This operational study extends and provides support for the findings of
earlier proof-of-principle studies6,7,22–37
demonstrates that the MODS assay outperforms the gold-standard reference methods of
developing and industrialized countries. For all three study groups, the MODS assay
detected M. tuberculosis
in sputum with greater sensitivity and
speed and reliably identified multidrug-resistant tuberculosis strains in less time
than did Löwenstein–Jensen or automated mycobacterial
cultures. These data indicate that the MODS assay could be considered for use in
Our study was designed to address conventional pitfalls.8,15,18,20,38
Specifically, it was
performed in an appropriately broad group of patients with or without disease and in
pertinent patient groups (without selection bias); all tests were performed in all
patients (preventing verification bias); and all results were interpreted by staff
members who were unaware of the other test results, using appropriate gold-standard
reference methods for comparison.
The robustness of our study derives from its operational, real-world design.
Meticulous resolution of discordant results is essential when an investigational
diagnostic method is more sensitive than existing reference standards. Use of two
established reference methods for comparison and two samples per patient facilitated
the rigorous definition of true positive results, addressing the problem of
incorporation bias. The high specificity and infrequent cross-contamination in the
relate to the containment of the
plates in ziplock bags and the absence of manipulation after inoculation, which also
improve biologic security.
The greater sensitivity and speed of detection in MODS culture than in the
gold standards were predicted on the basis of previous studies.6,7,37
The increased sensitivity of liquid medium has long
been known, and a discerning human eye can scrutinize cultures better than can
automated systems with their use of necessarily rigid cutoff values. It is simpler
to recognize the characteristic cord formation than to read a malarial smear; within
1 week, students training in our laboratory can comfortably and accurately read one
well per minute, considerably faster than the time it takes to read a smear for
acid-fast bacilli. Training in the MODS assay can be completed in less than 2 weeks
(similar to training in Löwenstein–Jensen and automated
mycobacterial cultures; training in drug-susceptibility testing with the proportion
method takes several months). Beyond standard laboratory equipment, automated
mycobacterial culture requires computer-linked automated culture incubators, whereas
MODS culture requires only an inverted light microscope. As purchased by us, the
cost of $2 per sample for MODS culture compares favorably with the
$6 cost per sample for Löwenstein–Jensen culture
and the proportion method and the cost of $52 per sample for automated
mycobacterial culture; however, labor costs may be higher for MODS culture.
Increased sensitivity carries the risk of increased bacterial overgrowth
(for MODS culture and automated mycobacterial culture), though even after repeated
decontamination, the sensitivity and specificity of MODS culture were unaffected.
High speed, sensitivity, and specificity, and the requirement for only one culture,
all enhance tuberculosis rule-out procedures and potentially simplify
tuberculosis-screening algorithms for use in patients with HIV infection before the
initiation of prophylactic treatment with isoniazid. If a MODS culture is negative
on day 15, there is a 99.7% chance that the sample is truly
culture-negative. Thus, we believe that a negative MODS culture can be discarded
after 3 weeks.
In settings with a high tuberculosis burden, the only susceptibility data
that are likely to effect a change in therapy at the programmatic level are those
for the detection of multidrug-resistant tuberculosis, for which the performance of
the MODS assay is highly reliable and rapid (median time to the results of
susceptibility testing, 7 days), providing clinically important information in a
meaningful time frame. Although direct drug-susceptibility testing is conventionally
viewed with suspicion — indirect testing of cultured strains is
preferred — our data refute that view for rifampin and isoniazid in the
MODS assay. However, susceptibility testing for M. tuberculosis
complex, and concordance among even regional laboratories performing gold-standard
testing is particularly variable for ethambutol and streptomycin.39
Our findings for these two drugs agreed with previous
data for the MODS assay,7
insufficient concordance of the assay (at least in its current format) with gold
standards to recommend usage.
Our study defines strengths and redundancies in the first-generation MODS
assay and should enable the development of a streamlined, clinically useful method.
The use of fewer wells per sample than we used — two wells with no drug
(to ensure high specificity), one with rifampin (1 μg per milliliter),
and one with isoniazid (0.4 μg per milliliter) — reduces
costs by 40% but does not affect performance. The MODS assay is
“laboratory freeware,” not a commercial product or a kit.
Any laboratory that is adequately biologically secured, has an incubator and a
centrifuge, and is capable of microscopy can safely perform MODS culture. All
ingredients are available from major laboratory suppliers.
Downstream effects on patient care are the litmus test of the utility of a
new method, and the added value will therefore depend on context and strategy for
implementation. In countries where smear-negative tuberculosis is frequently
diagnosed and treated empirically, the incremental benefit of MODS culture on case
detection, as compared with the smear alone, would be less than that in Peru, where
only 21% of treated cases are smear-negative and where MODS culture has
recently been incorporated into Ministry of Health guidelines (www.minsa.gob.pe/normaslegales/2006/RM383-2006.pdf
). However, the
high specificity rate would save patients and society money by minimizing
overtreatment, and the early detection and treatment of multidrug-resistant
tuberculosis would interrupt transmission. Equity in the access to high-performance
techniques to diagnose tuberculosis thus benefits both individual health40,41
and public health. Despite the low cost per sample, in many resource-limited
settings with a high tuberculosis burden, testing by the MODS assay of all patients
with suspected tuberculosis (<5% of whom have culture-positive
disease in Peru) would be a challenge financially and operationally. In our
targeted, high-risk groups, only one MODS culture (collected in one visit) is
required to achieve culture-positive rates of 20%, a good return on the
investment. Programmatic studies are now needed to determine the optimal
implementation strategy to maximize the effect and cost-effectiveness of this tool.
The MODS assay addresses two key gaps in resource-limited settings with a
high tuberculosis burden: rapid, accurate detection of M.
and simultaneous identification of multidrug-resistant
tuberculosis. The use of culture-based diagnostic techniques for case detection may
not be the future as envisaged by the International Union against Tuberculosis and
; promotion of such a strategy
may conflict with the view that the scale-up of coverage and improvement of smear
microscopy is currently a more important priority. However, we believe the MODS
assay could now be implemented in settings in which smear microscopy is being
optimally used and the augmentation of case detection is feasible and desirable.