Abstract
BackgroundLow income, high-tuberculosis burden, countries are considering selective deployment of Xpert MTB/RIF assay (Xpert) due to high cost per test. We compared the diagnostic gain of the Xpert add-on strategy with Xpert replacement strategy for pulmonary tuberculosis diagnosis among HIV-infected adults to inform its implementation.MethodsThe first diagnostic sputum sample of 424 HIV-infected adults (67% with CD4 counts ≤200/mm3) suspected for tuberculosis was tested by direct Ziehl-Neelsen (DZN) and direct fluorescent microscopy (DFM); concentrated fluorescent microscopy (CFM); Lowenstein-Jensen (LJ) and Mycobacterial Growth Indicator Tube (MGIT) culture; and Xpert. Overall diagnostic yield and sensitivity were calculated using MGIT as reference comparator. The sensitivity of Xpert in an add-on strategy was calculated as the number of smear negative but Xpert positive participants among MGIT positive participants.ResultsA total of 123 (29.0%) participants were MGIT culture positive for Mycobacterium tuberculosis. The sensitivity (95% confidence interval) was 31.7% (23.6–40.7%) for DZN, 35.0% (26.5–44.0%) for DFM, 43.9% (34.9–53.1%) for CFM, 76.4% (67.9–83.6) for Xpert and 81.3% (73.2–87.7%) for LJ culture. Add-on strategy Xpert showed an incremental sensitivity of 44.7% (35.7–53.9%) when added to DZN, 42.3% (33.4–51.5%) to DFM and 35.0% (26.5–44.0%) to CFM. This translated to an overall sensitivity of 76.4%, 77.3% and 79.0% for add-on strategies based on DZN, DFM and CFM, respectively, compared to 76.4% for Xpert done independently. From replacement to add-on strategy, the number of Xpert cartridges needed was reduced by approximately 10%.ConclusionsAmong HIV-infected TB suspects, doing smear microscopy prior to Xpert assay in add-on fashion only identifies a few additional TB cases.
Highlights
Low income, high-tuberculosis burden, countries are considering selective deployment of Xpert Mycobacterium tuberculosis complex (MTB)/RIF assay (Xpert) due to high cost per test
Xpert was negative for two specimens with direct Ziehl-Neelsen (DZN) positive smears of which one was culture negative, for three specimens with direct fluorescent microscopy (DFM) positive smears of which two were culture positive for M. tuberculosis, and for four specimens with concentrated fluorescent microscopy (CFM) positive smears of which three were M. tuberculosis culture positive
The total number of culture positive TB cases who were smear positive but Xpert negative were four of which DZN detected one, DFM detected two and CFM detected all those detected by DZN and DFM with additional two TB cases (Table S1)
Summary
High-tuberculosis burden, countries are considering selective deployment of Xpert MTB/RIF assay (Xpert) due to high cost per test. We compared the diagnostic gain of the Xpert add-on strategy with Xpert replacement strategy for pulmonary tuberculosis diagnosis among HIV-infected adults to inform its implementation. Tuberculosis (TB) remains the most important opportunistic infection causing death among HIV-infected individuals in SubSaharan Africa [1]. Microscopic examination of Ziehl-Neelsen (ZN)-stained sputum smears, the most commonly available diagnostic in resource-limited settings, has low sensitivity for TB detection, especially among HIV-infected individuals. At least two sputum specimens need to be tested. Generally considered the gold standard for TB diagnosis, takes several weeks to yield results and may not be available before individuals are lost to follow-up or even dead [10]. The use of culture for diagnosing TB was recommended by the
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