Transcript Slide 1

Growth Detection Failures
of MGIT 960 System
Julie Tans-Kersten, MS, BS-MT (ASCP)
Tuberculosis Laboratory Program Coordinator
Wisconsin State Laboratory of Hygiene
[email protected]
(608) 263-5364
WISCONSIN STATE
LABORATORY OF HYGIENE
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Culture of Primary Patient Specimens
• Mycobacteria Growth Indicator Tube (MGIT)
• 7 ml of broth + antibiotics
• Automated system that detects O2
consumption of growing organisms
• Instrument incubates tubes and checks for
growth hourly; flags positive tubes
• After 42 days, if no growth, tubes are
removed and reported as negative
WISCONSIN STATE
LABORATORY OF HYGIENE
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Growth Detection Failures in MGIT
960 System
• Pena, JA et al., J Clin Microbiol. 2012
Jun;50(6):2092-5
• Documents cases of growth detection failure in the
MGIT 960 system
• ∼1% of instrument-negative MGIT cultures
contained mycobacterial growth
• 10% of all cultures yielding mycobacteria were
instrument negative
• Isolates from instrument-negative MGIT cultures
included both tuberculous and nontuberculous
mycobacteria.
WISCONSIN STATE
LABORATORY OF HYGIENE
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Study at WSLH
• Checked instrument negative
tubes for signs of growth
– HPLC performed on growth
• For smear positive specimens that
were instrument-negative, we
sub-cultured a small amount to
fresh broth and held cultures and
additional 2 weeks
WISCONSIN STATE
LABORATORY OF HYGIENE
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Study at WSLH
• We found growth detection
failures in our lab
– Instrument negative tubes with visible
growth
– Instrument negative tubes that grew
mycobacteria after further incubation in
fresh broth culture
• In most cases, isolates were NTM
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LABORATORY OF HYGIENE
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But in some cases, the instrumentnegative tubes contained MTBC!
• WSLH reported results (8-10 weeks of
incubation)
• Caused confusion with public health
nurses who use culture results to
gauge treatment efficacy or to release
from isolation
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LABORATORY OF HYGIENE
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Questions
• When is the “real” report coming
out?
• Which report should I believe?
• How should I interpret this
information?
• Did I let an infectious patient back
out on the street?
WISCONSIN STATE
LABORATORY OF HYGIENE
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Study Abandoned
• Specimens were incubated “off label”
• Difficult to determine clinical significance of
very late positive broth culture results
– Isolates are very slow growing, possibly
attenuated due to patient therapy
– Although specimens were smear positive,
the number of viable organisms was
likely very low
– No evidence-based data on significance
of these isolations
WISCONSIN STATE
LABORATORY OF HYGIENE
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