Table 1.
Characteristics of Four Molecular Assays Designed for the Screening of Mycobacterium tuberculosis Disease, Including the Detection of RIF and INH Susceptibility
| Assay name | cobas MTB and cobas MTB-RIF/INH | Abbott RealTime (MTB, RIF/INH)17, 18, 19 | Xpert MTB/RIF20 | Xpert MTB/RIF Ultra21 |
|---|---|---|---|---|
| Regulatory status | CE-IVD | CE-IVD | CE-IVD | |
| Platform | cobas 6800/8800 | m2000 system | Xpert Infinity-48s/80 | |
| Target in MTBC | MTBC: 16S rRNA and 5 esx RIF-INH: rpoB, katG, and inhA |
MTBC: IS6110 and pab gene RIF-INH: rpoB, katG, and inhA |
rpoB with 5 probes | IS6110, IS1081, and rpoB with 4 probes |
| LOD | MTBC: 7.6–8.8 CFU/mL | MTBC: 2.5–35 CFU/mL RIF-INH: 60 CFU/mL |
50–165 CFU/mL | 5–25 CFU/mL |
| Specimen type and volume | Sputum: 0.4–1.2 mL Sediment: 0.2–0.6 mL |
Sputum or sediment: 0.5 mL | Sputum: 1 mL Sediment: 0.5 mL |
|
| Specimen processing and time | 1. Specimen + MIS (1:2) (1 minute/specimen) 2. Vortex (20–30 seconds) 3. Inactivation (1 hour) 4. Sonication (1 minute/specimen) 5. Centrifugation (1 minute at 3000 rcf) 4. Test (∼2 hours 20 minutes) |
1. Specimen + IR (1:3) (1 minute/specimen) 2. Vortex (20–30 seconds) 3. Inactivation (1 hour) 4. Test (∼3 hours for DNA extraction and 2 hours 30 minutes for amplification and detection) |
1. Specimen + SR (1:2) (1 minute/specimen) 2. Vortex (10 seconds) 3. Inactivation (15 minutes) 4. Test (1 hour 48 minutes) |
1. Specimen + SR (1:2) (2 minutes/specimen) 2. Vortex (10 seconds) 3. Inactivation (15 minutes) 4. Test (65 minutes negative results, 77 minutes positive results) |
| Extraction method | Chemical and mechanical (sonication) combined with magnetic glass particle technology for DNA capture | Chemical (guanidium thiocyanate) and magnetic microparticle technology for DNA capture | Closed cartridge, intact bacteria retained on a filter membrane, and DNA extracted with mechanical ultrasonication | |
| Amplification and detection | PCR volume = 25 μL Qualitative real-time PCR Fluorescent-labeled target-specific probes |
PCR volume = 50 μL Fluorescence-based real-time PCR Fluorescent-labeled target-specific probes |
PCR volume = 25 μL Heminested real-time PCR Molecular beacons |
PCR volume = 50 μL Semiquantitative, nested real-time PCR; SMB |
| Testing time | 8 hours/45 specimens (MTB detection and resistance profiling)∗ | 8 hours/93 specimens (MTB detection) + 3 hours/24 specimens (resistance detection) | ∼2 hours/specimen | ∼1–1.5 hours/specimen |
| Reported result | 1. MTBC: MTB positive/negative 2. RIF-INH: invalid/RIF positive/RIF negative/INH positive/INH negative |
1. MTBC: MTB positive/negative 2. RIF-INH: RIF R-; INH R-/RIF R det; INH high R/RIF R det; INH low R/RIF R det; INH low R/RIF R det; INH R/RIF R det; INH R/RIF Indet; INH R-/RIF R-; INH Indet/RIF R det; INH Indet/RIF Indet; INH Indet/Below LOD |
1. MTBC: MTB not detected/MTB detected (very low/low/medium/high) 2. RIF: RIF resistance (not detected/detected/indeterminate) 3. Error 4. Invalid 5. No result |
1. MTBC: MTB not detected/MTB detected (very low/low/medium/high/trace) 2. RIF: RIF resistance (not detected/detected/indeterminate) 3. Error 4. Invalid 5. No result |
CFU, colony-forming unit; Indet, indeterminate; INH, isoniazid; IR, inactivation reagent; LOD, limit of detection; MIS, mycobacterial inactivation solution; MTB, M. tuberculosis; MTBC, MTB complex; rcf, relative centrifugal force; R, resistant; R-, resistance not detected; R det, resistance detected; RIF, rifampicin; SMB, sticky molecular beacon; SR, sample reagent.
Throughput based on current study conditions, which can increase with scaled front-end sonication to system capacity (cobas 6800 system has capacity for 384 results/8 hours and cobas 8800 system has capacity for 960 results/8 hours).