Model Answers Microbiology Written examinations 2007 - RCPA

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.