We performed susceptibility testing by using an agar proportion method on enriched Middlebrook 7H10 medium (BBL Microbiology Systems, Cockeysville, MD, USA) at the following concentrations: rifampicin 1 μg/mL, isoniazid 0.2 μg/mL, streptomycin 2 μg/mL and 10 μg/mL, and ethambutol 5 μg/mL.
A standard agar plate proportion method was used for drug-susceptibility testing of M. tuberculosis isolates.
We included studies that met the following pre-determined criteria: (i) comparison of INNO-LiPA with a reference standard (including proportion method, radiometric BACTEC 460 method, and minimum inhibitory concentration method), (ii) evaluation of a minimum of ten RIF-sensitive and ten RIF-resistant samples.
The sensitivity and specificity of the BACTEC MGIT 960 system using the agar proportion method as a gold standard was 92% and 37% for EMB, 95%and37%7% for STR, 27% and 97% for KAN and 84% and 90% for OFX, respectively.
Indeed, the gold standard of second-line susceptibility on solid agar uses the proportion method and a single critical concentration of drug to determine qualitative susceptibility or resistance, contrary to the quantitative readout used in the testing of many other infectious pathogens.
Lastly, studies were included if the reference standard test in the report was an indirect assay i.e. proportion method (PM) on Lowenstein-Jensen (L-J) or 7H10 agar, BACTEC 460, BACTEC MGIT 960 or a MIC (minimum inhibitory concentration) test.
As a result, 364/373 samples with DST results by 1% proportion method were available for analysis.
The agar proportion method is not, however, designed to handle a large number of samples.
The proportion method calculates the proportion of resistant bacilli present in a strain.
On performing proportion method, 22 isolates were identified as MDR.
