Model Answers Microbiology Written examinations 2007 - RCPA

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Question 10, Marker 2 Multi resistant M tuberculosis (XDR) is being increasingly isolated world wide. Define XDR and discuss the detection and laboratory implications of the emergence of XDR mycobacteria. Answer XDR TB: detection and laboratory implications Drug resistant TB = isolate of MTB resistant to one of the 1 st line anti TB drugs; isoniazid, rifampicin, pytazinamide, ethambutol and streptomycin Multi drug resistant TB (MDR-TB) = isolate resistant to at least isoniazid and rifampicin Treatment for MDR TB patients use of 2 nd line drugs for greater than or equal to 12 months MDR TB patients whose isolates are resistant to any fluoroquinolone and resistant to at least on 2 nd line injectable drug (amikacin, capreomycin or kanamycin) have been classified as extensively drug resistant (XDR-TB) (revised definition from WHO task force Oct 2006). An earlier definition of XDR TB was MDR TB + resistant to greater than or equal to 3 of the 2 nd line drugs. 2 nd line drugs (SLD‟s) include aminoglycosides, capreomycin, fluorquinolones, thiomides, cycloserine, para-aminosalicyclic acid The rationale for the revised definition: (i) protocols for drug susceptibility testing of fluoroquinolones and injectable anti TB agents are established and there is good inter laboratory agreement; there is less agreement for the other SLD‟s and none whatsoever for cycloserine, (ii) the fluoroquinolones and injectable agents are the most potent SLD‟s and form the corner stones of most MDR TB treatment regimes and (iii) are often the only SLD‟s available in developing countries. Losing the FQs and the injectable agents means losing the most potent and least toxic options for the 2 nd line therapy XDR TB is now documented in many countries – highest known rates in Eastern Europe and Asia XDR TB is a microbiological diagnosis – documenting the emergence of XDR TB requires the collection and processing of adequate specimens for culture and susceptibility testing, prior to institution of therapy. If a patient cannot produce expectorate sputum, sputum induction should be performed with necessary protection of supervising HCW e.g sputum collection and cough-inducing procedures should be performed in negative pressure ventilation rooms. HCWs should wear respiratory protection when present in room or enclosure in which cough-inducing procedures are being performed on patients who my have infectious TB. These high-risk patients should also be managed appropriately before and after specimen collection to limit cross-infection to other patients and to HCWs.
Collection of tissue for those with extrapulmonary disease e.g. lymph node, bone omentum Smear results reported within 24 hours All specimens should be inoculated in broth-based culture system +/- onto solid media Susceptibility testing should be performed by a reference laboratory The DSTs must be performed using a broth-based culture system so that results are available promptly. Using these methods, laboratories should aim to report MTBC DST results within an average of 15-30 days from the time of the original specimen reception. The DSTs themselves can generally be completed within 7-14 days of obtaining the initial M. tuberculosis isolate from the primary cultures Drug susceptibility tests must be performed in the following circumstances: All initial isolates of M. tuberculosis Isolates from patients who remain culture-positive after 3 months of treatment Isolates from patients who are clinically failing treatment: or An initial isolated from a patient relapsing after previously successful TB treatment. 14,15 Second line drug susceptibility tests should be performed on: All MDR TB isolates (i.e. isolated demonstrating isoniazid and rifampicin resistance) All isolates demonstrating resistance to greater than or equal to 2 first line drugs and Isolates from patients experiencing severe adverse reactions to first line agents Laboratories performing susceptibility testing must be involved with an external quality assurance program Laboratories performing susceptibility tests should ideally have PC3 facilities At this time – WHO does not recommend testing of thioamides, cycloserine or PAS as reproducibility of testing is poor. Testing of a fluoriqyubikibe (cuori ir ifkixacub) then moxifloxacin if cipro/oflox is resistant, an aminoglycoside (amidacin and/or kanamycin) and capromycin
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Model Answers Microbiology Written examinations 2007 - RCPA

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