INFECTIOUS DISEASES
The ABCs of MIC Testing for Mycobacterium
tuberculosis complex
By Kimberly McCullor, PhD, APHL Antimicrobial Resistance Laboratory Fellow, Michigan Bureau of Laboratories
Susceptibility testing for Mycobacterium
tuberculosis complex (MTBC) plays
a vital role in protecting efficacy
of available therapeutics and in
identifying drug resistant cases to help
with epidemiological efforts to stop
transmission. Testing for MTBC drug
susceptibility is commonly performed
by methods such as agar proportion and
rapid broth culture systems, using fixed
antibiotic concentrations based upon
critical concentration breakpoints. Critical
concentration is the lowest concentration
of drug inhibiting 95% of growth of
wildtype isolates. The presence or absence
of growth is then used to classify isolates
as resistant or susceptible.
Microbroth dilution testing differs from
other tests because it is used to identify
the minimum inhibitory concentration
(MIC), or the lowest drug concentration
within a tested range that is required to
inhibit growth of an isolate. Although
widely used for susceptibility testing of
other pathogens, it is only utilized by a
small number of laboratories for MTBC.
The Case for MIC
In a clinical setting, MIC provides
susceptibility data that can aid physicians
with drug dosing and management
of complicated cases. MIC data are an
essential component to therapeutic drug
monitoring efforts that ensure appropriate
concentrations are achieved within the
patient, mitigating the potential for
treatment failure and development of
drug resistance.
Pre-made or customizable microtiter
plates by Sensititre TM are available and
include both first- and second-line
antimycobacterial drugs, excluding
pyrazinamide (PZA). Cost per plate
ranges from $30.00 to $50.00 and requires
minimum orders of 100 to 500 plates,
which may limit utility for sites with
lower testing volumes. Pure culture
isolates are required for testing (with
observations of better results using
inoculum from solid media). Plates are
generally read at seven, 10, 14 and 21
days, with many showing sufficient
growth after 14 days. While this causes a
delay in reporting for first line drugs—the
recommended CDC benchmark is 17 days
post MTBC culture identification—the
advantage is both first- and second-line
drugs are reported simultaneously.
Finding a Balance with MIC
Isoniazid, rifampin and ethambutol are
the only drugs with MIC breakpoints for
MTBC. MIC testing for the identification
of lower levels of resistance is beginning
to be better understood and has
provided important results to improve
patient management, especially for a
subset of rifampin-resistant MTBC. The
Comprehensive Resistance Prediction for
Tuberculosis International Consortium
(CRyPTIC) is conducting research to
correlate MIC values to genetic mutations
acquired from whole genome sequencing
data.
Further collaboration with clinical and
laboratory partners to correlate MIC
values of MTBC isolates to clinical
outcome and treatment response is
necessary to expand interpretative
breakpoints. Limited clinical data for
correlation and the inherent challenges
Above: Inoculating a Sensititre MycoTB plate for MIC testing.
Photo: Florida Bureau of Public Health Laboratories
Left: Reading the Sensititre MycoTB Plate to determine the
MIC(s) of patient isolates of Mycobacterium tuberculosis.
Photo: Florida Bureau of Public Health Laboratories
of MTBC susceptibility testing with
discrepancies between testing methods
(including genotypic screens), make
obtaining FDA approval for MTBC
microbroth dilution out of reach. Due
to the lack of interpretative guidelines
for MTBC MIC testing, consultation with
clinicians and TB Control Programs for
interpretation of results and appropriate
patient management are vital.
Susceptibility testing of MTBC remains
challenging regardless of the method.
Even with the limitations of interpretative
guidelines, MIC data provide detailed
information for susceptibility profiles of
clinical isolates that are not obtainable
from other methods—valuable data that
can alert physicians to potential issues
due to low-level resistance, can guide
dosing in patients with complicated cases
of infection or mitigate harmful side
effects.
The author thanks Dr. Marie-Claire Rowlinson
(Florida), Dr. Kimberlee Musser (New York),
Dr. Anne Gaynor (APHL) and the APHL TB
Subcommittee for their input. •
DIGITAL EXTRA: Find more
tuberculosis testing resources here.
24 LAB MATTERS Summer 2020
PublicHealthLabs
@APHL APHL.org