Neither genotyping nor contact tracing allow correct understanding of MDR TB transmission

Objectives: Active case tracing and strain genotyping are the main strategies used for identifying multidrug resistant tuberculosis (MDR TB) transmission. The objective of this study was to evaluate the performances of these strategies.Materials and Methods: We conducted a prospective study of active case finding of contacts of MDR TB cases in Paris between 2010 and 2013. Contact cases underwent clinical evaluation, chest X-ray and QuantiFERON® testing. MIRU VNTR genotyping was done for all MDR strains.Results: Investigations identified 84 contacts of 68 index MDR TB cases. Five had tuberculosis (6%). Of these, three (60%) had a strain sharing same genotype as the index case while 2 had drug susceptible TB. Genotyping allowed identification of 3 other possible transmissions not detected by contact investigation. However, retrospective investigation did not allow understanding how transmission occurred.Conclusion: Contact case tracing identified only 50% of secondary MDR cases identified by genotyping and misidentified 38% of cases as secondary to index case. However, by definition, genotyping identified secondary cases at the time tuberculosis had declared. As a consequence additional investigation conducted many months after transmission did not allow understanding how it occurred and didnot allow implementation of additional preventive measures.