INFECTIOUS DISEASES
A Collaborative Effort to Identify
Drug-Resistant Tuberculosis—The rpo B Alert
by Denise F. Dunbar, manager, Mycobacteriology-Mycology-Parasitology, Texas Department of State Health Services
and Anne M. Gaynor, PhD, manager, HIV, Viral Hepatitis, STD and TB Programs
On May 16, 2019, the Texas Department
of State Health Services Laboratory
(TX DSHS) received an “rpoB alert”
telephone call from the US Centers for
Disease Control and Prevention (CDC)
Division of Tuberculosis Elimination’s
(DTBE) Laboratory Branch. This call
initiated a cascade of activity that
resulted in confirmation of resistance
to rifampin, an important antibiotic
used in combination with three other
drugs to treat tuberculosis (TB), and
admission of the patient to the hospital
for appropriate treatment of drug-
resistant TB. But this story begins much
earlier for the patient and for the system
that was implemented to detect, alert
and ensure the appropriate treatment
of persons with drug-resistant TB.
TB). Over 95% of rifampin resistance is
the result of mutations in a well-defined
region within the rpoB gene, known as the
rifampin resistance determining region
(RRDR). Commercial assays can detect
the vast majority of common mutations.
However, there are mutations that occur
outside the RRDR yet within the rpoB
gene, which can only be identified by
methods that target the entire gene. There
are also mutations that can have clinical
significance but are not always identified
by growth-based drug susceptibility
testing (DST) methods. Therefore, if
a laboratory does not have access
to molecular testing methods, these
mutations and the associated rifampin
resistance may not be detected.
In August 2017, the Michigan Department
of Health and Human Services Laboratory
was selected to serve as the National
Tuberculosis Molecular Surveillance
Center (NTMSC), part of the Antibiotic
Resistance Laboratory Network (AR
Lab Network). This center performs
genotyping for one isolate from every
culture-confirmed TB patient in the
United States. Like many things in
public health, TB genotyping methods
are undergoing a transition to more
advanced methods such as whole genome
sequencing (WGS). As of March 2018,
all isolates sent to NTMSC are tested by
both traditional genotyping methods
(MIRU and spoligotyping) and WGS.
To expand the use of WGS data, DTBE
implemented the “rpoB alert” one year
after the implementation of NTMSC
to communicate predicted rifampin-
resistant results obtained through this
testing.
When Resistance is Undetected
Drug-resistant TB is complex and costly,
with treatment estimated at $164,000
for multidrug-resistant TB (MDR TB)
and upwards of half a million dollars
for extensively drug-resistant TB (XDR
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In this instance, the patient sample had
been previously tested by TX DSHS using
the Cepheid GeneXpert MTB/RIF assay
which detects mutations in the RRDR of
rpoB. However, in this case, resistance to
rifampin was not detected. The laboratory,
when reviewing patient results, noted
that their growth-based DST was unclear.
On May 16, after the rpoB alert from the
CDC DTBE Laboratory Branch, Texas
sent the isolate to CDC for Molecular
Detection of Drug Resistance (MDDR)
testing for further characterization and
confirmation of rifampin resistance using
CLIA-compliant testing. MDDR testing, a
nationally-available service, confirmed
the presence of a mutation in the rpoB
gene outside of the RRDR region. The
molecular assay performed by Texas did
not evaluate targets outside of the RRDR
of rpoB, explaining the initial inability to
detect the mutation. The rpoB mutation
identified is known to be clinically
significant; however, isolates with this
mutation often test susceptible by growth-
based methods. With the rpoB alert, TX
DSHS was made aware of an important
and previously undetected mutation
that impacted the course of the patient’s
subsequent treatment.
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Did you know?
Multi-Drug Resistant TB (MDR-TB):
• An isolate of tuberculosis with resistance to
at least isoniazid and rifampin, the two most
potent TB drugs
• US Cases Per Year: 2% of all cases
(123 in 2017)
• Treatment Cost for One Case: $164,000
Extensively Drug Resistant TB (XDR-TB):
• An isolate of tuberculosis that is resistant to
isoniazid, rifampin plus any fluoroquinolone
and at least one of the three injectable
second-line drugs (i.e., amikacin,
kanamycin, or capreomycin).
• US Cases Per Year: Two cases (2017)
• Treatment Cost for One Case: $526,000
Source: https://www.cdc.gov/tb/publications/infographic/
pdf/take-on-tuberculosis-infographic.pdf
Continuing Collaboration is
Required
As of June 2019, over 1,500 isolates
have been analyzed for rpoB mutations
with 44 alerts telephoned to 13 state or
local public health laboratories. In the
majority of cases, the laboratory had
already detected resistance to rifampin,
but in this case and six others, resistance
was previously unknown; of these,
80% were associated with low-level
rifampin resistance mutations, showing
that isolates are not always detected in
growth-based DST. Ongoing collaborations
will continue to evaluate how results
from molecular testing compare with
the results of traditional growth-based
DST for drug-resistant TB cases in
the US. With continued collaboration
and communication, future alerts for
resistance to additional drugs will be
implemented as systems evolve. n
Summer 2019 LAB MATTERS
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