An assessment of nucleic acid amplification testing for active mycobacterial infection

Meta-analysis of studies assessing the diagnostic accuracy of AFB plus NAAT compared with culture

Forest plots showing the sensitivity and specificity for the 38 studies that compared the diagnostic accuracy of AFB microscopy plus NAAT with culture in patients suspected of having TB are shown in Figure and Figure (Appendix ). A summary of meta-analysis of subgroups based on NAAT methodology, specimen type and incidence of TB in the country the study was conducted in is presented in Figure . AFB microscopy combined with NAAT was very sensitive (overall: 94%; 95%CI 91, 98) but the specificity was lower (88%; 95%CI 82, 92). Thus, 6% of patients will have a false-negative result for both AFB microscopy and NAAT, and 12% of patients (8% with sputum specimens and 17% with non-sputum specimens) will be falsely-positive for either AFB microscopy or NAAT.

Figure Forest plot showing the pooled sensitivity and specificity values for AFB plus NAAT compared with culture for studies grouped according to the NAAT comparator, specimen type and incidence of TB in the country in which the study was conducted

The sensitivity and specificity values shown in green represent median (range) values as meta-analysis could not be performed with that subgroup.

Incidence of TB based on WHO estimates from 2012: high incidence = > 100 cases per 100,000 people; medium incidence = 10–100 cases per 100,000 people; low incidence = ≤ 10 cases per 100,000 people

K = the number of studies; NAAT = nucleic acid amplification testing; TB = tuberculosis

The LR scattergram in Figure shows that the summary LR+ and LR– values for all studies investigating the ability of AFB microscopy plus NAAT to correctly identify patients with TB compared with culture were mostly within the green bands or the upper of the two left quadrants. This suggests that negative AFB and NAAT results correctly identified most patients who were culture-negative, and a positive result for either AFB or NAAT was more likely than not to indicate a culture-positive result. The reduced confidence in correctly diagnosing patients with culture-positive TB when AFB and NAAT were used together was due to the higher false-positive rate for the combined tests when compared with culture; 12% for AFB plus NAAT compared with 2% for AFB alone (Appendix ) and 6% for NAAT alone (Figure ). As discussed above, culture is an imperfect reference standard and it is likely that many of the patients with apparent false-positive results actually have TB.

Figure LR scattergram for diagnosis of MTB infection by AFB plus NAAT compared with culture in studies using either in-house NAAT or commercial Xpert NAAT

AFB microscopy plus NAAT was most effective at confirming and excluding the presence of culture-positive disease in sputum specimens but could only confidently exclude culture-positive disease in non-sputum specimens (Figure B and C). When studies using either an in-house NAAT or the commercial NAAT in combination with AFB were analysed separately, the summary LR+ and LR– estimates for the AFB plus commercial NAAT were more effective at confirming the presence of culture-positive disease than AFB plus in-house NAAT for all specimen types. Furthermore, in non-sputum specimens a positive AFB or in-house NAAT result did not provide any useful information, most likely due to the 14% false-positive rate in this population. A negative AFB and commercial NAAT result was only able to confidently exclude the presence of culture-positive disease in non-sputum specimens.

The SROC curve shows no threshold effect when AFB microscopy is combined with either in-house NAAT or commercial NAAT (Figure ). The SROC curves also show that when AFB microscopy plus NAAT was conducted in countries with a high incidence of TB, the results were less sensitive in sputum specimens and less specific in non-sputum specimens than when conducted in countries with an intermediate or low incidence of TB. The area under the curve (AUC) for AFB microscopy plus NAAT (0.97; 95%CI 0.95, 0.98) indicated that AFB plus NAAT performs well in predicting culture positivity (AUC > 0.9).

Figure SROC curve for all studies investigating the sensitivity and specificity of AFB plus NAAT versus culture in the diagnosis of TB for all studies based on NAAT methodology (A), and for sputum (B) and non-sputum (C) specimens based on incidence of TB

AUC = area under curve; SROC = summary receiver–operator characteristic