The relationship between the in vitro drug susceptibility of opportunist mycobacteria and their in vivo response to treatment.

SETTING

It is generally accepted that qualitative drug susceptibility tests established and validated for Mycobacterium tuberculosis are not applicable to opportunist (non-tuberculous) mycobacteria. Previous studies have shown that in vitro antimicrobial susceptibilities for opportunist mycobacteria, performed by the method of modal resistance (MR), correlate poorly with clinical response. Minimum inhibitory concentration (MIC) determination may provide better correlation with predicted clinical response than the conventional MR results.

OBJECTIVE

To determine the relationship between quantitative in vitro sensitivity results for opportunist mycobacteria and their in vivo response to treatment.

DESIGN

MICs were performed radiometrically with the Bactec TB-460 system; 35 M. avium complex isolates, 29 isolates of M. malmoense and 16 isolates of M. xenopi were tested.

RESULTS

Susceptibility results were analysed in comparison with therapeutic outcome by Fisher's exact probability test. Only one significant association was found; in vitro resistance to ethambutol correlated with treatment failure for M. malmoense infections (P = 0.027). There were no other significant correlations between in vitro results and treatment outcome.

CONCLUSION

Prediction of treatment outcome from in vitro susceptibility tests continues to be a problem in infections with opportunist mycobacteria.