The Second Zambian National Tuberculosis Drug Resistance survey - a comparison of conventional and molecular methods

The prevalence of MDR-TB in Zambia was estimated to be 1.8% in 2001. A second drug resistance survey was conducted in 2008 to determine trends; the use of the Genotype MTBDRplus assay was applied to compare results to the gold standard.

A two-stage cluster sampling, with health facilities as primary sampling units. Processed sputum specimens were inoculated on solid media for culture; heat-inactivated bacterial suspensions from sputum samples were tested on a commercial line probe assay for the identification of rifampicin and isoniazid resistance.

A total of 917 patients with TB were enrolled and 883 (96.3%) analysed. A total of 574 (65%) had LJ results and 824 (93.3%) had results from MTBDRplus assay. The median age was 32, and 63.3% were males. MDR-TB according to LJ-based DST was 1.1% (CI 0.1-2.4) whereas according to MDTBDRplus assay was 1.6% (CI 0.6-2.6). Isoniazid monoresistance in new cases was 2.4% (CI 0.613-4.26) based on LJ results and 5.0% (CI 3.2-6.7) based on the MTBDRplus; in retreatment cases, it was 4.4% (CI 0.3-8.6) and 2.40% (CI <0.1-5.1) on LJ and MTBDRplus, respectively. Rifampicin monoresistance in new cases was 0.1% (CI <0.1-0.4) based on LJ and 0.6% (CI 0.01-1.1) based on the MTBDRplus; in retreatment cases, it was 0% (CI 0-3.8) and 1.8% (CI <0.1-4.0) on LJ and MTBDRplus, respectively. There were no XDR-TB cases found and no association between MDR-TB and HIV.

There was no increase in MDR-TB prevalence in Zambia from 2001 to 2008; results from the two methods were similar. Molecular methods were quicker and simpler to use.