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