Second-line Phenotypic Drug Susceptibility Testing

Multidrug-resistant (MDR) TB is caused by strains of MTB resistant to at least isoniazid and rifampicin; for effective MDR patient management, optimized treatment regimens are required. These complex regimens can include a fluoroquinolone, aminoglycoside, cyclic peptide or compounds from other drug classes, and any remaining first-line susceptible drugs. 

Therefore, reliable drug susceptibility testing (DST) of these anti-TB drugs is crucial for the management of MDR-TB and for preventing the emergence of additional drug resistance in these patients.

Isolated cultures from TB patients are subjected to growth in the presence of a known concentration of a test drug. Growth control is also included without the addition of the drug. If the patient’s isolate grows in control but does not grow in the presence of the drug, it is considered susceptible. On the other hand, if it grows in both tubes, then it is considered to be resistant to that drug. Many methods are in use for susceptibility testing. The most commonly used method is the proportion method, where resistance is established at the 1% cut-off point for all the drugs. This means that if 1% or more of the total bacterial population is resistant to a drug, the strain is designated to be resistant to that drug.

Resource

Mycobacteriology Laboratory Manual    

Assessment