method relates to the concentration of residues and the TTSC, respectively. The
strength of the TTSC approach is that an assumption of linear depletion of residues
from milk is unnecessary. Conceptually, the TTSC method models the withdrawal
period as a function of concentration by assuming a distribution of the times to safe
concentration (i.e. MRL) and calculating an upper limit of the TTSC at MRL.
The JECFA does not assign withdrawal periods for veterinary drug products.
This function is performed by the relevant regulatory authority in member countries
that adopt Codex MRLs, and it ensures that the depletion kinetics of individual
products as well as regional practices and requirements are taken into account.
2.2.2 Residues of Veterinary Drugs in International Trade
Animal-derived foodstuffs in international trade must comply with the food stan-
dards of the importing country which, in general, are national MRLs or Codex
MRLs. Situations arise with certain veterinary drugs, however, where food stan-
dards have not been established by either the importing country or the Codex
Alimentarius Commission. This may reflect a prohibition on the use of the sub-
stance in food-producing animals, but more frequently, it is the result of no
evaluation of data having been performed. A zero-tolerance approach to residues
of such products is commonly adopted. Mechanisms for addressing this situation
have been developed: they include the establishment of Import MRLs for certain
substances; the development of trade agreements between trading partners; and
unique to Australia, the assignment of export slaughter intervals (ESI) to veterinary
products intended for use on food-producing animals destined for overseas markets
(see below).
Estimating an Export Slaughter Interval
Under its legislation, the Australian Pesticides and Veterinary Medicines Authority
(APVMA) must be satisfied that the registration of agricultural and veterinary
chemical products will not unduly prejudice trade with countries outside of Australia.
Discharge of this legislative obligation is guided by risk assessments conducted
prior to the granting of marketing approvals. Compliance with the withdrawal
period provides assurance that residues have depleted to below the Australian
MRL. However, food standards in some markets may be more stringent than the
Australian standard, or alternatively, no standard may have been established and
a zero-tolerance approach to residues of the substance applies. In both of these
situations, the ESI is a valuable tool for mitigating risks in trade that might other-
wise arise as the result of using a veterinary chemical product that is approved for
use in food-producing animals in Australia.
An ESI is the interval that elapses between administering, or applying, a veteri-
nary chemical product to a food-producing animal and slaughter of the animal for
export. An ESI may be equal to, or longer than, the withdrawal period for the
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