preliminary systematic review and meta-analysis

What is New in the Diagnosis of Childhood Tuberculosis?

The fact that almost half of the 1 million cases of childhood tuberculosis (TB) globally remain undiagnosed jeopardizes the TB elimination goal. Fortunately, there are new advances in this field which have the potential to bridge this diagnostic gap. Advances in imaging include computer assisted interpretation of chest X-rays (CXRs), point of care ultrasound (POCUS) and faster and superior computed tomography/ magnetic resonance imaging (CT/ MRI) protocols. The urine lipoarabinomannan test has proved to be a good point of care test for diagnosing TB in Human immunodeficiency virus (HIV) infected children. Stool and nasopharyngeal aspirates are emerging as acceptable alternatives for gastric lavage and induced sputum for diagnosing intrathoracic tuberculosis. Xpert MTB/RIF Ultra has improved sensitivity compared to Xpert MTB/RIF for diagnosing both pulmonary/ extrapulmonary TB. Xpert XDR is another commercially available accurate point of care test for detecting resistance to drugs other than rifampicin in smear positive samples. Other molecular methods including new line probe assays, pyrosequencing, whole genome sequencing, and targeted next generation sequencing are extremely promising but not available commercially at present. The C-Tb skin test is an acceptable alternative to the tuberculin skin test and interferon gamma release assays for diagnosis of latent infection. There is an urgent need to incorporate some of these advances in the existing diagnostic algorithms of childhood TB.

Evaluation of Xpert MTB/RIF Ultra for the Diagnosis of Extrapulmonary Tuberculosis: A Retrospective Analysis in Saudi Arabia

The incidence of extrapulmonary tuberculosis (EPTB) in low- and middle-income countries, as well as, high-income countries has increased over the last two decades. The acid-fast bacillus (AFB) smear test is easy to perform and cost-effective with a quick turnaround time but the test has low sensitivity. Culture remains the gold standard for detecting TB; however, it has low sensitivity and slow bacterial growth patterns, as it may take up to 6 to 8 weeks to grow. Therefore, a rapid detection tool is crucial for the early initiation of treatment and ensuring an improved therapeutic outcome. Here, the Xpert Ultra system was developed as a nucleic acid amplification technique to accelerate the detection of MTB in paucibacillary clinical samples and endorsed by the World Health Organization. From March 2020 to August 2021, Xpert Ultra was evaluated for its sensitivity and specificity against EPTB and compared with those of the routinely used Xpert, culture, and AFB tests in 845 clinical samples in Saudi Arabia. The results indicate the overall sensitivity and specificity of Xpert Ultra to be 91% and 95%, respectively, compared with the Xpert (82% and 99%, respectively) and AFB smear (18% and 100%, respectively) tests. The results also indicated that despite the low microbial loads that were categorized as trace, very low, or low on Xpert Ultra, yet, complete detection was achieved with some sample types (i.e., 100% detection). Consequently, Xpert Ultra has great potential to replace conventional diagnostic approaches as a standard detection method for EPTB.

Recent advances in microbiological and molecular biological detection techniques of tuberculous meningitis

Tuberculous meningitis (TBM) is the most common type of central nervous system tuberculosis (TB) and has the highest mortality and disability rate. Early diagnosis is key to improving the prognosis and survival rate of patients. However, laboratory diagnosis of TBM is often difficult due to its paucibacillary nature and sub optimal sensitivity of conventional microbiology and molecular tools which often fails to detect the pathogen. The gold standard for TBM diagnosis is the presence of MTB in the CSF. The recognised methods for the identification of MTB are acid-fast bacilli (AFB) detected under CSF smear microscopy, MTB cultured in CSF, and MTB detected by polymerase chain reaction (PCR). Currently, many studies consider that all diagnostic techniques for TBM are not perfect, and no single technique is considered simple, fast, cheap, and efficient. A definite diagnosis of TBM is still difficult in current clinical practice. In this review, we summarise the current state of microbiological and molecular biological diagnostics for TBM, the latest advances in research, and discuss the advantages of these techniques, as well as the issues and challenges faced in terms of diagnostic effectiveness, laboratory infrastructure, testing costs, and clinical expertise, for clinicians to select appropriate testing methods.

Clinical Characteristics of Pulmonary Tuberculosis in Children Tested by Xpert MTB/RIF Ultra

BACKGROUND

The Xpert MTB/rifampicin Ultra (Xpert Ultra) assay improves the early diagnosis of active tuberculosis (TB) in children. Clinical evaluation is paramount for the interpretation of any positive Xpert Ultra test, especially those with low quantities of DNA.

METHODS

In this study, 391 children with suspected TB who were tested with Xpert Ultra were enrolled. The clinical characteristics and Xpert Ultra results were further analyzed.

RESULTS

The sensitivity and specificity of Xpert Ultra were 45.0% (149/331) and 96.7% (58/60), respectively. Children with higher semiquantitative scales of Xpert Ultra showed higher percentages of a positive MTB culture, positive acid-fast bacilli staining, severe type of disease, fever, cough and expectoration, a higher white blood cell count and higher C-reactive protein concentrations (all P < 0.01). Among 44 children with an Xpert Ultra trace result, there were no differences in clinical characteristics between confirmed cases and unconfirmed TB cases.

CONCLUSIONS

The prevalence of trace is relatively high and can be considered positive in paucibacillary children. Clinical presentations are associated with bacterial load quantified by Xpert Ultra. The interpretation of Xpert Ultra trace results based on clinical information is important for the diagnosis of TB.

Xpert MTB/RIF Ultra assay for tuberculosis disease and rifampicin resistance in children

BACKGROUND

Every year, an estimated one million children and young adolescents become ill with tuberculosis, and around 226,000 of those children die. Xpert MTB/RIF Ultra (Xpert Ultra) is a molecular World Health Organization (WHO)-recommended rapid diagnostic test that simultaneously detects Mycobacterium tuberculosis complex and rifampicin resistance. We previously published a Cochrane Review 'Xpert MTB/RIF and Xpert MTB/RIF Ultra assays for tuberculosis disease and rifampicin resistance in children'. The current review updates evidence on the diagnostic accuracy of Xpert Ultra in children presumed to have tuberculosis disease. Parts of this review update informed the 2022 WHO updated guidance on management of tuberculosis in children and adolescents.

OBJECTIVES

To assess the diagnostic accuracy of Xpert Ultra for detecting: pulmonary tuberculosis, tuberculous meningitis, lymph node tuberculosis, and rifampicin resistance, in children with presumed tuberculosis. Secondary objectives To investigate potential sources of heterogeneity in accuracy estimates. For detection of tuberculosis, we considered age, comorbidity (HIV, severe pneumonia, and severe malnutrition), and specimen type as potential sources. To summarize the frequency of Xpert Ultra trace results.

SEARCH METHODS

We searched the Cochrane Infectious Diseases Group Specialized Register, MEDLINE, Embase, three other databases, and three trial registers without language restrictions to 9 March 2021.

SELECTION CRITERIA

Cross-sectional and cohort studies and randomized trials that evaluated Xpert Ultra in HIV-positive and HIV-negative children under 15 years of age. We included ongoing studies that helped us address the review objectives. We included studies evaluating sputum, gastric, stool, or nasopharyngeal specimens (pulmonary tuberculosis), cerebrospinal fluid (tuberculous meningitis), and fine needle aspirate or surgical biopsy tissue (lymph node tuberculosis). For detecting tuberculosis, reference standards were microbiological (culture) or composite reference standard; for stool, we also included Xpert Ultra performed on a routine respiratory specimen. For detecting rifampicin resistance, reference standards were drug susceptibility testing or MTBDRplus.

DATA COLLECTION AND ANALYSIS

Two review authors independently extracted data and, using QUADAS-2, assessed methodological quality judging risk of bias separately for each target condition and reference standard. For each target condition, we used the bivariate model to estimate summary sensitivity and specificity with 95% confidence intervals (CIs). We stratified all analyses by type of reference standard. We summarized the frequency of Xpert Ultra trace results; trace represents detection of a very low quantity of Mycobacterium tuberculosis DNA. We assessed certainty of evidence using GRADE.

MAIN RESULTS

We identified 14 studies (11 new studies since the previous review). For detection of pulmonary tuberculosis, 335 data sets (25,937 participants) were available for analysis. We did not identify any studies that evaluated Xpert Ultra accuracy for tuberculous meningitis or lymph node tuberculosis. Three studies evaluated Xpert Ultra for detection of rifampicin resistance. Ten studies (71%) took place in countries with a high tuberculosis burden based on WHO classification. Overall, risk of bias was low. Detection of pulmonary tuberculosis Sputum, 5 studies Xpert Ultra summary sensitivity verified by culture was 75.3% (95% CI 64.3 to 83.8; 127 participants; high-certainty evidence), and specificity was 97.1% (95% CI 94.7 to 98.5; 1054 participants; high-certainty evidence). Gastric aspirate, 7 studies Xpert Ultra summary sensitivity verified by culture was 70.4% (95% CI 53.9 to 82.9; 120 participants; moderate-certainty evidence), and specificity was 94.1% (95% CI 84.8 to 97.8; 870 participants; moderate-certainty evidence). Stool, 6 studies Xpert Ultra summary sensitivity verified by culture was 56.1% (95% CI 39.1 to 71.7; 200 participants; moderate-certainty evidence), and specificity was 98.0% (95% CI 93.3 to 99.4; 1232 participants; high certainty-evidence). Nasopharyngeal aspirate, 4 studies Xpert Ultra summary sensitivity verified by culture was 43.7% (95% CI 26.7 to 62.2; 46 participants; very low-certainty evidence), and specificity was 97.5% (95% CI 93.6 to 99.0; 489 participants; high-certainty evidence). Xpert Ultra sensitivity was lower against a composite than a culture reference standard for all specimen types other than nasopharyngeal aspirate, while specificity was similar against both reference standards. Interpretation of results In theory, for a population of 1000 children: • where 100 have pulmonary tuberculosis in sputum (by culture): - 101 would be Xpert Ultra-positive, and of these, 26 (26%) would not have pulmonary tuberculosis (false positive); and - 899 would be Xpert Ultra-negative, and of these, 25 (3%) would have tuberculosis (false negative). • where 100 have pulmonary tuberculosis in gastric aspirate (by culture): - 123 would be Xpert Ultra-positive, and of these, 53 (43%) would not have pulmonary tuberculosis (false positive); and - 877 would be Xpert Ultra-negative, and of these, 30 (3%) would have tuberculosis (false negative). • where 100 have pulmonary tuberculosis in stool (by culture): - 74 would be Xpert Ultra-positive, and of these, 18 (24%) would not have pulmonary tuberculosis (false positive); and - 926 would be Xpert Ultra-negative, and of these, 44 (5%) would have tuberculosis (false negative). • where 100 have pulmonary tuberculosis in nasopharyngeal aspirate (by culture): - 66 would be Xpert Ultra-positive, and of these, 22 (33%) would not have pulmonary tuberculosis (false positive); and - 934 would be Xpert Ultra-negative, and of these, 56 (6%) would have tuberculosis (false negative). Detection of rifampicin resistance Xpert Ultra sensitivity was 100% (3 studies, 3 participants; very low-certainty evidence), and specificity range was 97% to 100% (3 studies, 128 participants; low-certainty evidence). Trace results Xpert Ultra trace results, regarded as positive in children by WHO standards, were common. Xpert Ultra specificity remained high in children, despite the frequency of trace results.

AUTHORS' CONCLUSIONS

We found Xpert Ultra sensitivity to vary by specimen type, with sputum having the highest sensitivity, followed by gastric aspirate and stool. Nasopharyngeal aspirate had the lowest sensitivity. Xpert Ultra specificity was high against both microbiological and composite reference standards. However, the evidence base is still limited, and findings may be imprecise and vary by study setting. Although we found Xpert Ultra accurate for detection of rifampicin resistance, results were based on a very small number of studies that included only three children with rifampicin resistance. Therefore, findings should be interpreted with caution. Our findings provide support for the use of Xpert Ultra as an initial rapid molecular diagnostic in children being evaluated for tuberculosis.