R H Berhanu1, K Schnippel2, R Kularatne3, C Firnhaber4, K R Jacobson5, C R Horsburgh6, C K Lippincott7. 1. Division of Infectious Diseases, University of North Carolina at Chapel Hill, North Carolina, USA, Department of Medicine, Faculty of Health Sciences, Health Economics and Epidemiology Research Office, University of the Witwatersrand, Johannesburg, Right to Care, Johannesburg. 2. Right to Care, Johannesburg, Clinical HIV Research Unit, Department of Medicine, Faculty of Health Sciences. 3. National Institute for Communicable Diseases/National Health Laboratory Service and Department of Clinical Microbiology and Infectious Diseases, University of the Witwatersrand, Johannesburg, South Africa. 4. Clinical HIV Research Unit, Department of Medicine, Faculty of Health Sciences, Department of Medicine, Infectious Disease Division, University of Colorado, Denver, Colorado. 5. Section of Infectious Diseases, Boston University School of Medicine, Boston, Massachusetts. 6. Department of Epidemiology, Boston University School of Public Health, Boston, Massachusetts. 7. Department of Medicine, Division of Infectious Diseases, Johns Hopkins University, Baltimore, Maryland, USA.
Abstract
SETTING: Xpert® MTB/RIF is the first-line diagnostic test for Mycobacterium tuberculosis and rifampicin (RIF) resistance in South Africa. OBJECTIVE: To describe the rates of Xpert RIF resistance not confirmed on follow-up testing, as well as the patient and test characteristics associated with discordant results. DESIGN: Retrospective review of patients with isolates showing Xpert RIF resistance. Line-probe assay, phenotypic drug susceptibility testing or repeat Xpert were all considered confirmatory tests of RIF resistance. 'Discordance' was defined as a patient with RIF resistance identified on initial Xpert testing, with a subsequent confirmatory test indicating RIF susceptibility. Associations were analysed using Pearson χ² difference of proportions and modified Poisson regression. RESULTS: RIF discordance occurred in 22/263 subjects and was associated with Xpert probe B, probe binding delay, as opposed to probe dropout, and probe binding delays (ΔCt) of between 4 and 4.9. CONCLUSION: Discordant RIF resistance was common in our cohort and was associated with Xpert probe delay and use of probe B.
SETTING: Xpert® MTB/RIF is the first-line diagnostic test for Mycobacterium tuberculosis and rifampicin (RIF) resistance in South Africa. OBJECTIVE: To describe the rates of Xpert RIF resistance not confirmed on follow-up testing, as well as the patient and test characteristics associated with discordant results. DESIGN: Retrospective review of patients with isolates showing Xpert RIF resistance. Line-probe assay, phenotypic drug susceptibility testing or repeat Xpert were all considered confirmatory tests of RIF resistance. 'Discordance' was defined as a patient with RIF resistance identified on initial Xpert testing, with a subsequent confirmatory test indicating RIF susceptibility. Associations were analysed using Pearson χ² difference of proportions and modified Poisson regression. RESULTS:RIF discordance occurred in 22/263 subjects and was associated with Xpert probe B, probe binding delay, as opposed to probe dropout, and probe binding delays (ΔCt) of between 4 and 4.9. CONCLUSION: Discordant RIF resistance was common in our cohort and was associated with Xpert probe delay and use of probe B.
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