TABLE 2.
Initial susceptibility test results obtained with ESP II for CDC M. tuberculosis challenge isolates
| Drug concn (μg/ml) and expected resulta | No. of CDC challenge isolates | No. with result by ESP II (% agreement with CDC)
|
|
|---|---|---|---|
| McFarland-equivalent inoculum | Seed inoculum | ||
| Isoniazid (0.2) | |||
| Susceptible | 10 | 10 (100) | 11 (100) |
| Resistant | 20 | 20 (100) | 19 (95) |
| Isoniazid (1.0) | |||
| Susceptible | 15 | 15 (100) | 15 (100) |
| Resistant | 15 | 15 (100) | 15 (100) |
| Rifampin (1.0) | |||
| Susceptible | 19 | 21 (100) | 18 (95) |
| Resistant | 11 | 9 (82) | 12 (100) |
| Ethambutol (5.0) | |||
| Susceptible | 26 | 29 (100) | 30 (100) |
| Resistant | 4b | 1 (25) | 0 (0) |
| Streptomycin (2.0) | |||
| Susceptible | 25 | 27 (100) | 27 (100) |
| Resistant | 5c | 3 (60) | 3 (60) |
Method-of-proportion results provided by the CDC for M. tuberculosis challenge isolates with the drug concentrations indicated.
When these four isolates were tested by ESP II with 5.0 μg of ethambutol per ml (rather than 8.0 μg/ml), two were identified as resistant with the inoculum prepared from the McFarland equivalent and one was identified as resistant with the inoculum prepared from the seed bottle.
When the two isolates yielding discrepant results were tested by ESP II with 2.0 μg of streptomycin per ml (rather than 8.0 μg/ml), both were identified as resistant with the inoculum prepared from the McFarland equivalent and one was identified as resistant with the inoculum prepared from the seed bottle.