U. S. Food and Drug Administration
Center for Food Safety and Applied Nutrition
FDA Prime Connection
FDA - Evaluation and Use of Milk Antimicrobial Drug Screening Tests
FOOD AND DRUG ADMINISTRATION
EVALUATION AND USE OF MILK
ANTIMICROBIAL DRUG SCREENING TESTS
I. BACKGROUND
The use, in accordance with label directions, of the approximately 40
animal drugs approved for use in lactating dairy cows presents
consumers with a negligible risk from residues in milk 1. The eleven
approved drugs used to treat mastitis and respiratory infections --
penicillin, ampicillin, cephapirin, hetacillin 2, amoxicillin,
ceftiofur, oxytetracycline, chlortetracycline, novobiocin,
erythromycin, and sulfadimethoxine -- are among the most widely used
drugs to treat dairy cows. The first five can cause hypersensitivity
reactions in some people. Because of this, residues of beta-lactam
type antibiotics are of greatest concern from the standpoint of human
food safety.
Prior to 1991, the only required monitoring of raw milk for animal
drug residues was described in Section 6 of the Grade A Pasteurized
Milk Ordinance (PMO). It states, "...drug tests on each producer's
milk shall be conducted by State regulatory agencies at least four
times during any consecutive 6 months." The tests accepted at that
time by the National Conference on Interstate Milk Shipments (NCIMS)
for Section 6, PMO, testing were the Difco Bacillus stearothermophilus
Disc Assay (BSDA), the generic BSDA described in Appendix G of the
PMO, the Charm I and II Liquid Tests, and the Delvo Test P.
II. DEFINITIONS
The following definitions are important to the understanding of the
evaluation and use of milk antimicrobial drug screening tests:
A. A truly positive test result is a positive test result on a milk
sample in which the actual drug concentration is at or above the
established tolerance or safe level.
B. A false violative test result is a positive test result on a
milk sample in which the actual drug concentration is at or above
the detectable concentration, but below the established tolerance
or safe level.
_____________________
1 Additional animal drugs may be given to lactating cattle by
veterinarians under the Center for Veterinary Medicine's Extra
Label Drug Use Policy.
2 Residues of hetacillin are almost immediately and completely
metabolized to ampicillin following administration to animals,
and for this reason, are not normally considered in drug residue
monitoring programs.
C. A false positive test result is a positive test result on a milk
sample which actually contains no drug residue at a detectable
concentration.
D. A false negative test result is a negative test result on a milk
sample which contains drug residue at a detectable concentration.
E. The sensitivity rate is the percentage of truly positive samples
that are found by the test to be positive.
F. The selectivity rate is the percentage of truly negative samples
that are found by the test to be negative.
G. The estimated minimum ninety percent sensitivity level is the
estimated lowest drug concentration which gives, with 95 percent
confidence, a positive result with the test at least 90 percent
of the time on samples which are truly positive.
III. APPENDIX N, PASTEURIZED MILK ORDINANCE
At their 1991 biennial meeting, the National Conference on Interstate
Milk Shipments (NCIMS) passed a resolution modifying the PMO to
require the contents of every bulk milk pick-up truck tanker to be
tested for beta-lactam residues prior to entering the food supply.
This change in the PMO, which was identified as Appendix N, created a
need for rapid, reliable screening tests that could detect residues of
beta-lactam antibiotics in milk when present at or above the
concentrations established by the U.S. Food and Drug Administration
(FDA) to be safe.
In part because Appendix N, PMO, required methods used in monitoring
programs to "have been evaluated through AOAC and accepted by FDA,"
the Agency began the evaluation program in cooperation with the AOAC
International-Research Institute (AOACI-RI) and the A. D. Little, Inc.
(A. D. Little, Inc. is no longer participating in the evaluation
program). These firms served as independent third parties in the
initial evaluation.
A protocol was developed for the evaluation of the tests for detecting
antibiotics in raw, bulk, commingled milk. This protocol was
developed with cooperation and input of representatives of all
interests of the milk industry. Since the NCIMS (Appendix N, PMO)
required monitoring at the truck tanker, the protocol was only for
raw, bulk, commingled milk and not for milk from individual cows.
The selectivity and sensitivity data developed by the test sponsors
were verified in an independent laboratory. The selectivity standard
used requires tests to correctly identify, with 95 percent confidence,
milk containing no drug residues in 90 percent of the samples (90/95).
The initial sensitivity standard required the tests to detect milk
containing residues of claimed drugs at their tolerance/safe levels 90
percent of the time with 95 percent confidence. A test could meet
these standards by correctly identifying 30 of 30 zero control samples
and 30 of 30 samples containing each claimed drug at its
tolerance/safe level. With this selectivity standard for acceptance,
the probability is low that an accepted screening test would produce a
positive result on a milk sample that does not contain any of the drug
it is designed to detect.
Although the only standards FDA established for acceptance of tests
were those for selectivity rate and sensitivity rate, other parameters
of test performance were also evaluated in the joint program, and the
sensitivity was validated with incurred residues in milk. Parameters
studied include: effect of the presence in milk of PMO-permitted
levels of bacteria and somatic cells; possible interference of other
classes of animal drugs which might be present as residues in milk;
the percent of positive results to be expected from a test when milk
containing residue levels below a drug's tolerance are assayed (dose
response)(false violative test results); and ruggedness testing
provided for the evaluation of variation in times, volumes,
temperatures, etc. in the directions for operating the test that would
impact the performance of each test. In requiring the incurred
residue parameter, milk from animals receiving the different drugs are
evaluated using the tests. This requires that the tests detect the
incurred residues at the tolerance/safe level with the concentration
in the milk being quantitatively determined by a chemical test. The
objective of including these parameters of test performance is to
insure that manufacturers of FDA accepted tests provide potential
users with as much information as possible about the performance of a
given test. This enables individual users to make informed decisions
about which test is the best one for them to use and also to encourage
test manufacturers to develop improved tests.
To date, 17 screening tests for beta-lactam residues, one test for
chloramphenicol, and one test for sulfonamide residues in milk have
been accepted by FDA, certified as "performance tested" by AOACI-RI,
and/or accepted by A.D. Little, Inc. These tests are also accepted by
the NCIMS for Appendix N, PMO, testing. The attached Chart contains
the tolerance/safe levels for approved beta-lactam drugs and the 90/95
percent sensitivity levels for accepted tests. 3
_____________________
3 The BSDA described in Appendix G and discussed above as
having
been evaluated earlier by AOAC, was widely used in the years
prior to 1991. The test was available as a complete kit from
Difco Laboratories, but some testing facilities used a generic
BSDA made in-house from agar, spores, standards, etc. obtained
from Difco or from other sources. The BSDA listed in the
attached Chart is a complete kit manufactured by Charm
Sciences,
Inc. Neither the Difco BSDA nor any generic BSDA was
submitted
for inclusion in the Agency's initial evaluation of the BSDA
test. Under the provisions of Appendix N, any version of the
BSDA that has not been accepted by FDA should no longer be
used
to monitor tanker truck milk.
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IV. BACILLUS STEAROTHERMOPHILUS DISC ASSAY (BSDA)
Appendix G of the PMO presently recognizes a generic BSDA. The
BSDA
is a microbiological test based on growth inhibition to detect
antibiotic presence. Performance of the BSDA was evaluated in 1982
in
an AOAC collaborative study using only penicillin. A 16 mm zone
was
accepted as the standard for a violative result. A positive test
result with this test indicates that an inhibitor is present in the
milk. It does not provide the identity nor quantity of the inhibitor.
In addition to penicillin, this BSDA procedure has been demonstrated
to detect residues of ampicillin, cephapirin, and amoxicillin at or
below safe levels. The test also detects residues of ceftiofur at
slightly above its level of concern for misuse.
Charm Sciences and Difco Laboratories market a complete BSDA kit.
The
Charm Sciences' test has been evaluated and accepted for Appendix
N
monitoring. Difco recently decided to have its test evaluated. At
this time, the Difco BSDA is not accepted for Appendix N, PMO,
testing.
At the 1995 meeting of the NCIMS, all the tests accepted for milk
monitoring under the provisions of Appendix N, PMO, were also
accepted
for Section 6, PMO, testing (producer bulk tank). The Difco BSDA,
the
Delvo Test P 5 Pack, the generic BSDA described in Appendix G,
PMO,
and the Charm I and II Liquid tests are accepted for use on farm bulk
tank samples but not Appendix N, PMO, tanker truck samples.
These
tests have not passed the performance standards of the Appendix N,
PMO, tests, and their use in regulatory decisions is an issue that is
expected to be an item for consideration by the NCIMS. (Note: The
Charm I and II Liquid tests are no longer manufactured).
V. SENSITIVITY STANDARD
Since the screening tests give positive results at drug concentrations
below the tolerance/safe level, there have been a number of questions
regarding the acceptance of the tests. These comments have inferred
that tests should not have been accepted by the NCIMS as they were
to
have been evaluated exactly "at the safe level or tolerance" (Appendix
N, PMO).
The original evaluation required a determination of a 90/95 percent
concentration at or below the tolerance/safe level. All the
evaluations have included the tolerance/safe level. Had FDA not
accepted tests which give a positive result at drug concentrations
below the tolerance/safe level, none of the tests would have been
accepted (including the current Section 6, PMO, tests). The current
state of screening test technology is inadequate for this level of
precision.
All the tests have a characteristic sigmoid response curve. This
means that as drug concentration increases, there is also a
corresponding increase in percent positives until you reach a
concentration plateau after which all samples will be positive. In
the original evaluations, only a minimal number of assays were
required at the lower concentrations. The Food and Drug
Administration originally established a sensitivity rate standard
requiring a test to give 90 percent positive results with 95 percent
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confidence on milk samples containing drug concentrations at or
below
the tolerance/safe level. All the tests met this standard for each
drug claimed on the label. When the data were presented to the
NCIMS,
they requested that the test sponsors provide data which define the
dose response at the lower drug concentrations with equal confidence
as the 90/95 percent level. These data provide the means to
determine
the estimated minimal 90 percent level, with 95 percent confidence.
The development of these data is nearing completion and will be
reflected on revised labeling for each test. The FDA remains
convinced that the dose response information is important to the user
in deciding on the test to use. This information is on the label of
each accepted test.
Ideally, the accepted tests should give no positive results when there
are drug residues below the tolerance/safe level (false violative test
result). Equally desired is for the tests to give a positive result
100 percent of the time when the drug concentration is exactly at or
above the tolerance/safe level (no false negative test results). The
screening tests evaluated to date do not perform in this manner.
They
do not "turn on and off" precisely at the tolerance/safe level.
VI. QUESTIONS/ISSUES ON PERFORMANCE OF THE
CURRENTLY ACCEPTED TESTS
There is no ideal screening test currently available for detecting
antimicrobial drug residues in milk (see attached Chart). None of the
beta-lactam tests detect all the beta-lactam drugs. Some have the
90/95 percent sensitivity level for specific drugs below the
tolerance/safe level, and all give false violative test results.
These multi-residue tests are developed to be sensitive to at least
four of the beta-lactam drugs at their tolerance/safe level. Each
multi-residue test, including the BSDA, detects one or more drugs at
levels below their tolerance, and each test fails to detect one or
more approved beta-lactam drugs at their tolerances. There is a
consumer concern when a test is used which does not detect all the
beta-lactam drugs because milk might contain residue that has not
been
shown to be safe. The above is also true for the sulfonamide test.
The rejection of safe milk is an economic issue for the milk producer.
Currently the NCIMS accepted screening tests, including the BSDA,
produce positive results on a certain proportion of samples which
contain below tolerance/safe level concentrations of residues (false
violative test results). For example, a test that demonstrates a 90/95
percent sensitivity of 10 ppb for residues of ampicillin, might
produce a positive result on 40 percent of the samples with 4 ppb,
etc. Of course, the number of truck tankers that would be rejected
because of this detection characteristic of screening tests is a
function of the number of truck tankers that arrive at a processor
containing 4 ppb of ampicillin. Our best estimates of this number is
that it is low. To help test users select the most appropriate test
for their situation and minimize the number of false violative test
results, the Agency requires concentration-response data to be placed
on the labels of all accepted screening tests.
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Furthermore, it is obvious from Chart that different tests demonstrate
different sensitivities toward the six approved beta-lactams. It is
not surprising therefore, that the action of replacing the single
official BSDA (Appendix G, PMO) with 17 different, more reliable,
and
some more sensitive, accepted beta-lactam tests has resulted in some
confusion. For example, the same sample of milk could produce
different results when tested by different receiving stations,
processors, or State regulatory agencies, that do not employ the same
test.
Recognizing the economic importance of false violative test results to
the milk producer, the Agency and the NCIMS agreed to a procedure
whereby all truck tankers found to have a positive test result in
their initial screening test must be confirmed using a procedure to
reduce the incidence of false positive and false violative test
results. This is actually a retesting of the positive sample using
the same test when the initial test is conducted by the industry
analyst. This retesting includes the following:
1. Positive control sample -- Ensures that truly positive milk will
test positive.
2. Negative control sample -- Ensures that truly negative milk will
test negative.
3. Two truck tanker samples -- For the truck tanker to be rejected,
one of the retests must be positive.
In the example of the test described above, the follow-up testing
procedure would reduce the chance of finding a positive truck tanker
with milk containing 4 ppb of ampicillin from 40 to 26 percent.
Some
comments have criticized this procedure because of use of the term
"confirm." The purpose of the retesting is to reduce the number of
false violatives.
The following are other issues for consideration in forming an
opinion
regarding the use of milk residue screening tests:
1. Milk producers rarely, if ever, treat their entire herd with an
antibiotic. Rather, in most instances, a few cows are treated at
the same time. Following the milk discard time, the milk from a
few treated cows is commingled in the farm bulk tank with a
greater amount of milk from untreated animals. Farm bulk tank
milk is normally further diluted with milk from several farms
while being transported to a milk processing plant where it is
tested. The effect of this dilution virtually eliminates the
possibility that truly positive truck tanker milk will occur when
drugs are used in an approved manner. For a test to give a
positive result, even if it is a false violative test result, the
drug generally must have been misused in the cow(s) that
contributed the milk containing residues at a high enough
concentration to contaminate the truck tanker. A positive test
result on a truck tanker sample is an indication that the
producer whose milk caused the positive truck tanker test offered
milk for sale which was above the recommended tolerance/safe
level (regulatory action level). Follow-up testing on producer
samples and on-farm investigations confirm this conclusion. The
proper use of approved animal drugs in individual dairy cows
does
not cause the accepted screening tests to produce a truly
positive truck tanker result as some may suggest.
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In accordance with the provisions of Appendix N, PMO, all
positive test results must be investigated. As a false violative
test result is a possible outcome from the monitoring of milk
with screening tests and based on a case by case review of each
investigation, additional testing and/or investigation may be
justified.
2. An objective of the National Drug Residue Milk Monitoring
Program
(NDRMMP) is to estimate the incidence of non-beta-lactam drug
residues. This objective is being accomplished through use of
screening tests for other animal drugs which have neither been
evaluated nor accepted for use in Appendix N type monitoring
programs.
Some have misinterpreted data from FDA's NDRMMP to
conclude that
the screening tests used in this program produced an 85.6 percent
false positive rate. This is a misrepresentation of the data.
These quarterly reports include any screening test samples
submitted by the states to FDA for confirmation as well as the
NDRMMP samples. Those samples previously found to be
positive by
screening tests are reported as violative as determined by
chemical tests only when the drug concentration is found to be at
or above the tolerance/safe level. The majority of the screening
test positive samples (presumptive positive result) have been
found to contain detectable concentrations of drug; these are
false violative test results. This misleading report of a 85.6
percent false positive results includes those samples found to
contain drug concentrations below the tolerance/safe level but
not reported as violative. Except for the beta/lactam samples,
all confirmed positive test results are based on a chemical test.
3. The FDA funds a contract under which State milk regulatory
agencies report the results of milk monitoring in their states.
The proportion of reported positive loads throughout the country
has been very low. In 1994, there was 3.2 million truck tanker
loads tested with 0.063 percent found positive (63 of each
100,000 truck tankers). The milk rejected for human food totaled
approximately 68 million pounds which is approximately 0.044
percent of the milk supply. These numbers represent the
maximum
percent of positive test results at or above the tolerance/safe
level plus false violative and false positive test results.
4. Several reports in the scientific literature describe the
presence of "natural defense secretions" in milk from mastitic
cows that will produce positive screening test results on milk
from cows that have not been treated with any animal drug.
These
data indicate that certain screening tests produce false positive
test results in milk from individual cows secreting these
natural inhibitors. Some authors have attempted to apply the
false positive rates obtained in milk from individual cows to
samples from truck tankers containing commingled milk derived
from many cows. The FDA believes these are invalid
comparisons
as the data reported would not support a conclusion that false
positive milk from one or several cows produces a false positive
test result in truck tanker milk. Using the level of somatic
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cells as an indicator of the presence of natural
inhibitors, none of the research and test evaluations conducted
at FDA has revealed an effect on test performance from natural
inhibitors unless the milk has a somatic cell count that exceeds
several million - many times the level of these cells permitted
under the PMO.
VII. TRULY POSITIVE, FALSE POSITIVE, OR FALSE
VIOLATIVE TEST RESULT
Prior to the implementation of milk testing in accordance with the
provision of Appendix N, PMO, the milk processors, who carry out
the
testing, acknowledged that some positives could result from residue
concentrations that are below the tolerance/safe level. However,
since they advised FDA that they do not want any antibiotic residues
in their products, they indicated they were prepared to accept some
false violative results.
Based on the levels of drug residues expected to be in milk from
individual cows following recommended milk discard periods for
approved drugs, the use of animal drugs in accordance with label
directions does not cause a truly violative result in truck tanker
samples when using an accepted screening test. There is the small
possibility that when using an approved beta-lactam drug in
accordance
with label directions on an entire herd with that herd being the only
one represented on a given truck tanker, a positive result is
possible. The FDA believes the probability of this scenario
happening
is remote as herd treatment for mastitis is a rare event. There is
also the possibility of a positive screening test result for
chlortetracycline when administered via the feed on a herd basis.
It is important to understand that a truly false positive, false
negative, or false violative test result can only be determined by a
chemical analysis of the milk sample. A determination of the true
status of a positive sample, before the milk enters the food chain,
requires that:
1. Each positive tanker be held for further chemical analysis to
determine the drug and concentration. This would prove to be
impractical because of the time and cost involved.
2. Screening tests of increased capability be accepted for screening
truck tankers. Current state of the art for practical,
inexpensive tests, does not permit an identification of the drug
nor determination of concentration. In the meantime, the NCIMS
screening tests are the best technology available.
VIII. MILK DISCARD TIME ON APPROVED DRUG LABELS
Milk discard time is the number of hours after treatment that are
required for residues in milk to reach the tolerance/safe level. The
milk discard time is not the point at which residues can no longer be
detected with the chemical tests or some screening tests. Currently,
milk discard times are established using a chemical test; the accepted
milk screening tests do not have the required analytical
characteristics to establish official milk discard times.
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IX. EVALUATION OF SCREENING TESTS FOR USE IN MILK
FROM INDIVIDUAL
COWS
The FDA announced in 1993 its interest in developing a protocol for
the evaluation of screening tests for milk from individual cows. A
number of meetings were held involving representatives of the
industry, including the National Mastitis Council. A protocol was
designed using the same administrative procedures as for the accepted
tests for commingled milk with the test manufacturers paying for the
evaluation. As there is no regulatory requirement for the testing of
milk from individual cows, the manufacturers saw no economic
incentive
to have their tests evaluated; and consequently, no tests have been
evaluated under this protocol.
Given FDA's view of the importance of having reliable,
independently
evaluated tests available for use by the producer/practitioner in milk
from individual cows, the Agency decided to fund an evaluation of
the
tests. Recognizing the screening tests which had already been
evaluated for commingled milk and the parameters included in that
evaluation, FDA developed a different protocol using the selectivity
and mastitis concerns as parameters for additional evaluation. This
revised protocol was also reviewed by representative of the National
Mastitis Council. The protocol addresses the issues of false positive
test results in milk from individual cows which are healthy and have
not been treated with drugs and also a mastitis model to address the
issue of cows recovering from mastitis. In the mastitis model study,
the tests must give the correct result on a milk sample from healthy
cows and also on visually normal milk following recovery from an
endotoxin challenge in the udder. The tests must also give the
correct response on the visually normal milk when claimed drugs are
added. The Agricultural Research Service at the United States
Department of Agriculture, Beltsville, MD is conducting the studies
under an Interagency Agreement with the Center for Veterinary
Medicine. Test manufacturers requested that nine of the accepted
beta-lactam tests be evaluated. The selectivity study section of the
protocol has been completed. The mastitis model study was
completed
in 1995. The FDA expects to issue a report on these studies in the
spring of 1996.
As screening tests can detect drug concentrations below the
tolerance/safe level, a positive test result is possible with a
screening test on a milk sample collected from cows after the labeled
milk discard time. For this reason, FDA does not recommend the
use of
screening tests on cows which have been individually treated in
accordance with label directions. It should be noted, however, that
these lower concentrations will not cause a violative or non-violative
positive truck tanker except in those exceptions described previously.
If it is desirable to have no detectable residue in milk from
individual cows, then testing with a screening test until the sample
is negative is a viable option. To preclude the possibility of a
truck tanker positive result, screening tests may also be used in milk
from individual cows treated in an extra-label manner.
X. STATE OF THE ART
The FDA recognizes that screening tests for detecting antimicrobial
drugs in milk are neither drug specific nor quantitative in their
performance. A positive test result means that something in the milk
caused this outcome. The evaluation protocol used
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provides ample assurance that a positive test will result when the
drug claimed on the label is in the sample at concentrations at or
above the tolerance/safe level. Similarly, FDA is confident that the
test result will be negative when there is no drug in the sample. The
residue concentration between no detectable concentration and the
90/95 percent concentration (false violative test result) is a
c
oncern, particularly for the producer, as these concentrations may
cause a positive test result and the truck tanker of milk deemed
unacceptable for human food. From a human safety perspective,
FDA is
also concerned for the false negative potential of the tests.
It is important to understand that under Section 6 of the PMO,
producer bulk tank milk has been monitored for a number of years,
before implementation of Appendix N, PMO, using some of the
accepted
Appendix N, PMO, tests. These screening tests have the same
limitations whether used as a Section 6, PMO, test or an Appendix
N,
PMO, test. The difference is that now the PMO requires the
monitoring
of each tanker truck for beta-lactam residues before entry into the
food chain.
XI. SUMMARY
Some have inaccurately reported results of milk monitoring to
advance
the proposition that the use of the accepted milk residue screening
tests be discontinued until more accurate tests are available. The
percentage of truck tankers found positive in 1994 (National Milk
Drug
Residue Third Party Data Base) was 0.063 percent. This low
incidence
of positive truck tankers supports our standard for selectivity (false
positive test result). Further, this low incidence of positive truck
tankers also demonstrates that the majority of the milk producers are
using drugs in a responsible manner. The FDA has found no
evidence
which indicates that the use of approved drugs in accordance with
label directions will cause a positive test result in a truck tanker
with the possible exception when chlortetracycline is administered in
feed on a herd basis. The FDA has concluded that despite the
inherent
limitations of screening tests, the issue remains one of proper drug
use. The FDA believes the use of the accepted tests under the
provisions of Appendix N, PMO, has reduced the amount of milk
containing antimicrobial drugs entering the food supply.
The low incidence of positive truck tankers and the rejection of those
truck tanker loads for human food is recognition of the milk
industry's focus on and commitment to the human safety of milk.
This
low incidence and Appendix N, PMO, monitoring is a significant
factor
for the milk industry in maintaining consumer confidence in the
safety
of milk. The reliability of the accepted tests to detect drug
residues adds credibility to the testing program, and the small
percentage of positive tankers confirms the validity of the acceptance
standards for the tests.
The FDA recognizes the limitations of the screening tests. These
are:
1. The tests cannot identify the drug residue nor the concentration
of the residue. A safety determination cannot be determined
from
a positive result.
2. The screening tests produce some positive results when the drug
concentration is below the tolerance/safe level. Under the
provisions of Appendix N, PMO, this limitation provides for a
few
positive truck tanker loads of milk to be rejected for human
consumption when the milk is safe.
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While these accepted tests have their limitations in drug
identification and quantification, they work well in detecting
violative drug residues in milk. The low probability of a false
positive and a false negative finding is inherent in the acceptability
standards of the evaluation. The FDA has found no evidence which
would indicate that the accepted tests are not performing in this
manner.
The FDA recognizes the economic losses to the milk producer which
would result from false violative and false positive test results.
With this issue in mind, the NCIMS and the FDA agreed to retest all
original positive truck tanker samples using the same test when tested
by an industry analyst. Only after the results from retesting
indicate a positive finding is a truck tanker rejected. Retesting
increases the probability of acceptance of a non-violative milk tanker
and decreases the number of non-violative truck tanker rejections.
The FDA must also be concerned with the incidence of false negative
results to ensure the public health.
Based on reports from the states, the FDA has concluded that misuse
of
animal drugs is the cause of most positive test results from truck
tanker testing even when residue concentrations are below the
tolerance/safe level. The FDA has found no evidence which indicates
that treating lactating cows in accordance with labeled directions
will cause a positive truck tanker. The low incidence of positive
truck tanker results do not appear to be caused by unreliable tests.
The follow-up by the State regulatory agencies on positive truck
tankers indicates that the positive test results are primarily the
result of misuse of animal drugs.
Office of Science, HFV-500
CVM/FDA
7500 Standish Place
Rockville, MD 20855
January, 1996
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