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The developer of a new quantitative method applies a
similar concept of an experimental comparison between candidate
and reference methods, but the statistical evaluation
is based not on differences in detection but on differences in
enumeration. Thus, the experimental design is necessarily
different and can be reviewed in the AOAC (1) or ISO (9)
validation protocols.
The discussion in this paper will focus on comparison of
certified, validated, qualitative methods because these are
readily available and multiple test kit providers offer methods
for significant pathogens such as Salmonella and Listeria. For
pathogen testing, laboratories typically resort to internally
developed or internally validated methods only when certified
methods are not available or not suitable for a particular
matrix. Nevertheless, we will give some examples pertaining
to quantitative methods.
ARE CERTIFIED METHODS EQUIVALENT TO
EACH OTHER?
Method evaluations
The process of validation outlined above, with certification
by an independent certifying body such as AOAC or AFNOR,
results in a choice of several test kits that accomplish
the same testing goals with respect to the target analyte and
food matrix. How does one compare among methods that
are all equivalent to the same reference method? Providing a
rationale to follow in answering this question allows laboratories
to create a " toolbox " from which a variety of appropriate
methods could be used interchangeably, thereby reducing the
burden of validation or verification for many matrices and
facilitating rapid switching between methods in the event of
supply disruptions.
Certified methods must have successfully demonstrated
equivalence to the reference method. However, all reference
methods are not the same, and accreditation protocols are
not harmonized. For example, ISO methods are consensus
methods for which the validation data are publicly available;
however, methods in the FDA BAM are internal methods
that have been shared publicly but for which validation data
are not generally public. Hence, careful consideration is needed
to determine whether certified microbiological methods
are interchangeable:
* The methods must be shown to be equivalent to the
same reference method by a recognized accreditation
protocol.
* The validation study must have assessed the same target
analyte in the same matrix and test portion size.
Given these criteria are met, and allowing that the testing
lab is accredited and proficient in performing the method,
selection could then be based on reasons other than proven
pathogen or indicator organism detection. The choice could
be based upon ease of use, turnaround time (TAT; also
known as time to result), throughput, and ruggedness (16),
along with cost of labor, consumables, and training, capital
expenditures for equipment, and quality of vendor support.
There are several examples of a " toolbox " approach being
used in regulatory laboratories where a selection of methods
are available for use for the same matrix.
The USDA Food Safety and Inspection Service (FSIS)
makes available the list " Foodborne Pathogen Test Kits
Validated by Independent Organizations " for the detection of
several foodborne pathogen targets (i.e., Salmonella, Campylobacter,
Listeria spp., L. monocytogenes, E. coli O157:H7, and
non-O157 STEC) (12). The introduction to that document
states that
Establishments and laboratories should choose test kits that are:
* Validated for testing relevant foods by a process from:
◊ [A] recognized independent body (i.e., AOAC, AFNOR,
MicroVal, NordVal);
◊ [A] U.S. regulatory body (i.e., USDA FSIS, or FDA);
◊ [The] International Organization for Standardization
(ISO).
Outside of the United States, laboratories should be free to
consider test kits validated by their appropriate national bodies
in addition to the organizations listed above, as long as they are
not generating data for USDA-regulated foods for sale in the
United States. The FSIS document further states that
* The validated method should be:
◊ Fit for the intended purpose and application (e.g.,
validated for the appropriate matrix and sample size
to detect the appropriate foodborne pathogen);
◊ Performed per the conditions of the validated protocol
by a laboratory that assures the quality of the
analytical results.
Similarly, the FDA notified stakeholders that they had confirmed
the equivalency of several rapid testing methodologies
for Listeria species and Listeria monocytogenes in environmental
samples (2-4, 14). The agency determined that the
methods were " scientifically valid " and " at least equivalent to
the reference method of analysis in [CFR] § 112.152(a)(b)
in accuracy, precision, and sensitivity " for detecting Listeria
species and L. monocytogenes (13). In confirming equivalency
to the reference method, any of the rapid methods evaluated
could be selected for environmental sample analyses within
their laboratories.
Application of the proposed selection criteria in practical
use is shown using several examples. Information important
in determining whether two methods would be interchangeable
can be found in method validation studies.
Examples of assessing equivalence
Example 1. A company is considering a switch in methods
and would like to know whether the alternative methods being
considered for use to assay peanut butter are equivalent to
the test methods currently in use (Table 3). Their first interest
is in changing from a cultural method for aerobic plate count
(APC) to a rapid method. On considering the certification of
the rapid method, they find that it was validated against their
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