Comparison of same day diagnostic tools including Gene Xpert and unstimulated IFN-γ for the evaluation of pleural tuberculosis: a prospective cohort study

Abstract

Background: The accuracy of currently available same-day diagnostic tools (smear microscopy and conventional nucleic acid amplification tests) for pleural tuberculosis (TB) is sub-optimal. Newer technologies may offer improved detection.

Methods: Smear-microscopy, adenosine deaminase (ADA), interferon gamma (IFN-γ), and Xpert MTB/RIF [using an unprocessed (1 ml) and centrifuged (~20 ml) sample] test accuracy was evaluated in pleural fluid from 103 consecutive patients with suspected pleural TB. Culture for M.tuberculosis and/or histopathology (pleural biopsy) served as the reference standard. Patients were followed prospectively to determine their diagnostic categorisation.

Results: Of 93 evaluable participants, 40 had definite-TB (reference positive), 5 probable-TB (not definite but treated for TB) and 48 non-TB (culture and histology negative, and not treated for TB). Xpert MTB/RIF sensitivity and specificity (95% CI) was 22.5% (12.4 - 37.6) and 98% (89.2 - 99.7), respectively, and centrifugation did not improve sensitivity (23.7%). The Xpert MTB/RIF internal positive control showed no evidence of inhibition. Biomarker specific sensitivity, specificity, PPV, and NPVs were: ADA (48.85 IU/L; rule-in cut-point) 55.3% (39.8 - 69.9), 95.2% (83.9 - 98.7), 91.4 (73.4 - 95.4), 69.7% (56.7 - 80.1); ADA (30 IU/L; clinically used cut-point) 79% (63.7 - 89), 92.7% (80.6 - 97.5), 91.0 (73.4 - 95.4), 82.7% (69.3 - 90.1); and IFN-γ (107.7 pg/ml; rule-in cut-point) 92.5% (80.2 - 97.5), 95.9% (86.1 - 98.9), 94.9% (83.2 - 98.6), 93.9% (83.5 - 97.9), respectively (IFN-γ sensitivity and NPV better than Xpert [p < 0.05] and rule-in ADA [p < 0.05]).

Conclusion: The usefulness of Xpert MTB/RIF to diagnose pleural TB is limited by its poor sensitivity. IFN-γ is an excellent rule-in test and, compared to ADA, has significantly better sensitivity and rule-out value in a TB-endemic setting.

Figure 1

Patient flow and test results available for final diagnoses. ^$biopsy not performed (n = 1); *no fluid smear requested (n = 15); ^#liquid culture not requested, pleural fluid (n = 17) and pleural biopsy (n = 12); ^§indeterminate liquid culture: contamination (n = 1) and sample rejected (n = 1); ^◊histology not requested (n = 1); ^□ADA measurements not requested (n = 9); ^†sample lost during processing (n = 1), ^‡indeterminate Xpert MTB/RIFF result (n = 7).

Figure 2

Scatter plot of interferon gamma (IFN-γ) and adenosine deaminase (ADA) using pleural fluid from patients with TB and non-TB pleural effusions. *: p < 0.0001 (Mann-Whitney). See Additional file 1: Table S6 in the online supplementary data for final diagnosis of non-TB patients with ADA and/or IFN-γ levels above cut points.

Figure 3

Area under the receiver operator characteristic (ROC) curves for interferon gamma (IFN-γ) and adenosine deaminase (ADA) were 0.94 and 0.91, respectively.

Figure 4

The determination of the: a) limit of detection of Xpert MTB/RIF and b) analysis of possible inhibition of Xpert MTB/RIF internal probe in pleural fluid samples. a) To determine the limit of detection, known amounts of colony forming units of stock H37Rv were added to 1 ml aliquots of pleural fluid in duplicate for two repeat experiments. b) CT values for Xpert MTB/RIF internal probe are represented as box and whiskers (min and max) for pleural fluid, concentrated pleural fluid (20 ml pellet resuspended in 1 ml) and sputum. *: p < 0.0001 (Mann-Whitney).