Tuberculosis (TB) is a major health problem across the world. Thailand is one of the high TB burden countries. In 2012, World Health Organization (WHO) estimated that the incidence of TB in Thailand was 119 cases per 100,000 populations with 9,200 annual deaths. Among all TB cases, 12,000 were infected with HIV, and there were 2,200 annual deaths [
Despite low sensitivity in detection of
Recently, in 2011, WHO endorsed the wide use of Xpert MTB/RIF assay, a fully automated diagnostic molecular test using real-time polymerase chain reaction (PCR) technology to simultaneously detect
However, Xpert assay is not widely recognized as a diagnostic test for TB in Thailand. This study, therefore, was conducted to evaluate diagnostic performance of Xpert assay in northern Thai patients with clinically suspected pulmonary tuberculosis, using tuberculosis culture as a reference standard.
We conducted a cross-sectional study at Chiang Mai University Hospital, Chiang Mai, Thailand, between September 2012 and November 2013. The inclusion criteria were male or female patients with: (1) age ≥15 years; (2) clinically suspected pulmonary tuberculosis, which was defined as having 2 or more of the following symptoms: fever, chronic cough, weight loss, pleuritic chest pain, hemoptysis, and with or without abnormal chest radiograph compatible with pulmonary tuberculosis (cavitary lesion, infiltration, and miliary pattern); and (3) no history of receiving antituberculous drug within 3 months before enrollment.
The eligible patients were asked to provide at least 1 expectorated sputum specimen to the maximum of 3 specimens. Sputum acid-fast smear was performed on fresh specimen at the Central Diagnostic Laboratory, Chiang Mai University Hospital, Faculty of Medicine. The specimen was then decontaminated and separated into 2 samples which were blindly tested at 2 laboratory sites. Liquid-media mycobacterial culture of all sputum specimens using Mycobacteria Growth Indicator Tube (MGIT) method as a reference standard was performed at the Central Diagnostic Laboratory, Chiang Mai University Hospital. Xpert MTB/RIF assay (Cepheid) was performed at Mycobacterial Laboratory, Research Institute for Health Sciences (RIHES), Chiang Mai University. The method was performed according to manufacturer’s instruction of Xpert MTB/RIF [
The specimen was excluded from the analysis if (1) it was an invalid sample for Xpert assay or sample error according to Cepheid package insert, or (2) the culture was contaminated.
Demographic data including age, sex, past medical history of lung diseases, and HIV serostatus were recorded. Clinical data including fever, days of fever, chest pain, dyspnea, hemoptysis, weight loss, cough, and the extrapulmonary sites of tuberculosis were collected. Laboratory results included sputum acid-fast smear, sputum mycobacterial culture results, sputum Xpert MTB/RIF results, and chest radiographic findings were collected. Sputum acid-fast smear results were categorized into positive (reported AFB found per field as 1+, 2+, 3+, and 4+) or negative. Sputum Xpert MTB/RIF results were also categorized into positive (reported Xpert detected as very low, low, medium, and high), or negative. Sputum cultures were reported as
Descriptive data were presented in percentages, mean ± SD, and median (IQR) as appropriate. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of Xpert and sputum acid-fast smear were compared to tuberculosis culture as a reference standard using contingency
All statistical analyses were performed using Stata statistical software version 10.0 (Stata Statistical Software: Release 10.0, Stata Corporation, College Station, TX, 2007).
This study was approved from the Faculty of Medicine, Chiang Mai University ethical committee.
Sixty consecutive patients who met the inclusion criteria providing 127 sputum specimens were enrolled to the study. Of 127 specimens, 2 were invalid for Xpert, 2 were sample error, 14 were contaminated, and 11 grew NTM. After exclusion of 18 specimens, 109 sputum specimens from 57 patients were included in the analysis. Twenty-nine, 15, and 12 patients provided 1, 2, and 3 specimens, respectively.
Demographic data of 57 patients are shown in Table
Demographic data of 57 patients with clinically suspected pulmonary tuberculosis.
Demographic data | Patients ( |
---|---|
Female | 23 (40.4%) |
Age (mean ± SD) | 55.6 ± 20.1 |
Underlying lung diseases | |
Chronic obstructive pulmonary |
6 (10.7%) |
Bronchiectasis | 4 (7.1%) |
Other chronic lung diseases | 3 (5.4%) |
HIV serostatus | |
Positive | 15 (26.3%) |
Negative | 40 (71.9%) |
Not tested | 1 (1.8%) |
Among 109 sputum specimens,
From MGIT culture, 3 specimens from 1 patient showed INH resistance, 4 specimens from 2 patients showed streptomycin resistance, and 3 specimens from 1 patient showed both INH and rifampin resistance. Xpert assay failed to detect rifampin resistance in all 3 specimens with rifampin resistance.
Table
Two by two contingency tables showing true positive, false positive, true negative, and false negative comparing Xpert assay (a) and sputum acid-fast smear (b) with the reference standard.
Positive Xpert assay | Negative Xpert assay | Total | |
---|---|---|---|
Culture grew |
41 | 2 | 43 |
Culture grew NTM or no growth | 9 | 57 | 66 |
|
|||
Total | 50 | 59 | 109 |
NTM: nontuberculous mycobacteria, 4 in positive Xpert assay and 6 in negative Xpert assay.
Positive sputum acid-fast smear | Negative sputum acid-fast smear | Total | |
---|---|---|---|
Culture grew |
26 | 17 | 43 |
Culture grew NTM or no growth | 1 | 65 | 66 |
|
|||
Total | 27 | 82 | 109 |
NTM: nontuberculous mycobacteria, 10 in negative sputum acid-fast smear.
Table
The sensitivity, specificity, PPV, and NPV and their correspondent 95% CIs are shown in Table
Sensitivity, specificity, positive predictive value, and negative predictive value of Xpert assay and sputum acid-fast smear comparing to sputum culture.
Xpert assay | 95% confidence interval | Acid-fast smear | 95% confidence interval | |
---|---|---|---|---|
Specimens from all patients ( | ||||
SEN | 41/43 = 95.3% | 84.2–99.4% | 26/43 = 60.5% | 44.4–75.0% |
SPEC | 57/66 = 86.4% | 75.7–93.6% | 65/66 = 98.5% | 91.8–100% |
PPV | 41/50 = 82.0% | 68.6–91.4% | 26/27 = 96.3% | 81.0–99.9% |
NPV | 57/59 = 96.6% | 88.3–99.6% | 65/82 = 79.3% | 68.9–87.4% |
|
||||
Specimens from HIV-infected patients ( | ||||
SEN | 10/10 = 100% | 69.2–100% | 6/10 = 60% | 26.2–87.8% |
SPEC | 16/16 = 100% | 79.4–100% | 19/19 = 100% | 82.4–100% |
PPV | 10/13 = 76.9% | 46.2–95.0% | 6/6 = 100% | 54.1–100% |
NPV | 16/16 = 100% | 79.4–100% | 19/23 = 80% | 61.2–95.0% |
|
||||
Specimens from HIV-uninfected patients ( | ||||
SEN | 28/30 = 93.3% | 77.9–99.2% | 20/30 = 66.7% | 47.2–82.7% |
SPEC | 41/43 = 95.3% | 84.2–99.4% | 46/47 = 97.9% | 88.7–99.9% |
PPV | 28/34 = 82.4% | 65.5–93.2% | 20/21 = 95.2% | 76.2–99.9% |
NPV | 41/43 = 95.3% | 84.2–99.4% | 46/56 = 82.1% | 69.6–91.1% |
SEN: sensitivity; SPEC: specificity; PPV: positive predictive value; NPV: negative predictive value.
Among 43 culture-proven
Xpert assay, by subgroup analysis in HIV-infected patients (29 specimens), had sensitivity of 100%, specificity of 100.0%, PPV of 76.9%, and NPV of 100% (Table
Among 27 patients with culture-confirmed pulmonary tuberculosis, sputum acid-fast smear was positive in 18 patients (66.7%). Table
Demographic and clinical characteristics of 27 TB culture-proven patients with positive and negative sputum acid-fast smears.
Characteristic | Positive acid-fast smear ( |
Negative acid-fast smear ( |
|
---|---|---|---|
Female | 5 (27.8) | 6 (66.7) | 0.097 |
Age | 48.3 (±17.2) | 61.7 (±22.8) | 0.136 |
HIV infection | 13 (72.2) | 7 (77.8) | 0.301 |
Presence of underlying lung disease | 5 (27.8) | 2 (22.2) | 0.395 |
Presence of fever | 14 (77.8) | 4 (44.4) | 0.083 |
Duration of illness >7 days | 8 (44.4) | 4 (44.4) | 1.000 |
Presence of chest pain | 3 (16.7) | 2 (22.2) | 0.726 |
Presence of dyspnea | 4 (22.2) | 1 (11.1) | 0.636 |
Presence of cough | 17 (94.4) | 6 (66.6) | 0.093 |
Hemoptysis | 1 (5.6) | 1 (11.1) | 1.000 |
Significant weight loss | 5 (27.8) | 2 (22.2) | 0.263 |
Concurrent extrapulmonary infection | 1 (5.6) | 2 (22.2) | 0.250 |
Abnormal CXR | 17 (94.4) | 9 (100) | 0.694 |
Data are presented in number (%), or mean ± SD.
A recent meta-analysis that included 9,557 participants from 27 studies showed that Xpert assay of respiratory specimens had a pooled sensitivity of 89% (95% CI 85%, 92%) and specificity of 99% (95% CI 98%-99%) in the diagnosis of pulmonary tuberculosis [
Data from several studies in culture-proven TB patients with acid-fast positive sputum specimens showed that Xpert assay had a pooled sensitivity of 98% (95% CI 97%, 99%) in the detection of
Although WHO endorsed the wide use of Xpert MTB/RIF assay, this test is expensive and available only in large medical centers in Thailand. Using this assay in all patients with clinically suspected tuberculosis may neither be possible nor cost-effective in resource-limited settings in particular where the laboratory technicians have high experience in microscopy. As mentioned above, our study and other studies all confirm that positive acid-fast smear correlates well with Xpert assay and TB culture; patients with positive acid-fast smear may not have benefit from Xpert assay in detecting TB but may have benefit in detecting rifampin resistance if it exists. In order to identify predicting factors for having negative sputum acid-fast smear, we compared demographic and clinical characteristics among patients with culture-confirmed pulmonary tuberculosis who had positive and negative sputum acid-fast smears. Patient who has one of these factors, if any, and has negative sputum acid-fast smear may have benefit from Xpert assay. Unfortunately, we could not identify any factors associated with negative sputum acid-fast smear in patients with tuberculosis. Failure to demonstrate that may be due to small number of patients in this study and further studies with larger sample size may be warranted.
Our study demonstrated the overall specificity of 86.4% (75.7–93.6%), which was slightly lower than those in previous studies that had a range from 94% to 100% [
Interestingly, 10 samples from 6 patients in our study grew NTM. Four samples from 3 patients were weakly positive by Xpert assay. In one patient who produced 2 sputum specimens, one specimen grew
Of note, invalid and error results from Xpert assay occurred in 4 of 127 specimens (3.1%) whereas contaminated culture occurred in 14 of 127 specimens (11.0%). In addition, median turnaround time from sputum collection to get Xpert results was faster than from culture results. Due to the time-taking process to prepare and run the test for each specimen, we decide to run the test once we have four samples ready instead of one at a time. Therefore, the median turnaround time for Xpert assay in our study was 6 days, comparing to 45 days for culture results. These demonstrated that Xpert assay was more reliable than sputum culture for
Our study has several limitations. First, there was a high rate of contaminated culture specimens (11.0%). This may indicated poor technique of specimen collection and decontamination. Second, the relatively small numbers of participants limited the study power. Finally, since there were only 3 specimens from one patient with rifampin resistance in this study, data was not sufficient to evaluate Xpert performance in detecting rifampin resistance.
This study demonstrated good sensitivity and specificity of Xpert assay in detecting
All authors declared no conflict of interests.
This study was supported by the National Research University Project under Thailand’s Office of the Higher Education Commission. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the paper.