The value of xpert MTB/RIF-generated CT values for predicting the smear status of patients with pulmonary tuberculosis

Abstract

BackgroundSmear microscopy is used to assess the patient's infectiousness at the time of initial diagnosis of pulmonary tuberculosis. However, its limited sensitivity and specificity highlights the need for new diagnostic strategies. The aim of our study was to assess the diagnostic accuracy of GX Ct value as a predictor of smear status and its usefulness to quantify mycobacterial load.MethodsAll GX-positive sputum samples during a seven-year period were included in the study. Correlations among Ct values, smear status and TTD on liquid culture were calculated. An optimal Ct value for ruling in infectious patients was established. Clinical and radiological variables were also analyzed.ResultsSixty-eight samples from 65 patients were included. Ct value and TTD yielded a positive correlation (ρ = 0.714; p < 0.05), while Ct and smear grade yielded an inverse correlation (r = −0.71). An optimal Ct value for ruling in smear positive patients was established at 21.1 cycles (90.5% sensitivity, 61% specificity, 81% PPV and 78% NPV).ConclusionsOur study confirms the value of GX Ct levels for quantifying mycobacterial load and demonstrates the added value of Ct as a predictor of positive smear status, especially at Ct values below 21.

Association of Cartridge Based Nucleic Acid Amplification Test (CBNAAT) Grading on the Basis of Cycle Threshold Value with Conventional Microbiological Diagnosis in Pediatric Tuberculosis

“…About 75.0% specimens with CT value less than 22 were detected to have AFB on direct microscopy. Similar results were obtained in a recent study on patients with pulmonary TB, where 81.0% of the cases with cut off value of CT as 21.1 were identified as smear positive 7 . Hence, low CT value can identify even smear negative specimens with high bacillary load.…”

“…Hence, low CT value can identify even smear negative specimens with high bacillary load. Since smear negative cases are a possible source of transmission, grading or quantification in CBNAAT could be utilized for identifying such potentially infectious cases 7 . Other studies have also reported strong association of CBNAAT CT values and smear positivity with cut off values ranging from 27.7 to 31.8 with variable sensitivities and specificities [17][18][19] .…”

“…6 Smear microscopy is usually the first test available for suspected cases of TB. Cultures take a long time to show growth 7 . The use of CBNAAT has reduced the median time to start treatment for AFB smear negative TB from 56 days to 5 days 8 .…”

“…The use of CBNAAT has reduced the median time to start treatment for AFB smear negative TB from 56 days to 5 days 8 . Few studies have been done to compare conventional microbiological techniques with CBNAAT 7,[9][10] . But similar studies in pediatric age group are lacking.…”

Factors affecting outcome of longer regimen multidrug-resistant tuberculosis treatment in West Java Indonesia: A retrospective cohort study

“…The degree of smear positivity is operator dependent so it has varying sensitivity. Therefore, WHO recommends Xpert MTB/RIF as the first test for TB diagnosis but does not replace the role of AFB smear [ 31 ]. AFB smear proved to play an independent risk factor in determining the successful treatment of MDR-TB.…”